Request for Comments: 6797
Category: Standards Track
Carnegie Mellon University
HTTP Strict Transport Security (HSTS)
This specification defines a mechanism enabling web sites to declare themselves accessible only via secure connections and/or for users to be able to direct their user agent(s) to interact with given sites only over secure connections. This overall policy is referred to as HTTP Strict Transport Security (HSTS). The policy is declared by web sites via the Strict-Transport-Security HTTP response header field and/or by other means, such as user agent configuration, for example.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6797.
Copyright © 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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Table of Contents
1. Introduction ....................................................4 1.1. Organization of This Specification .........................6 1.2. Document Conventions .......................................6 2. Overview ........................................................6 2.1. Use Cases ..................................................6 2.2. HTTP Strict Transport Security Policy Effects ..............6 2.3. Threat Model ...............................................6 2.3.1. Threats Addressed ...................................7 188.8.131.52. Passive Network Attackers ..................7 184.108.40.206. Active Network Attackers ...................7 220.127.116.11. Web Site Development and Deployment Bugs ...8 2.3.2. Threats Not Addressed ...............................8 18.104.22.168. Phishing ...................................8 22.214.171.124. Malware and Browser Vulnerabilities ........8 2.4. Requirements ...............................................9 2.4.1. Overall Requirement .................................9 126.96.36.199. Detailed Core Requirements .................9 188.8.131.52. Detailed Ancillary Requirements ...........10 3. Conformance Criteria ...........................................10 4. Terminology ....................................................11 5. HSTS Mechanism Overview ........................................13 5.1. HSTS Host Declaration .....................................13 5.2. HSTS Policy ...............................................13 5.3. HSTS Policy Storage and Maintenance by User Agents ........14 5.4. User Agent HSTS Policy Enforcement ........................14 6. Syntax .........................................................14 6.1. Strict-Transport-Security HTTP Response Header Field ......15 6.1.1. The max-age Directive ..............................16 6.1.2. The includeSubDomains Directive ....................16 6.2. Examples ..................................................16 7. Server Processing Model ........................................17 7.1. HTTP-over-Secure-Transport Request Type ...................17 7.2. HTTP Request Type .........................................18 8. User Agent Processing Model ....................................18 8.1. Strict-Transport-Security Response Header Field Processing ................................................19 8.1.1. Noting an HSTS Host - Storage Model ................20 8.2. Known HSTS Host Domain Name Matching ......................20 8.3. URI Loading and Port Mapping ..............................21 8.4. Errors in Secure Transport Establishment ..................22 8.5. HTTP-Equiv <Meta> Element Attribute .......................22 8.6. Missing Strict-Transport-Security Response Header Field ...23 9. Constructing an Effective Request URI ..........................23 9.1. ERU Fundamental Definitions ...............................23 9.2. Determining the Effective Request URI .....................24 9.2.1. Effective Request URI Examples .....................24 10. Domain Name IDNA-Canonicalization .............................25 11. Server Implementation and Deployment Advice ...................26 11.1. Non-Conformant User Agent Considerations .................26 11.2. HSTS Policy Expiration Time Considerations ...............26 11.3. Using HSTS in Conjunction with Self-Signed Public-Key Certificates .............................................27 11.4. Implications of includeSubDomains ........................28 11.4.1. Considerations for Offering Unsecured HTTP Services at Alternate Ports or Subdomains of an HSTS Host ........................................28 11.4.2. Considerations for Offering Web Applications at Subdomains of an HSTS Host .......................29 12. User Agent Implementation Advice ..............................30 12.1. No User Recourse .........................................30 12.2. User-Declared HSTS Policy ................................30 12.3. HSTS Pre-Loaded List .....................................31 12.4. Disallow Mixed Security Context Loads ....................31 12.5. HSTS Policy Deletion .....................................31 13. Internationalized Domain Names for Applications (IDNA): Dependency and Migration ......................................32 14. Security Considerations .......................................32 14.1. Underlying Secure Transport Considerations ...............32 14.2. Non-Conformant User Agent Implications ...................33 14.3. Ramifications of HSTS Policy Establishment Only over Error-Free Secure Transport ..............................33 14.4. The Need for includeSubDomains ...........................34 14.5. Denial of Service ........................................35 14.6. Bootstrap MITM Vulnerability .............................36 14.7. Network Time Attacks .....................................37 14.8. Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack .........................................37 14.9. Creative Manipulation of HSTS Policy Store ...............37 14.10. Internationalized Domain Names ..........................38 15. IANA Considerations ...........................................39 16. References ....................................................39 16.1. Normative References .....................................39 16.2. Informative References ...................................40 Appendix A. Design Decision Notes .................................44 Appendix B. Differences between HSTS Policy and Same-Origin Policy ................................................45 Appendix C. Acknowledgments .......................................46
HTTP [RFC2616] may be used over various transports, typically the Transmission Control Protocol (TCP). However, TCP does not provide channel integrity protection, confidentiality, or secure host identification. Thus, the Secure Sockets Layer (SSL) protocol [RFC6101] and its successor, Transport Layer Security (TLS) [RFC5246] were developed in order to provide channel-oriented security and are typically layered between application protocols and TCP. [RFC2818] specifies how HTTP is layered onto TLS and defines the Uniform Resource Identifier (URI) scheme of "https" (in practice, however, HTTP user agents (UAs) typically use either TLS or SSL3, depending upon a combination of negotiation with the server and user preferences).
UAs employ various local security policies with respect to the characteristics of their interactions with web resources, depending on (in part) whether they are communicating with a given web resource's host using HTTP or HTTP-over-Secure-Transport. For example, cookies ([RFC6265]) may be flagged as Secure. UAs are to send such Secure cookies to their addressed host only over a secure transport. This is in contrast to non-Secure cookies, which are returned to the host regardless of transport (although subject to other rules).
UAs typically announce to their users any issues with secure connection establishment, such as being unable to validate a TLS server certificate trust chain, or if a TLS server certificate is expired, or if a TLS host's domain name appears incorrectly in the TLS server certificate (see Section 3.1 of [RFC2818]). Often, UAs enable users to elect to continue to interact with a web resource's host in the face of such issues. This behavior is sometimes referred to as "click(ing) through" security [GoodDhamijaEtAl05] [SunshineEgelmanEtAl09]; thus, it can be described as "click-through insecurity".
A key vulnerability enabled by click-through insecurity is the leaking of any cookies the web resource may be using to manage a user's session. The threat here is that an attacker could obtain the cookies and then interact with the legitimate web resource while impersonating the user.
Jackson and Barth proposed an approach, in [ForceHTTPS], to enable web resources to declare that any interactions by UAs with the web resource must be conducted securely and that any issues with establishing a secure transport session are to be treated as fatal and without direct user recourse. The aim is to prevent click- through insecurity and address other potential threats.
This specification embodies and refines the approach proposed in [ForceHTTPS]. For example, rather than using a cookie to convey policy from a web resource's host to a UA, it defines an HTTP response header field for this purpose. Additionally, a web resource's host may declare its policy to apply to the entire domain name subtree rooted at its host name. This enables HTTP Strict Transport Security (HSTS) to protect so-called "domain cookies", which are applied to all subdomains of a given web resource's host name.
This specification also incorporates notions from [JacksonBarth2008] in that policy is applied on an "entire-host" basis: it applies to HTTP (only) over any TCP port of the issuing host.
Note that the policy defined by this specification is distinctly different than the "same-origin policy" defined in "The Web Origin Concept" [RFC6454]. These differences are summarized in Appendix B.
1.1. Organization of This Specification
This specification begins with an overview of the use cases, policy effects, threat models, and requirements for HSTS (in Section 2). Then, Section 3 defines conformance requirements. Section 4 defines terminology relevant to this document. The HSTS mechanism itself is formally specified in Sections 5 through 15.
1.2. Document Conventions
NOTE: This is a note to the reader. These are points that should be expressly kept in mind and/or considered.
This section discusses the use cases, summarizes the HSTS Policy, and continues with a discussion of the threat model, non-addressed threats, and derived requirements.
2.1. Use Cases
The high-level use case is a combination of:
- Web browser user wishes to interact with various web sites (some arbitrary, some known) in a secure fashion.
- Web site deployer wishes to offer their site in an explicitly secure fashion for their own, as well as their users', benefit.
2.2. HTTP Strict Transport Security Policy Effects
The effects of the HSTS Policy, as applied by a conformant UA in interactions with a web resource host wielding such policy (known as an HSTS Host), are summarized as follows:
- UAs transform insecure URI references to an HSTS Host into secure URI references before dereferencing them.
- The UA terminates any secure transport connection attempts upon any and all secure transport errors or warnings.
