Threat Model for BGP Path Security

Abstract

This document describes a threat model for the context in which External Border Gateway Protocol (EBGP) path security mechanisms will be developed. The threat model includes an analysis of the Resource Public Key Infrastructure (RPKI) and focuses on the ability of an Autonomous System (AS) to verify the authenticity of the AS path info received in a BGP update. We use the term "PATHSEC" to refer to any BGP path security technology that makes use of the RPKI. PATHSEC will secure BGP, consistent with the inter-AS security focus of the RPKI.

The document characterizes classes of potential adversaries that are considered to be threats and examines classes of attacks that might be launched against PATHSEC. It does not revisit attacks against unprotected BGP, as that topic has already been addressed in the BGP-4 standard. It concludes with a brief discussion of residual vulnerabilities.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.

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). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see 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/rfc7132.

Copyright Notice

Copyright © 2014 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Threat Characterization . . . . . . . . . . . . . . . . . . .   6
   4.  Attack Characterization . . . . . . . . . . . . . . . . . . .   8
     4.1.  Active Wiretapping of Sessions between Routers  . . . . .   8
     4.2.  Attacks on a BGP Router . . . . . . . . . . . . . . . . .   9
     4.3.  Attacks on Network Operator Management Computers (Non-CA
           Computers)  . . . . . . . . . . . . . . . . . . . . . . .  11
     4.4.  Attacks on a Repository Publication Point . . . . . . . .  12
     4.5.  Attacks on an RPKI CA . . . . . . . . . . . . . . . . . .  14
   5.  Residual Vulnerabilities  . . . . . . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  18

1. Introduction

This document describes the security context in which PATHSEC is intended to operate. The term "PATHSEC" (for path security) refers to any design used to preserve the integrity and authenticity of the AS_PATH attribute carried in a BGP update message [RFC4271]. The security context used throughout this document is established by the Secure Inter-Domain Routing (SIDR) working group charter [SIDR-CH]. The charter requires that solutions that afford PATHSEC make use of the Resource Public Key Infrastructure (RPKI) [RFC6480]. It also calls for protecting only the information required to verify that a received route traversed the Autonomous Systems (ASes) in question, and that the Network Layer Reachability Information (NLRI) in the route is what was advertised.

Thus, the goal of PATHSEC is to enable a BGP speaker to verify that the ASes enumerated in this path attribute represent the sequence of ASes that the NLRI traversed. The term "PATHSEC" is thus consistent with the goal described above. (Other SIDR documents use the term "BGPSEC" to refer to a specific design; we avoid use of that term here.)

This document discusses classes of potential adversaries that are considered to be threats, and classes of attacks that might be launched against PATHSEC. Because PATHSEC will rely on the RPKI, threats and attacks against the RPKI are included. This model also takes into consideration classes of attacks that are enabled by the use of PATHSEC (e.g., based on use of the RPKI).

The motivation for developing PATHSEC, i.e., residual security concerns for BGP, is well described in several documents, including "BGP Security Vulnerabilities Analysis" [RFC4272] and "Design and Analysis of the Secure Border Gateway Protocol (S-BGP)" [Kent2000]. All of these documents note that BGP does not include mechanisms that allow an AS to verify the legitimacy and authenticity of BGP route advertisements. (BGP now mandates support for mechanisms to secure peer-to-peer communication, i.e., for the links that connect BGP routers. There are several secure protocol options to address this security concern, e.g., IPsec [RFC4301] and TCP Authentication Option (TCP-AO) [RFC5925]. This document briefly notes the need to address this aspect of BGP security, but focuses on application layer BGP security issues that must be addressed by PATHSEC.)

RFC 4272 [RFC4272] succinctly notes:

BGP speakers themselves can inject bogus routing information, either by masquerading as any other legitimate BGP speaker, or by distributing unauthorized routing information as themselves. Historically, misconfigured and faulty routers have been responsible for widespread disruptions in the Internet. The legitimate BGP peers have the context and information to produce believable, yet bogus, routing information, and therefore have the opportunity to cause great damage. The cryptographic protections of [TCPMD5] and operational protections cannot exclude the bogus information arising from a legitimate peer. The risk of disruptions caused by legitimate BGP speakers is real and cannot be ignored.