2.3. Threat Model
HSTS is concerned with three threat classes: passive network attackers, active network attackers, and imperfect web developers. However, it is explicitly not a remedy for two other classes of threats: phishing and malware. Threats that are addressed, as well as threats that are not addressed, are briefly discussed below.
Readers may wish to refer to Section 2 of [ForceHTTPS] for details as well as relevant citations.
2.3.1. Threats Addressed
184.108.40.206. Passive Network Attackers
When a user browses the web on a local wireless network (e.g., an 802.11-based wireless local area network) a nearby attacker can possibly eavesdrop on the user's unencrypted Internet Protocol-based connections, such as HTTP, regardless of whether or not the local wireless network itself is secured [BeckTews09]. Freely available wireless sniffing toolkits (e.g., [Aircrack-ng]) enable such passive eavesdropping attacks, even if the local wireless network is operating in a secure fashion. A passive network attacker using such tools can steal session identifiers/cookies and hijack the user's web session(s) by obtaining cookies containing authentication credentials [ForceHTTPS]. For example, there exist widely available tools, such as Firesheep (a web browser extension) [Firesheep], that enable their wielder to obtain other local users' session cookies for various web applications.
To mitigate such threats, some web sites support, but usually do not force, access using end-to-end secure transport -- e.g., signaled through URIs constructed with the "https" scheme [RFC2818]. This can lead users to believe that accessing such services using secure transport protects them from passive network attackers. Unfortunately, this is often not the case in real-world deployments, as session identifiers are often stored in non-Secure cookies to permit interoperability with versions of the service offered over insecure transport ("Secure cookies" are those cookies containing the "Secure" attribute [RFC6265]). For example, if the session identifier for a web site (an email service, say) is stored in a non-Secure cookie, it permits an attacker to hijack the user's session if the user's UA makes a single insecure HTTP request to the site.
220.127.116.11. Active Network Attackers
A determined attacker can mount an active attack, either by impersonating a user's DNS server or, in a wireless network, by spoofing network frames or offering a similarly named evil twin access point. If the user is behind a wireless home router, an attacker can attempt to reconfigure the router using default passwords and other vulnerabilities. Some sites, such as banks, rely on end-to-end secure transport to protect themselves and their users from such active attackers. Unfortunately, browsers allow their users to easily opt out of these protections in order to be usable for sites that incorrectly deploy secure transport, for example by generating and self-signing their own certificates (without also distributing their certification authority (CA) certificate to their users' browsers).
18.104.22.168. Web Site Development and Deployment Bugs
The security of an otherwise uniformly secure site (i.e., all of its content is materialized via "https" URIs) can be compromised completely by an active attacker exploiting a simple mistake, such as the loading of a cascading style sheet or a SWF (Shockwave Flash) movie over an insecure connection (both cascading style sheets and SWF movies can script the embedding page, to the surprise of many web developers, plus some browsers do not issue so-called "mixed content warnings" when SWF files are embedded via insecure connections). Even if the site's developers carefully scrutinize their login page for "mixed content", a single insecure embedding anywhere on the overall site compromises the security of their login page because an attacker can script (i.e., control) the login page by injecting code (e.g., a script) into another, insecurely loaded, site page.
NOTE: "Mixed content" as used above (see also Section 5.3 in [W3C.REC-wsc-ui-20100812]) refers to the notion termed "mixed security context" in this specification and should not be confused with the same "mixed content" term used in the context of markup languages such as XML and HTML.
2.3.2. Threats Not Addressed
Phishing attacks occur when an attacker solicits authentication credentials from the user by hosting a fake site located on a different domain than the real site, perhaps driving traffic to the fake site by sending a link in an email message. Phishing attacks can be very effective because users find it difficult to distinguish the real site from a fake site. HSTS is not a defense against phishing per se; rather, it complements many existing phishing defenses by instructing the browser to protect session integrity and long-lived authentication tokens [ForceHTTPS].
22.214.171.124. Malware and Browser Vulnerabilities
Because HSTS is implemented as a browser security mechanism, it relies on the trustworthiness of the user's system to protect the session. Malicious code executing on the user's system can compromise a browser session, regardless of whether HSTS is used.
This section identifies and enumerates various requirements derived from the use cases and the threats discussed above and also lists the detailed core requirements that HTTP Strict Transport Security addresses, as well as ancillary requirements that are not directly addressed.
2.4.1. Overall Requirement
- Minimize, for web browser users and web site deployers, the risks that are derived from passive and active network attackers, web site development and deployment bugs, and insecure user actions.
126.96.36.199. Detailed Core Requirements
These core requirements are derived from the overall requirement and are addressed by this specification.
- Web sites need to be able to declare to UAs that they should be accessed using a strict security policy.
- Web sites need to be able to instruct UAs that contact them insecurely to do so securely.
- UAs need to retain persistent data about web sites that signal strict security policy enablement, for time spans declared by the web sites. Additionally, UAs need to cache the "freshest" strict security policy information, in order to allow web sites to update the information.
- UAs need to rewrite all insecure UA "http" URI loads to use the "https" secure scheme for those web sites for which secure policy is enabled.
- Web site administrators need to be able to signal strict security policy application to subdomains of higher-level domains for which strict security policy is enabled, and UAs need to enforce such policy.
For example, both example.com and foo.example.com could set policy for bar.foo.example.com.
- UAs need to disallow security policy application to peer domains, and/or higher-level domains, by domains for which strict security policy is enabled.
For example, neither bar.foo.example.com nor foo.example.com can set policy for example.com, nor can bar.foo.example.com set policy for foo.example.com. Also, foo.example.com cannot set policy for sibling.example.com.
- UAs need to prevent users from "clicking through" security warnings. Halting connection attempts in the face of secure transport exceptions is acceptable. See also Section 12.1 ("No User Recourse").
NOTE: A means for uniformly securely meeting the first core requirement above is not specifically addressed by this specification (see Section 14.6 ("Bootstrap MITM Vulnerability")). It may be addressed by a future revision of this specification or some other specification. Note also that there are means by which UA implementations may more fully meet the first core requirement; see Section 12 ("User Agent Implementation Advice").
188.8.131.52. Detailed Ancillary Requirements
These ancillary requirements are also derived from the overall requirement. They are not normatively addressed in this specification but could be met by UA implementations at their implementor's discretion, although meeting these requirements may be complex.
- Disallow "mixed security context" loads (see Section 184.108.40.206).
- Facilitate user declaration of web sites for which strict security policy is enabled, regardless of whether the sites signal HSTS Policy.
3. Conformance Criteria
This specification is written for hosts and user agents.
A conformant host is one that implements all the requirements listed in this specification that are applicable to hosts.
A conformant user agent is one that implements all the requirements listed in this specification that are applicable to user agents.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
Terminology is defined in this section.
ASCII case-insensitive comparison:
means comparing two strings exactly, codepoint for codepoint, except that the characters in the range U+0041 .. U+005A (i.e., LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and the corresponding characters in the range U+0061 .. U+007A (i.e., LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are considered to also match. See [Unicode] for details.
is a colloquial contraction of Code Point, which is any value in the Unicode codespace; that is, the range of integers from 0 to 10FFFF(hex) [Unicode].
is also referred to as "DNS name" and is defined in [RFC1035] to be represented outside of the DNS protocol itself (and implementations thereof) as a series of labels separated by dots, e.g., "example.com" or "yet.another.example.org". In the context of this specification, domain names appear in that portion of a URI satisfying the reg-name production in "Appendix A. Collected ABNF for URI" in [RFC3986], and the host component from the Host HTTP header field production in Section 14.23 of [RFC2616].
NOTE: The domain names appearing in actual URI instances and matching the aforementioned production components may or may not be a fully qualified domain name.
domain name label:
is that portion of a domain name appearing "between the dots", i.e., consider "foo.example.com": "foo", "example", and "com" are all domain name labels.
Effective Request URI:
is a URI, identifying the target resource, that can be inferred by an HTTP host for any given HTTP request it receives. Such inference is necessary because HTTP requests often do not contain a complete "absolute" URI identifying the target resource. See Section 9 ("Constructing an Effective Request URI").
HTTP Strict Transport Security:
is the overall name for the combined UA- and server-side security policy defined by this specification.
HTTP Strict Transport Security Host:
is a conformant host implementing the HTTP server aspects of the HSTS Policy. This means that an HSTS Host returns the "Strict-Transport-Security" HTTP response header field in its HTTP response messages sent over secure transport.
HTTP Strict Transport Security Policy:
is the name of the combined overall UA- and server-side facets of the behavior defined in this specification.
See HTTP Strict Transport Security.
See HTTP Strict Transport Security Host.
See HTTP Strict Transport Security Policy.
Known HSTS Host:
is an HSTS Host for which the UA has an HSTS Policy in effect; i.e., the UA has noted this host as a Known HSTS Host. See Section 8.1.1 ("Noting an HSTS Host - Storage Model") for particulars.
comprises policy rules that deployers specify and that are often manifested as configuration settings.
is an acronym for "man in the middle". See "man-in-the-middle attack" in [RFC4949].
is the URI used to cause a UA to issue an HTTP request message. See also "Effective Request URI".
is an acronym for "user agent". For the purposes of this specification, a UA is an HTTP client application typically actively manipulated by a user [RFC2616].
unknown HSTS Host:
is an HSTS Host that the user agent has not noted.
5. HSTS Mechanism Overview
This section provides an overview of the mechanism by which an HSTS Host conveys its HSTS Policy to UAs and how UAs process the HSTS Policies received from HSTS Hosts. The mechanism details are specified in Sections 6 through 15.
5.1. HSTS Host Declaration
An HTTP host declares itself an HSTS Host by issuing to UAs an HSTS Policy, which is represented by and conveyed via the Strict-Transport-Security HTTP response header field over secure transport (e.g., TLS). Upon error-free receipt and processing of this header by a conformant UA, the UA regards the host as a Known HSTS Host.
5.2. HSTS Policy
An HSTS Policy directs UAs to communicate with a Known HSTS Host only over secure transport and specifies policy retention time duration.
HSTS Policy explicitly overrides the UA processing of URI references, user input (e.g., via the "location bar"), or other information that, in the absence of HSTS Policy, might otherwise cause UAs to communicate insecurely with the Known HSTS Host.
An HSTS Policy may contain an optional directive -- includeSubDomains -- specifying that this HSTS Policy also applies to any hosts whose domain names are subdomains of the Known HSTS Host's domain name.
5.3. HSTS Policy Storage and Maintenance by User Agents
UAs store and index HSTS Policies based strictly upon the domain names of the issuing HSTS Hosts.
This means that UAs will maintain the HSTS Policy of any given HSTS Host separately from any HSTS Policies issued by any other HSTS Hosts whose domain names are superdomains or subdomains of the given HSTS Host's domain name. Only the given HSTS Host can update or can cause deletion of its issued HSTS Policy. It accomplishes this by sending Strict-Transport-Security HTTP response header fields to UAs with new values for policy time duration and subdomain applicability. Thus, UAs cache the "freshest" HSTS Policy information on behalf of an HSTS Host. Specifying a zero time duration signals the UA to delete the HSTS Policy (including any asserted includeSubDomains directive) for that HSTS Host. See Section 8.1 ("Strict-Transport-Security Response Header Field Processing") for details. Additionally, Section 6.2 presents examples of Strict-Transport-Security HTTP response header fields.
5.4. User Agent HSTS Policy Enforcement
When establishing an HTTP connection to a given host, however instigated, the UA examines its cache of Known HSTS Hosts to see if there are any with domain names that are superdomains of the given host's domain name. If any are found, and of those if any have the includeSubDomains directive asserted, then HSTS Policy applies to the given host. Otherwise, HSTS Policy applies to the given host only if the given host is itself known to the UA as an HSTS Host. See Section 8.3 ("URI Loading and Port Mapping") for details.
This section defines the syntax of the Strict-Transport-Security HTTP response header field and its directives, and presents some examples.
Section 7 ("Server Processing Model") then details how hosts employ this header field to declare their HSTS Policy, and Section 8 ("User Agent Processing Model") details how user agents process the header field and apply the HSTS Policy.
6.1. Strict-Transport-Security HTTP Response Header Field
The Strict-Transport-Security HTTP response header field (STS header field) indicates to a UA that it MUST enforce the HSTS Policy in regards to the host emitting the response message containing this header field.
The ABNF (Augmented Backus-Naur Form) syntax for the STS header field is given below. It is based on the Generic Grammar defined in Section 2 of [RFC2616] (which includes a notion of "implied linear whitespace", also known as "implied *LWS").
Strict-Transport-Security = "Strict-Transport-Security" ":"
[ directive ] *( ";" [ directive ] ) directive = directive-name [ "=" directive-value ] directive-name = token directive-value = token | quoted-string
token = <token, defined in [RFC2616], Section 2.2> quoted-string = <quoted-string, defined in [RFC2616], Section 2.2>
The two directives defined in this specification are described below. The overall requirements for directives are:
- The order of appearance of directives is not significant.
- All directives MUST appear only once in an STS header field. Directives are either optional or required, as stipulated in their definitions.
- Directive names are case-insensitive.
- UAs MUST ignore any STS header field containing directives, or other header field value data, that does not conform to the syntax defined in this specification.
- If an STS header field contains directive(s) not recognized by the UA, the UA MUST ignore the unrecognized directives, and if the STS header field otherwise satisfies the above requirements (1 through 4), the UA MUST process the recognized directives.
Additional directives extending the semantic functionality of the STS header field can be defined in other specifications, with a registry (having an IANA policy definition of IETF Review [RFC5226]) defined for them at such time.
NOTE: Such future directives will be ignored by UAs implementing only this specification, as well as by generally non-conforming UAs. See Section 14.2 ("Non-Conformant User Agent Implications") for further discussion.
6.1.1. The max-age Directive
The REQUIRED "max-age" directive specifies the number of seconds, after the reception of the STS header field, during which the UA regards the host (from whom the message was received) as a Known HSTS Host. See also Section 8.1.1 ("Noting an HSTS Host - Storage Model"). The delta-seconds production is specified in [RFC2616].
The syntax of the max-age directive's REQUIRED value (after quoted-string unescaping, if necessary) is defined as:
max-age-value = delta-seconds delta-seconds = <1*DIGIT, defined in [RFC2616], Section 3.3.2> NOTE: A max-age value of zero (i.e., "max-age=0") signals the UA to cease regarding the host as a Known HSTS Host, including the includeSubDomains directive (if asserted for that HSTS Host). See also Section 8.1 ("Strict-Transport-Security Response Header Field Processing").
6.1.2. The includeSubDomains Directive
The OPTIONAL "includeSubDomains" directive is a valueless directive which, if present (i.e., it is "asserted"), signals the UA that the HSTS Policy applies to this HSTS Host as well as any subdomains of the host's domain name.
The HSTS header field below stipulates that the HSTS Policy is to remain in effect for one year (there are approximately 31536000 seconds in a year), and the policy applies only to the domain of the HSTS Host issuing it:
The HSTS header field below stipulates that the HSTS Policy is to remain in effect for approximately six months and that the policy applies to the domain of the issuing HSTS Host and all of its subdomains:
Strict-Transport-Security: max-age=15768000 ; includeSubDomains
The max-age directive value can optionally be quoted:
The HSTS header field below indicates that the UA must delete the entire HSTS Policy associated with the HSTS Host that sent the header field:
The HSTS header field below has exactly the same effect as the one immediately above because the includeSubDomains directive's presence in the HSTS header field is ignored when max-age is zero:
Strict-Transport-Security: max-age=0; includeSubDomains
7. Server Processing Model
This section describes the processing model that HSTS Hosts implement. The model comprises two facets: the first being the processing rules for HTTP request messages received over a secure transport (TLS [RFC5246] or SSL [RFC6101]; see also Section 14.1 ("Underlying Secure Transport Considerations")), and the second being the processing rules for HTTP request messages received over non-secure transports, such as TCP.
7.1. HTTP-over-Secure-Transport Request Type
When replying to an HTTP request that was conveyed over a secure transport, an HSTS Host SHOULD include in its response message an STS header field that MUST satisfy the grammar specified above in Section 6.1 ("Strict-Transport-Security HTTP Response Header Field"). If an STS header field is included, the HSTS Host MUST include only one such header field.
Establishing a given host as a Known HSTS Host, in the context of a given UA, MAY be accomplished over HTTP, which is in turn running over secure transport, by correctly returning (per this specification) at least one valid STS header field to the UA. Other mechanisms, such as a client-side pre-loaded Known HSTS Host list, MAY also be used; e.g., see Section 12 ("User Agent Implementation Advice").
NOTE: Including the STS header field is stipulated as a "SHOULD" in order to accommodate various server- and network-side caches and load-balancing configurations where it may be difficult to uniformly emit STS header fields on behalf of a given HSTS Host.
7.2. HTTP Request Type
If an HSTS Host receives an HTTP request message over a non-secure transport, it SHOULD send an HTTP response message containing a status code indicating a permanent redirect, such as status code 301 (Section 10.3.2 of [RFC2616]), and a Location header field value containing either the HTTP request's original Effective Request URI (see Section 9 ("Constructing an Effective Request URI")) altered as necessary to have a URI scheme of "https", or a URI generated according to local policy with a URI scheme of "https".