PATHSEC is intended to address the concerns cited above, to provide significantly improved path security, which builds upon the route origination validation capability offered by use of the RPKI [RFC6810]. Specifically, the RPKI enables relying parties (RPs) to determine if the origin AS for a path was authorized to advertise the prefix contained in a BGP update message. This security feature is enabled by the use of two types of digitally signed data: a PKI [RFC6487] that associates one or more prefixes with the public key(s) of an address space holder, and Route Origin Authorizations (ROAs) [RFC6482] that allow a prefix holder to specify one or more ASes that are authorized to originate routes for a prefix.

The security model adopted for PATHSEC does not assume an "oracle" that can see all of the BGP inputs and outputs associated with every AS or every BGP router. Instead, the model is based on a local notion of what constitutes legitimate, authorized behavior by the BGP routers associated with an AS. This is an AS-centric model of secure operation, consistent with the AS-centric model that BGP employs for routing. This model forms the basis for the discussion that follows.

This document begins with a brief set of definitions relevant to the subsequent sections. It then discusses classes of adversaries that are perceived as viable threats against routing in the public Internet. It continues to explore a range of attacks that might be effected by these adversaries against both path security and the infrastructure upon which PATHSEC relies. It concludes with a brief review of residual vulnerabilities, i.e., vulnerabilities that are not addressed by use of the RPKI and that appear likely to be outside the scope of PATHSEC mechanisms.

2. Terminology

The following security and routing terminology definitions are employed in this document.

   Adversary:  An adversary is an entity (e.g., a person or an
   
      organization) that is perceived as malicious, relative to the
      security policy of a system.  The decision to characterize an
      entity as an adversary is made by those responsible for the
      security of a system.  Often, one describes classes of adversaries
      with similar capabilities or motivations rather than specific
      individuals or organizations.
   
   Attack:  An attack is an action that attempts to violate the security
      policy of a system, e.g., by exploiting a vulnerability.  There is
      often a many-to-one mapping of attacks to vulnerabilities because
      many different attacks may be used to exploit a vulnerability.
   
   Autonomous System (AS):  An AS is a set of one or more IP networks
      operated by a single administrative entity.
   
   AS Number (ASN):  An ASN is a 2- or 4-byte number issued by a
      registry to identify an AS in BGP.
   
   Certification Authority (CA):  An entity that issues digital
      certificates (e.g., X.509 certificates) and vouches for the
      binding between the data items in a certificate.
   
   Countermeasure:  A countermeasure is a procedure or technique that
      thwarts an attack, preventing it from being successful.  Often,
      countermeasures are specific to attacks or classes of attacks.
   
   Border Gateway Protocol (BGP):  A path vector protocol used to convey
      "reachability" information among ASes in support of inter-domain
      routing.
   
   False (Route) Origination:  If a network operator originates a route
      for a prefix that the operator does not hold (and that has not
      been authorized to originate by the prefix holder), this is termed
      false route origination.
   
   Internet Service Provider (ISP):  An organization managing (and
      typically selling) Internet services to other organizations or
      individuals.
   
   Internet Number Resources (INRs):  IPv4 or IPv6 address space and
      ASNs.
   
   Internet Registry:  An organization that manages the allocation or
      distribution of INRs.  This encompasses the Internet Assigned
      Number Authority (IANA), Regional Internet Registries (RIRs),
      National Internet Registries (NIRs), and Local Internet Registries
      (LIRs) (network operators).
   
   Man in the Middle (MITM):  A MITM is an entity that is able to
      examine and modify traffic between two (or more) parties on a
      communication path.
   
   Network Operator:  An entity that manages an AS and thus emits (E)BGP
      updates, e.g., an ISP.
   
   Network Operations Center (NOC):  A network operator employs a set of
      equipment and a staff to manage a network, typically on a 24/7
      basis.  The equipment and staff are often referred to as the NOC
      for the network.
   
   Prefix:  A prefix is an IP address and a mask used to specify a set
      of addresses that are grouped together for purposes of routing.
   
   Public Key Infrastructure (PKI):  A PKI is a collection of hardware,
      software, people, policies, and procedures used to create, manage,
      distribute, store, and revoke digital certificates.
   
   Relying Parties (RPs):  An RP is an entity that makes use of signed
      products from a PKI, i.e., it relies on signed data that is
      verified using certificates and Certificate Revocation Lists
      (CRLs) from a PKI.
   