NOTE: The above behavior is a "SHOULD" rather than a "MUST" due to: * Risks in server-side non-secure-to-secure redirects [OWASP-TLSGuide].
- Site deployment characteristics. For example, a site that incorporates third-party components may not behave correctly when doing server-side non-secure-to-secure redirects in the case of being accessed over non-secure transport but does behave correctly when accessed uniformly over secure transport. The latter is the case given an HSTS-capable UA that has already noted the site as a Known HSTS Host (by whatever means, e.g., prior interaction or UA configuration).
An HSTS Host MUST NOT include the STS header field in HTTP responses conveyed over non-secure transport.
8. User Agent Processing Model
This section describes the HTTP Strict Transport Security processing model for UAs. There are several facets to the model, enumerated by the following subsections.
This processing model assumes that the UA implements IDNA2008 [RFC5890], or possibly IDNA2003 [RFC3490], as noted in Section 13 ("Internationalized Domain Names for Applications (IDNA): Dependency and Migration"). It also assumes that all domain names manipulated in this specification's context are already IDNA-canonicalized as outlined in Section 10 ("Domain Name IDNA-Canonicalization") prior to the processing specified in this section.
NOTE: [RFC3490] is referenced due to its ongoing relevance to actual deployments for the foreseeable future.
The above assumptions mean that this processing model also specifically assumes that appropriate IDNA and Unicode validations and character list testing have occurred on the domain names, in conjunction with their IDNA-canonicalization, prior to the processing specified in this section. See the IDNA-specific security considerations in Section 14.10 ("Internationalized Domain Names") for rationale and further details.
8.1. Strict-Transport-Security Response Header Field Processing
If an HTTP response, received over a secure transport, includes an STS header field, conforming to the grammar specified in Section 6.1 ("Strict-Transport-Security HTTP Response Header Field"), and there are no underlying secure transport errors or warnings (see Section 8.4), the UA MUST either:
- Note the host as a Known HSTS Host if it is not already so noted (see Section 8.1.1 ("Noting an HSTS Host - Storage Model")),
- Update the UA's cached information for the Known HSTS Host if either or both of the max-age and includeSubDomains header field value tokens are conveying information different than that already maintained by the UA.
The max-age value is essentially a "time to live" value relative to the reception time of the STS header field.
If the max-age header field value token has a value of zero, the UA MUST remove its cached HSTS Policy information (including the includeSubDomains directive, if asserted) if the HSTS Host is known, or the UA MUST NOT note this HSTS Host if it is not yet known.
If a UA receives more than one STS header field in an HTTP response message over secure transport, then the UA MUST process only the first such header field.
- If an HTTP response is received over insecure transport, the UA MUST ignore any present STS header field(s).
- The UA MUST ignore any STS header fields not conforming to the grammar specified in Section 6.1 ("Strict-Transport-Security HTTP Response Header Field").
8.1.1. Noting an HSTS Host - Storage Model
If the substring matching the host production from the Request-URI (of the message to which the host responded) syntactically matches the IP-literal or IPv4address productions from Section 3.2.2 of [RFC3986], then the UA MUST NOT note this host as a Known HSTS Host.
Otherwise, if the substring does not congruently match a Known HSTS Host's domain name, per the matching procedure specified in Section 8.2 ("Known HSTS Host Domain Name Matching"), then the UA MUST note this host as a Known HSTS Host, caching the HSTS Host's domain name and noting along with it the expiry time of this information, as effectively stipulated per the given max-age value, as well as whether the includeSubDomains directive is asserted or not. See also Section 11.2 ("HSTS Policy Expiration Time Considerations").
The UA MUST NOT modify the expiry time or the includeSubDomains directive of any superdomain matched Known HSTS Host.
A Known HSTS Host is "expired" if its cache entry has an expiry date in the past. The UA MUST evict all expired Known HSTS Hosts from its cache if, at any time, an expired Known HSTS Host exists in the cache.
8.2. Known HSTS Host Domain Name Matching
A given domain name may match a Known HSTS Host's domain name in one or both of two fashions: a congruent match, or a superdomain match. Alternatively, there may be no match.
The steps below determine whether there are any matches, and if so, of which fashion:
Compare the given domain name with the domain name of each of the UA's unexpired Known HSTS Hosts. For each Known HSTS Host's domain name, the comparison is done with the given domain name label-by-label (comparing only labels) using an ASCII case- insensitive comparison beginning with the rightmost label, and continuing right-to-left. See also Section 220.127.116.11 of [RFC5890].
- Superdomain Match
If a label-for-label match between an entire Known HSTS Host's domain name and a right-hand portion of the given domain name is found, then this Known HSTS Host's domain name is a superdomain match for the given domain name. There could be multiple superdomain matches for a given domain name.
Given domain name (DN): qaz.bar.foo.example.com
Superdomain matched Known HSTS Host DN: bar.foo.example.com Superdomain matched Known HSTS Host DN: foo.example.com
- Congruent Match
If a label-for-label match between a Known HSTS Host's domain name and the given domain name is found -- i.e., there are no further labels to compare -- then the given domain name congruently matches this Known HSTS Host.
Given domain name: foo.example.com
Congruently matched Known HSTS Host DN: foo.example.com
- Otherwise, if no matches are found, the given domain name does not represent a Known HSTS Host.
8.3. URI Loading and Port Mapping
Whenever the UA prepares to "load" (also known as "dereference") any "http" URI [RFC3986] (including when following HTTP redirects [RFC2616]), the UA MUST first determine whether a domain name is given in the URI and whether it matches a Known HSTS Host, using these steps:
- Extract from the URI any substring described by the host component of the authority component of the URI.
- If the substring is null, then there is no match with any Known HSTS Host.
- Else, if the substring is non-null and syntactically matches the IP-literal or IPv4address productions from Section 3.2.2 of [RFC3986], then there is no match with any Known HSTS Host.
- Otherwise, the substring is a given domain name, which MUST be matched against the UA's Known HSTS Hosts using the procedure in Section 8.2 ("Known HSTS Host Domain Name Matching").
- If, when performing domain name matching any superdomain match with an asserted includeSubDomains directive is found, or, if no superdomain matches with asserted includeSubDomains directives are found and a congruent match is found (with or without an asserted includeSubDomains directive), then before proceeding with the load:
The UA MUST replace the URI scheme with "https" [RFC2818], and
if the URI contains an explicit port component of "80", then the UA MUST convert the port component to be "443", or
if the URI contains an explicit port component that is not equal to "80", the port component value MUST be preserved; otherwise,
if the URI does not contain an explicit port component, the UA MUST NOT add one.
NOTE: These steps ensure that the HSTS Policy applies to HTTP over any TCP port of an HSTS Host. NOTE: In the case where an explicit port is provided (and to a lesser extent with subdomains), it is reasonably likely that there is actually an HTTP (i.e., non-secure) server running on the specified port and that an HTTPS request will thus fail (see item 6 in Appendix A ("Design Decision Notes")).
8.4. Errors in Secure Transport Establishment
When connecting to a Known HSTS Host, the UA MUST terminate the connection (see also Section 12 ("User Agent Implementation Advice")) if there are any errors, whether "warning" or "fatal" or any other error level, with the underlying secure transport. For example, this includes any errors found in certificate validity checking that UAs employ, such as via Certificate Revocation Lists (CRLs) [RFC5280], or via the Online Certificate Status Protocol (OCSP) [RFC2560], as well as via TLS server identity checking [RFC6125].
8.5. HTTP-Equiv <Meta> Element Attribute
UAs MUST NOT heed http-equiv="Strict-Transport-Security" attribute settings on <meta> elements [W3C.REC-html401-19991224] in received content.
8.6. Missing Strict-Transport-Security Response Header Field
If a UA receives HTTP responses from a Known HSTS Host over a secure channel but the responses are missing the STS header field, the UA MUST continue to treat the host as a Known HSTS Host until the max-age value for the knowledge of that Known HSTS Host is reached. Note that the max-age value could be effectively infinite for a given Known HSTS Host. For example, this would be the case if the Known HSTS Host is part of a pre-configured list that is implemented such that the list entries never "age out".
9. Constructing an Effective Request URI
This section specifies how an HSTS Host must construct the Effective Request URI for a received HTTP request.
HTTP requests often do not carry an absoluteURI for the target resource; instead, the URI needs to be inferred from the Request-URI, Host header field, and connection context ([RFC2616], Sections 3.2.1, 5.1.2, and 5.2). The result of this process is called the "effective request URI (ERU)". The "target resource" is the resource identified by the effective request URI.
9.1. ERU Fundamental Definitions
The first line of an HTTP request message, Request-Line, is specified by the following ABNF from [RFC2616], Section 5.1:
Request-Line = Method SP Request-URI SP HTTP-Version CRLF The Request-URI, within the Request-Line, is specified by the following ABNF from [RFC2616], Section 5.1.2: Request-URI = "*" | absoluteURI | abs_path | authority
The Host request header field is specified by the following ABNF from [RFC2616], Section 14.23:
Host = "Host" ":" host [ ":" port ]
9.2. Determining the Effective Request URI
If the Request-URI is an absoluteURI, then the effective request URI is the Request-URI.
If the Request-URI uses the abs_path form or the asterisk form, and the Host header field is present, then the effective request URI is constructed by concatenating:
- the scheme name: "http" if the request was received over an insecure TCP connection, or "https" when received over a TLS/ SSL-secured TCP connection, and
- the octet sequence "://", and
- the host, and the port (if present), from the Host header field, and
- the Request-URI obtained from the Request-Line, unless the Request-URI is just the asterisk "*".
If the Request-URI uses the abs_path form or the asterisk form, and the Host header field is not present, then the effective request URI is undefined.
Otherwise, when Request-URI uses the authority form, the effective request URI is undefined.
Effective request URIs are compared using the rules described in [RFC2616] Section 3.2.3, except that empty path components MUST NOT be treated as equivalent to an absolute path of "/".
9.2.1. Effective Request URI Examples
Example 1: the effective request URI for the message
GET /pub/WWW/TheProject.html HTTP/1.1 Host: www.example.org:8080
(received over an insecure TCP connection) is "http", plus "://", plus the authority component "www.example.org:8080", plus the request-target "/pub/WWW/TheProject.html". Thus, it is "http://www.example.org:8080/pub/WWW/TheProject.html".
Example 2: the effective request URI for the message
OPTIONS * HTTP/1.1 Host: www.example.org
(received over an SSL/TLS secured TCP connection) is "https", plus "://", plus the authority component "www.example.org". Thus, it is "https://www.example.org".
10. Domain Name IDNA-Canonicalization
An IDNA-canonicalized domain name is the output string generated by the following steps. The input is a putative domain name string ostensibly composed of any combination of "A-labels", "U-labels", and "NR-LDH labels" (see Section 2 of [RFC5890]) concatenated using some separator character (typically ".").
- Convert the input putative domain name string to an order- preserving sequence of individual label strings.
2. When implementing IDNA2008, convert, validate, and test each A-label and U-label found among the sequence of individual label strings, using the procedures defined in Sections 5.3 through 5.5 of [RFC5891].
Otherwise, when implementing IDNA2003, convert each label using the "ToASCII" conversion in Section 4 of [RFC3490] (see also the definition of "equivalence of labels" in Section 2 of [RFC3490]).
- If no errors occurred during the foregoing step, concatenate all the labels in the sequence, in order, into a string, separating each label from the next with a %x2E (".") character. The resulting string, known as an IDNA-canonicalized domain name, is appropriate for use in the context of Section 8 ("User Agent Processing Model").
Otherwise, errors occurred. The input putative domain name string was not successfully IDNA-canonicalized. Invokers of this procedure should attempt appropriate error recovery.
See also Sections 13 ("Internationalized Domain Names for Applications (IDNA): Dependency and Migration") and 14.10 ("Internationalized Domain Names") of this specification for further details and considerations.
11. Server Implementation and Deployment Advice
This section is non-normative.
11.1. Non-Conformant User Agent Considerations
Non-conformant UAs ignore the Strict-Transport-Security header field; thus, non-conformant user agents do not address the threats described in Section 2.3.1 ("Threats Addressed"). Please refer to Section 14.2 ("Non-Conformant User Agent Implications") for further discussion.
11.2. HSTS Policy Expiration Time Considerations
Server implementations and deploying web sites need to consider whether they are setting an expiry time that is a constant value into the future, or whether they are setting an expiry time that is a fixed point in time.
The "constant value into the future" approach can be accomplished by constantly sending the same max-age value to UAs.
For example, a max-age value of 7776000 seconds is 90 days:
Note that each receipt of this header by a UA will require the UA to update its notion of when it must delete its knowledge of this Known HSTS Host.
The "fixed point in time" approach can be accomplished by sending max-age values that represent the remaining time until the desired expiry time. This would require the HSTS Host to send a newly calculated max-age value in each HTTP response.
A consideration here is whether a deployer wishes to have the signaled HSTS Policy expiry time match that for the web site's domain certificate.
Additionally, server implementers should consider employing a default max-age value of zero in their deployment configuration systems. This will require deployers to willfully set max-age in order to have UAs enforce the HSTS Policy for their host and will protect them from inadvertently enabling HSTS with some arbitrary non-zero duration.
11.3. Using HSTS in Conjunction with Self-Signed Public-Key
If all four of the following conditions are true...
- a web site/organization/enterprise is generating its own secure transport public-key certificates for web sites, and
- that organization's root certification authority (CA) certificate is not typically embedded by default in browser and/or operating system CA certificate stores, and
- HSTS Policy is enabled on a host identifying itself using a certificate signed by the organization's CA (i.e., a "self-signed certificate"), and
- this certificate does not match a usable TLS certificate association (as defined by Section 4 of the TLSA protocol specification [RFC6698]),
...then secure connections to that site will fail, per the HSTS design. This is to protect against various active attacks, as discussed above.
However, if said organization wishes to employ its own CA, and self- signed certificates, in concert with HSTS, it can do so by deploying its root CA certificate to its users' browsers or operating system CA root certificate stores. It can also, in addition or instead, distribute to its users' browsers the end-entity certificate(s) for specific hosts. There are various ways in which this can be accomplished (details are out of scope for this specification). Once its root CA certificate is installed in the browsers, it may employ HSTS Policy on its site(s).
Alternatively, that organization can deploy the TLSA protocol; all browsers that also use TLSA will then be able to trust the certificates identified by usable TLS certificate associations as denoted via TLSA.
NOTE: Interactively distributing root CA certificates to users, e.g., via email, and having the users install them, is arguably training the users to be susceptible to a possible form of phishing attack. See Section 14.8 ("Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack"). Thus, care should be taken in the manner in which such certificates are distributed and installed on users' systems and browsers.
11.4. Implications of includeSubDomains
The includeSubDomains directive has practical implications meriting careful consideration; two example scenarios are:
- An HSTS Host offers unsecured HTTP-based services on alternate ports or at various subdomains of its HSTS Host domain name.
- Distinct web applications are offered at distinct subdomains of an HSTS Host, such that UAs often interact directly with these subdomain web applications without having necessarily interacted with a web application offered at the HSTS Host's domain name (if any).
The sections below discuss each of these scenarios in turn.
11.4.1. Considerations for Offering Unsecured HTTP Services at
Alternate Ports or Subdomains of an HSTS Host
For example, certification authorities often offer their CRL distribution and OCSP services [RFC2560] over plain HTTP, and sometimes at a subdomain of a publicly available web application that may be secured by TLS/SSL. For example, <https://ca.example.com/> is a publicly available web application for "Example CA", a certification authority. Customers use this web application to register their public keys and obtain certificates. "Example CA" generates certificates for customers containing <http://crl-and-ocsp.ca.example.com/> as the value for the "CRL Distribution Points" and "Authority Information Access:OCSP" certificate fields.
If ca.example.com were to issue an HSTS Policy with the includeSubDomains directive, then HTTP-based user agents implementing HSTS that have interacted with the ca.example.com web application would fail to retrieve CRLs and fail to check OCSP for certificates, because these services are offered over plain HTTP.
In this case, Example CA can either:
- not use the includeSubDomains directive, or
- ensure that HTTP-based services offered at subdomains of ca.example.com are also uniformly offered over TLS/SSL, or
- offer plain HTTP-based services at a different domain name, e.g., crl-and-ocsp.ca.example.NET, or
- utilize an alternative approach to distributing certificate status information, obviating the need to offer CRL distribution and OCSP services over plain HTTP (e.g., the "Certificate Status Request" TLS extension [RFC6066], often colloquially referred to as "OCSP Stapling").
NOTE: The above points are expressly only an example and do not purport to address all the involved complexities. For instance, there are many considerations -- on the part of CAs, certificate deployers, and applications (e.g., browsers) -- involved in deploying an approach such as "OCSP Stapling". Such issues are out of scope for this specification.
11.4.2. Considerations for Offering Web Applications at Subdomains of
an HSTS Host
In this scenario, an HSTS Host declares an HSTS Policy with an includeSubDomains directive, and there also exist distinct web applications offered at distinct subdomains of the HSTS Host such that UAs often interact directly with these subdomain web applications without having necessarily interacted with the HSTS Host. In such a case, the UAs will not receive or enforce the HSTS Policy.
For example, the HSTS Host is "example.com", and it is configured to emit the STS header field with the includeSubDomains directive. However, example.com's actual web application is addressed at "www.example.com", and example.com simply redirects user agents to "https://www.example.com/".
If the STS header field is only emitted by "example.com" but UAs typically bookmark -- and links (from anywhere on the web) are typically established to -- "www.example.com", and "example.com" is not contacted directly by all user agents in some non-zero percentage of interactions, then some number of UAs will not note "example.com" as an HSTS Host, and some number of users of "www.example.com" will be unprotected by HSTS Policy.
To address this, HSTS Hosts should be configured such that the STS header field is emitted directly at each HSTS Host domain or subdomain name that constitutes a well-known "entry point" to one's web application(s), whether or not the includeSubDomains directive is employed.
Thus, in our example, if the STS header field is emitted from both "example.com" and "www.example.com", this issue will be addressed. Also, if there are any other well-known entry points to web applications offered by "example.com", such as "foo.example.com", they should also be configured to emit the STS header field.
12. User Agent Implementation Advice
This section is non-normative.
In order to provide users and web sites more effective protection, as well as controls for managing their UA's caching of HSTS Policy, UA implementers should consider including features such as the following:
12.1. No User Recourse
Failing secure connection establishment on any warnings or errors (per Section 8.4 ("Errors in Secure Transport Establishment")) should be done with "no user recourse". This means that the user should not be presented with a dialog giving her the option to proceed. Rather, it should be treated similarly to a server error where there is nothing further the user can do with respect to interacting with the target web application, other than wait and retry.
Essentially, "any warnings or errors" means anything that would cause the UA implementation to announce to the user that something is not entirely correct with the connection establishment.
Not doing this, i.e., allowing user recourse such as "clicking through warning/error dialogs", is a recipe for a man-in-the-middle attack. If a web application issues an HSTS Policy, then it is implicitly opting into the "no user recourse" approach, whereby all certificate errors or warnings cause a connection termination, with no chance to "fool" users into making the wrong decision and compromising themselves.
12.2. User-Declared HSTS Policy
A user-declared HSTS Policy is the ability for users to explicitly declare a given domain name as representing an HSTS Host, thus seeding it as a Known HSTS Host before any actual interaction with it. This would help protect against the bootstrap MITM vulnerability as discussed in Section 14.6 ("Bootstrap MITM Vulnerability").
NOTE: Such a feature is difficult to get right on a per-site basis. See the discussion of "rewrite rules" in Section 5.5 of [ForceHTTPS]. For example, arbitrary web sites may not materialize all their URIs using the "https" scheme and thus could "break" if a UA were to attempt to access the site exclusively using such URIs. Also note that this feature would complement, but is independent of, an "HSTS pre-loaded list" feature (see Section 12.3).
12.3. HSTS Pre-Loaded List
An HSTS pre-loaded list is a facility whereby web site administrators can have UAs pre-configured with HSTS Policy for their site(s) by the UA vendor(s) -- a so-called "pre-loaded list" -- in a manner similar to how root CA certificates are embedded in browsers "at the factory". This would help protect against the bootstrap MITM vulnerability (Section 14.6).
NOTE: Such a facility would complement a "user-declared HSTS Policy" feature (Section 12.2).
12.4. Disallow Mixed Security Context Loads
"Mixed security context" loads happen when a web application resource, fetched by the UA over a secure transport, subsequently causes the fetching of one or more other resources without using secure transport. This is also generally referred to as "mixed content" loads (see Section 5.3 ("Mixed Content") in [W3C.REC-wsc-ui-20100812]) but should not be confused with the same "mixed content" term that is also used in the context of markup languages such as XML and HTML.
NOTE: In order to provide behavioral uniformity across UA implementations, the notion of mixed security context will require further standardization work, e.g., to define the term(s) more clearly and to define specific behaviors with respect to it.
12.5. HSTS Policy Deletion
HSTS Policy deletion is the ability to delete a UA's cached HSTS Policy on a per-HSTS Host basis.
NOTE: Adding such a feature should be done very carefully in both the user interface and security senses. Deleting a cache entry for a Known HSTS Host should be a very deliberate and well-considered act -- it shouldn't be something that users get used to doing as a matter of course: e.g., just "clicking
through" in order to get work done. Also, implementations need to guard against allowing an attacker to inject code, e.g., ECMAscript, into the UA that silently and programmatically removes entries from the UA's cache of Known HSTS Hosts.
13. Internationalized Domain Names for Applications (IDNA): Dependency
Textual domain names on the modern Internet may contain one or more "internationalized" domain name labels. Such domain names are referred to as "internationalized domain names" (IDNs). The specification suites defining IDNs and the protocols for their use are named "Internationalized Domain Names for Applications (IDNA)". At this time, there are two such specification suites: IDNA2008 [RFC5890] and its predecessor IDNA2003 [RFC3490].
IDNA2008 obsoletes IDNA2003, but there are differences between the two specifications, and thus there can be differences in processing (e.g., converting) domain name labels that have been registered under one from those registered under the other. There will be a transition period of some time during which IDNA2003-based domain name labels will exist in the wild. In order to facilitate their IDNA transition, user agents SHOULD implement IDNA2008 [RFC5890] and MAY implement [RFC5895] (see also Section 7 of [RFC5894]) or [UTS46]. If a user agent does not implement IDNA2008, the user agent MUST implement IDNA2003.
14. Security Considerations
This specification concerns the expression, conveyance, and enforcement of the HSTS Policy. The overall HSTS Policy threat model, including addressed and unaddressed threats, is given in Section 2.3 ("Threat Model").
Additionally, the sections below discuss operational ramifications of the HSTS Policy, provide feature rationale, discuss potential HSTS Policy misuse, and highlight some known vulnerabilities in the HSTS Policy regime.
14.1. Underlying Secure Transport Considerations
This specification is fashioned to be independent of the secure transport underlying HTTP. However, the threat analysis and requirements in Section 2 ("Overview") in fact presume TLS or SSL as the underlying secure transport. Thus, employment of HSTS in the context of HTTP running over some other secure transport protocol would require assessment of that secure transport protocol's security model in conjunction with the specifics of how HTTP is layered over it in order to assess HSTS's subsequent security properties in that context.
14.2. Non-Conformant User Agent Implications
Non-conformant user agents ignore the Strict-Transport-Security header field; thus, non-conformant user agents do not address the threats described in Section 2.3.1 ("Threats Addressed").
This means that the web application and its users wielding non-conformant UAs will be vulnerable to both of the following:
- Passive network attacks due to web site development and deployment bugs:
For example, if the web application contains any insecure references (e.g., "http") to the web application server, and if not all of its cookies are flagged as "Secure", then its cookies will be vulnerable to passive network sniffing and, potentially, subsequent misuse of user credentials.
- Active network attacks:
For example, if an attacker is able to place a "man in the middle", secure transport connection attempts will likely yield warnings to the user, but without HSTS Policy being enforced, the present common practice is to allow the user to "click through" and proceed. This renders the user and possibly the web application open to abuse by such an attacker.
This is essentially the status quo for all web applications and their users in the absence of HSTS Policy. Since web application providers typically do not control the type or version of UAs their web applications interact with, the implication is that HSTS Host deployers must generally exercise the same level of care to avoid web site development and deployment bugs (see Section 18.104.22.168) as they would if they were not asserting HSTS Policy.
14.3. Ramifications of HSTS Policy Establishment Only over Error-Free
The user agent processing model defined in Section 8 ("User Agent Processing Model") stipulates that a host is initially noted as a Known HSTS Host, or that updates are made to a Known HSTS Host's cached information, only if the UA receives the STS header field over a secure transport connection having no underlying secure transport errors or warnings.
The rationale behind this is that if there is a "man in the middle" (MITM) -- whether a legitimately deployed proxy or an illegitimate entity -- it could cause various mischief (see also Appendix A ("Design Decision Notes") item 3, as well as Section 14.6 ("Bootstrap MITM Vulnerability")); for example:
- Unauthorized notation of the host as a Known HSTS Host, potentially leading to a denial-of-service situation if the host does not uniformly offer its services over secure transport (see also Section 14.5 ("Denial of Service")).
- Resetting the time to live for the host's designation as a Known HSTS Host by manipulating the max-age header field parameter value that is returned to the UA. If max-age is returned as zero, this will cause the host to cease being regarded as a Known HSTS Host by the UA, leading to either insecure connections to the host or possibly denial of service if the host delivers its services only over secure transport.
However, this means that if a UA is "behind" a MITM non-transparent TLS proxy -- within a corporate intranet, for example -- and interacts with an unknown HSTS Host beyond the proxy, the user could possibly be presented with the legacy secure connection error dialogs. Even if the risk is accepted and the user "clicks through", the host will not be noted as an HSTS Host. Thus, as long as the UA is behind such a proxy, the user will be vulnerable and will possibly be presented with the legacy secure connection error dialogs for as-yet unknown HSTS Hosts.
Once the UA successfully connects to an unknown HSTS Host over error- free secure transport, the host will be noted as a Known HSTS Host. This will result in the failure of subsequent connection attempts from behind interfering proxies.
The above discussion relates to the recommendation in Section 12 ("User Agent Implementation Advice") that the secure connection be terminated with "no user recourse" whenever there are warnings and errors and the host is a Known HSTS Host. Such a posture protects users from "clicking through" security warnings and putting themselves at risk.
14.4. The Need for includeSubDomains
Without the includeSubDomains directive, a web application would not be able to adequately protect so-called "domain cookies" (even if these cookies have their "Secure" flag set and thus are conveyed only on secure channels). These are cookies the web application expects UAs to return to any and all subdomains of the web application.
For example, suppose example.com represents the top-level DNS name for a web application. Further suppose that this cookie is set for the entire example.com domain, i.e., it is a "domain cookie", and it has its Secure flag set. Suppose example.com is a Known HSTS Host for this UA, but the includeSubDomains directive is not set.
Now, if an attacker causes the UA to request a subdomain name that is unlikely to already exist in the web application, such as "https://uxdhbpahpdsf.example.com/", but that the attacker has managed to register in the DNS and point at an HTTP server under the attacker's control, then:
- The UA is unlikely to already have an HSTS Policy established for "uxdhbpahpdsf.example.com".
- The HTTP request sent to uxdhbpahpdsf.example.com will include the Secure-flagged domain cookie.
- If "uxdhbpahpdsf.example.com" returns a certificate during TLS establishment, and the user "clicks through" any warning that might be presented (it is possible, but not certain, that one may obtain a requisite certificate for such a domain name such that a warning may or may not appear), then the attacker can obtain the Secure-flagged domain cookie that's ostensibly being protected.
Without the "includeSubDomains" directive, HSTS is unable to protect such Secure-flagged domain cookies.
14.5. Denial of Service
HSTS could be used to mount certain forms of Denial-of-Service (DoS) attacks against web sites. A DoS attack is an attack in which one or more network entities target a victim entity and attempt to prevent the victim from doing useful work. This section discusses such scenarios in terms of HSTS, though this list is not exhaustive. See also [RFC4732] for a discussion of overall Internet DoS considerations.
- Web applications available over HTTP
There is an opportunity for perpetrating DoS attacks with web applications (or critical portions of them) that are available only over HTTP without secure transport, if attackers can cause UAs to set HSTS Policy for such web applications' host(s).
This is because once the HSTS Policy is set for a web application's host in a UA, the UA will only use secure transport to communicate with the host. If the host is not using secure transport or is not using it for critical portions of its web application, then the web application will be rendered unusable for the UA's user.
NOTE: This is a use case for UAs to offer an "HSTS Policy deletion" feature as noted in Section 12.5 ("HSTS Policy Deletion").
An HSTS Policy can be set for a victim host in various ways:
- If the web application has an HTTP response splitting vulnerability [CWE-113] (which can be abused in order to facilitate "HTTP header injection").
- If an attacker can spoof a redirect from an insecure victim site, e.g., <http://example.com/> to <https://example.com/>, where the latter is attacker-controlled and has an apparently valid certificate. In this situation, the attacker can then set an HSTS Policy for example.com and also for all subdomains of example.com.
- If an attacker can convince users to manually configure HSTS Policy for a victim host. This assumes that their UAs offer such a capability (see Section 12 ("User Agent Implementation Advice")). Alternatively, if such UA configuration is scriptable, then an attacker can cause UAs to execute his script and set HSTS Policies for whichever desired domains.
- Inadvertent use of includeSubDomains
The includeSubDomains directive instructs UAs to automatically regard all subdomains of the given HSTS Host as Known HSTS Hosts. If any such subdomains do not support properly configured secure transport, then they will be rendered unreachable from such UAs.
14.6. Bootstrap MITM Vulnerability
Bootstrap MITM (man-in-the-middle) vulnerability is a vulnerability that users and HSTS Hosts encounter in the situation where the user manually enters, or follows a link, to an unknown HSTS Host using an "http" URI rather than an "https" URI. Because the UA uses an insecure channel in the initial attempt to interact with the specified server, such an initial interaction is vulnerable to various attacks (see Section 5.3 of [ForceHTTPS]).
NOTE: There are various features/facilities that UA implementations may employ in order to mitigate this vulnerability. Please see Section 12 ("User Agent Implementation Advice").
14.7. Network Time Attacks
Active network attacks can subvert network time protocols (such as the Network Time Protocol (NTP) [RFC5905]) -- making HSTS less effective against clients that trust NTP or lack a real time clock. Network time attacks are beyond the scope of this specification. Note that modern operating systems use NTP by default. See also Section 2.10 of [RFC4732].
14.8. Bogus Root CA Certificate Phish plus DNS Cache Poisoning Attack
An attacker could conceivably obtain users' login credentials belonging to a victim HSTS-protected web application via a bogus root CA certificate phish plus DNS cache poisoning attack.
For example, the attacker could first convince users of a victim web application (which is protected by HSTS Policy) to install the attacker's version of a root CA certificate purporting (falsely) to represent the CA of the victim web application. This might be accomplished by sending the users a phishing email message with a link to such a certificate, which their browsers may offer to install if clicked on.
Then, if the attacker can perform an attack on the users' DNS servers, (e.g., via cache poisoning) and turn on HSTS Policy for their fake web application, the affected users' browsers would access the attacker's web application rather than the legitimate web application.
This type of attack leverages vectors that are outside of the scope of HSTS. However, the feasibility of such threats can be mitigated by including in a web application's overall deployment approach appropriate employment, in addition to HSTS, of security facilities such as DNS Security Extensions [RFC4033], plus techniques to block email phishing and fake certificate injection.
14.9. Creative Manipulation of HSTS Policy Store
Since an HSTS Host may select its own host name and subdomains thereof, and this information is cached in the HSTS Policy store of conforming UAs, it is possible for those who control one or more HSTS Hosts to encode information into domain names they control and cause such UAs to cache this information as a matter of course in the process of noting the HSTS Host. This information can be retrieved by other hosts through cleverly constructed and loaded web resources, causing the UA to send queries to (variations of) the encoded domain names. Such queries can reveal whether the UA had previously visited the original HSTS Host (and subdomains).
Such a technique could potentially be abused as yet another form of "web tracking" [WebTracking].
14.10. Internationalized Domain Names
Internet security relies in part on the DNS and the domain names it hosts. Domain names are used by users to identify and connect to Internet hosts and other network resources. For example, Internet security is compromised if a user entering an internationalized domain name (IDN) is connected to different hosts based on different interpretations of the IDN.
The processing models specified in this specification assume that the domain names they manipulate are IDNA-canonicalized, and that the canonicalization process correctly performed all appropriate IDNA and Unicode validations and character list testing per the requisite specifications (e.g., as noted in Section 10 ("Domain Name IDNA- Canonicalization")). These steps are necessary in order to avoid various potentially compromising situations.
In brief, examples of issues that could stem from lack of careful and consistent Unicode and IDNA validations include unexpected processing exceptions, truncation errors, and buffer overflows, as well as false-positive and/or false-negative domain name matching results. Any of the foregoing issues could possibly be leveraged by attackers in various ways.
Additionally, IDNA2008 [RFC5890] differs from IDNA2003 [RFC3490] in terms of disallowed characters and character mapping conventions. This situation can also lead to false-positive and/or false-negative domain name matching results, resulting in, for example, users possibly communicating with unintended hosts or not being able to reach intended hosts.
For details, refer to the Security Considerations sections of [RFC5890], [RFC5891], and [RFC3490], as well as the specifications they normatively reference. Additionally, [RFC5894] provides detailed background and rationale for IDNA2008 in particular, as well as IDNA and its issues in general, and should be consulted in conjunction with the former specifications.
15. IANA Considerations
Below is the Internet Assigned Numbers Authority (IANA) Permanent Message Header Field registration information per [RFC3864].
Header field name: Strict-Transport-Security Applicable protocol: http Status: standard Author/Change controller: IETF Specification document(s): this one
16.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, "Internationalizing Domain Names in Applications (IDNA)", RFC 3490, March 2003. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. [RFC5890] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework", RFC 5890, August 2010. [RFC5891] Klensin, J., "Internationalized Domain Names in Applications (IDNA): Protocol", RFC 5891, August 2010. [RFC5895] Resnick, P. and P. Hoffman, "Mapping Characters for Internationalized Domain Names in Applications (IDNA) 2008", RFC 5895, September 2010. [RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA", RFC 6698, August 2012. [UTS46] Davis, M. and M. Suignard, "Unicode IDNA Compatibility Processing", Unicode Technical Standard #46, <http://unicode.org/reports/tr46/>. [Unicode] The Unicode Consortium, "The Unicode Standard", <http://www.unicode.org/versions/latest/>.
Raggett, D., Le Hors, A., and I. Jacobs, "HTML 4.01 Specification", World Wide Web Consortium Recommendation REC-html401-19991224, December 1999, <http://www.w3.org/TR/1999/REC-html401-19991224/>.
16.2. Informative References
d'Otreppe, T., "Aircrack-ng", Accessed: 11-Jul-2010, <http://www.aircrack-ng.org/>.
Beck, M. and E. Tews, "Practical Attacks Against WEP and WPA", Second ACM Conference on Wireless Network Security Zurich, Switzerland, 2009, <http://dl.acm.org/citation.cfm?id=1514286>. [CWE-113] "CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Response Splitting')", Common Weakness Enumeration <http://cwe.mitre.org/>, The Mitre Corporation <http://www.mitre.org/>, <http://cwe.mitre.org/data/definitions/113.html>.
Various, "Firesheep", Wikipedia Online, ongoing, <https:// secure.wikimedia.org/wikipedia/en/w/ index.php?title=Firesheep&oldid=517474182>.
Jackson, C. and A. Barth, "ForceHTTPS: Protecting High- Security Web Sites from Network Attacks", In Proceedings of the 17th International World Wide Web Conference (WWW2008) , 2008, <https://crypto.stanford.edu/forcehttps/>.
Good, N., Dhamija, R., Grossklags, J., Thaw, D., Aronowitz, S., Mulligan, D., and J. Konstan, "Stopping Spyware at the Gate: A User Study of Privacy, Notice and Spyware", In Proceedings of Symposium On Usable Privacy and Security (SOUPS) Pittsburgh, PA, USA, July 2005, <http://www.law.berkeley.edu/files/ Spyware_at_the_Gate.pdf>.
Fielding, R., Ed., and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", Work in Progress, October 2012.
Jackson, C. and A. Barth, "Beware of Finer-Grained Origins", Web 2.0 Security and Privacy Workshop, Oakland, CA, USA, 2008, <http://seclab.stanford.edu/websec/origins/fgo.pdf>.
Coates, M., Wichers, D., Boberski, M., and T. Reguly, "Transport Layer Protection Cheat Sheet", Accessed: 11-Jul-2010, <http://www.owasp.org/index.php/ Transport_Layer_Protection_Cheat_Sheet>. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 2560, June 1999. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "DNS Security Introduction and Requirements", RFC 4033, March 2005. [RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of- Service Considerations", RFC 4732, December 2006. [RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC 4949, August 2007. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [RFC5894] Klensin, J., "Internationalized Domain Names for Applications (IDNA): Background, Explanation, and Rationale", RFC 5894, August 2010. [RFC5905] Mills, D., Martin, J., Burbank, J., and W. Kasch, "Network Time Protocol Version 4: Protocol and Algorithms Specification", RFC 5905, June 2010. [RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions: Extension Definitions", RFC 6066, January 2011. [RFC6101] Freier, A., Karlton, P., and P. Kocher, "The Secure Sockets Layer (SSL) Protocol Version 3.0", RFC 6101, August 2011. [RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, March 2011. [RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265, April 2011. [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December 2011.
Sunshine, J., Egelman, S., Almuhimedi, H., Atri, N., and L. Cranor, "Crying Wolf: An Empirical Study of SSL Warning Effectiveness", In Proceedings of 18th USENIX Security Symposium Montreal, Canada, August 2009, <http:// www.usenix.org/events/sec09/tech/full_papers/ sunshine.pdf>.
Roessler, T. and A. Saldhana, "Web Security Context: User Interface Guidelines", World Wide Web Consortium Recommendation REC-wsc-ui-20100812, August 2010, <http://www.w3.org/TR/2010/REC-wsc-ui-20100812>.
Schmucker, N., "Web Tracking", SNET2 Seminar Paper - Summer Term, 2011, <http://www.snet.tu-berlin.de/ fileadmin/fg220/courses/SS11/snet-project/ web-tracking_schmuecker.pdf>.
Appendix A. Design Decision Notes
This appendix documents various design decisions.
- Cookies aren't appropriate for HSTS Policy expression, as they are potentially mutable (while stored in the UA); therefore, an HTTP header field is employed.
- We chose to not attempt to specify how "mixed security context loads" (also known as "mixed content loads") are handled, due to UA implementation considerations as well as classification difficulties.
- An HSTS Host may update UA notions of HSTS Policy via new HSTS header field parameter values. We chose to have UAs honor the "freshest" information received from a server because there is the chance of a web site sending out an erroneous HSTS Policy, such as a multi-year max-age value, and/or an incorrect includeSubDomains directive. If the HSTS Host couldn't correct such errors over protocol, it would require some form of annunciation to users and manual intervention on the users' part, which could be a non-trivial problem for both web application providers and their users.
- HSTS Hosts are identified only via domain names -- explicit IP address identification of all forms is excluded. This is for simplification and also is in recognition of various issues with using direct IP address identification in concert with PKI-based security.
- The max-age approach of having the HSTS Host provide a simple integer number of seconds for a cached HSTS Policy time-to-live value, as opposed to an approach of stating an expiration time in the future, was chosen for various reasons. Amongst the reasons are no need for clock synchronization, no need to define date and time value syntaxes (specification simplicity), and implementation simplicity.
- In determining whether port mapping was to be employed, the option of merely refusing to dereference any URL with an explicit port was considered. It was felt, though, that the possibility that the URI to be dereferenced is incorrect (and there is indeed a valid HTTPS server at that port) is worth the small cost of possibly wasted HTTPS fetches to HTTP servers.
Appendix B. Differences between HSTS Policy and Same-Origin Policy
HSTS Policy has the following primary characteristics:
HSTS Policy stipulates requirements for the security characteristics of UA-to-host connection establishment, on a per-host basis.
Hosts explicitly declare HSTS Policy to UAs. Conformant UAs are obliged to implement hosts' declared HSTS Policies.
HSTS Policy is conveyed over protocol from the host to the UA.
The UA maintains a cache of Known HSTS Hosts.
UAs apply HSTS Policy whenever making an HTTP connection to a Known HSTS Host, regardless of host port number; i.e., it applies to all ports on a Known HSTS Host. Hosts are unable to affect this aspect of HSTS Policy.
Hosts may optionally declare that their HSTS Policy applies to all subdomains of their host domain name.
In contrast, the Same-Origin Policy (SOP) [RFC6454] has the following primary characteristics:
An origin is the scheme, host, and port of a URI identifying a resource.
A UA may dereference a URI, thus loading a representation of the resource the URI identifies. UAs label resource representations with their origins, which are derived from their URIs.
The SOP refers to a collection of principles, implemented within UAs, governing the isolation of and communication between resource representations within the UA, as well as resource representations' access to network resources.
In summary, although both HSTS Policy and SOP are enforced by UAs, HSTS Policy is optionally declared by hosts and is not origin-based, while the SOP applies to all resource representations loaded from all hosts by conformant UAs.
Appendix C. Acknowledgments
The authors thank Devdatta Akhawe, Michael Barrett, Ben Campbell, Tobias Gondrom, Paul Hoffman, Murray Kucherawy, Barry Leiba, James Manger, Alexey Melnikov, Haevard Molland, Yoav Nir, Yngve N. Pettersen, Laksh Raghavan, Marsh Ray, Julian Reschke, Eric Rescorla, Tom Ritter, Peter Saint-Andre, Brian Smith, Robert Sparks, Maciej Stachowiak, Sid Stamm, Andy Steingrubl, Brandon Sterne, Martin Thomson, Daniel Veditz, and Jan Wrobel, as well as all the websec working group participants and others for their various reviews and helpful contributions.
Thanks to Julian Reschke for his elegant rewriting of the effective request URI text, which he did when incorporating the ERU notion into the updates to HTTP/1.1 [HTTP1_1-UPD]. Subsequently, the ERU text in this spec was lifted from Julian's work in the updated HTTP/1.1 (part 1) specification and adapted to the [RFC2616] ABNF.
Jeff Hodges PayPal 2211 North First Street San Jose, California 95131 US EMail: Jeff.Hodges@PayPal.com
Carnegie Mellon University
EMail: email@example.com URI: http://www.adambarth.com/