   RPKI Repository System:  The RPKI repository system consists of a
      distributed set of loosely synchronized databases.
   
   Resource PKI (RPKI):  A PKI operated by the entities that manage INRs
      and that issue X.509 certificates (and CRLs) that attest to the
      holdings of INRs.
   
   RPKI Signed Object:  An RPKI signed object is a data object
      encapsulated with Cryptographic Message Syntax (CMS) that complies
      with the format and semantics defined in [RFC6488].
   
   Route:  In the Internet, a route is a prefix and an associated
      sequence of ASNs that indicates a path via which traffic destined
      for the prefix can be directed.  (The route includes the origin
      AS.)
   
   Route Leak:  A route leak is said to occur when AS-A advertises
      routes that it has received from AS-B to the neighbors of AS-A,
      but AS-A is not viewed as a transit provider for the prefixes in
      the route.
   
   Threat:  A threat is a motivated, capable adversary.  An adversary
      that is not motivated to launch an attack is not a threat.  An
      adversary that is motivated but not capable of launching an attack
      also is not a threat.
   
   Vulnerability:  A vulnerability is a flaw or weakness in a system's
      design, implementation, or operation and management that could be
      exploited to violate the security policy of a system.

3. Threat Characterization

As noted in Section 2 above, a threat is defined as a motivated, capable adversary. The following classes of threats represent classes of adversaries viewed as relevant to this environment.

Network Operators: A network operator may be a threat. An operator may be motivated to cause BGP routers it controls to emit update messages with inaccurate routing info, e.g., to cause traffic to flow via paths that are economically advantageous for the operator. Such updates might cause traffic to flow via paths that would otherwise be rejected as less advantageous by other network operators. Because an operator controls the BGP routers in its network, it is in a position to modify their operation in arbitrary ways. Routers managed by a network operator are vehicles for mounting MITM attacks on both control and data plane traffic. If an operator participates in the RPKI, it will have at least one CA resource certificate and may be able to generate an arbitrary number of subordinate CA certificates and ROAs. It will be authorized to populate (and may even host) its own repository publication point. If it implements PATHSEC, and if PATHSEC makes use of certificates associated with routers or ASes, it will have the ability to issue such certificates for itself. If PATHSEC digitally signs updates, it will be able to do so in a fashion that will be accepted by PATHSEC-enabled neighbors.

Hackers: Hackers are considered a threat. A hacker might assume control of network management computers and routers controlled by operators, including operators that implement PATHSEC. In such cases, hackers would be able to act as rogue network operators (see above). It is assumed that hackers generally do not have the capability to effect MITM attacks on most links between networks (links used to transmit BGP and subscriber traffic). A hacker might be recruited, without his/her knowledge, by criminals or by nations, to act on their behalf. Hackers may be motivated by a desire for "bragging rights", for profit, or to express support for a cause ("hacktivists" [Sam04]). We view hackers as possibly distinct from criminals in that the former are presumed to effect attacks only remotely (not via a physical presence associated with a target) and not necessarily for monetary gain. Some hackers may commit criminal acts (depending on the jurisdiction), and thus there is a potential for overlap between this adversary group and criminals.

Criminals: Criminals may be a threat. Criminals might persuade (via threats or extortion) a network operator to act as a rogue operator (see above) and thus be able to effect a wide range of attacks. Criminals might persuade the staff of a telecommunications provider to enable MITM attacks on links between routers. Motivations for criminals may include the ability to extort money from network operators or network operator clients, e.g., by adversely affecting routing for these network operators or their clients. Criminals also may wish to manipulate routing to conceal the sources of spam, DoS attacks, or other criminal activities.

Registries: Any registry in the RPKI could be a threat. Staff at the registry are capable of manipulating repository content or mismanaging the RPKI certificates that they issue. These actions could adversely affect a network operator or a client of a network operator. The staff could be motivated to do this based on political pressure from the nation in which the registry operates (see below) or due to criminal influence (see above).

Nations: A nation may be a threat. A nation may control one or more network operators that operate in the nation, and thus can cause them to act as rogue network operators. A nation may have a technical active wiretapping capability (e.g., within its territory) that enables it to effect MITM attacks on inter-network traffic. (This capability may be facilitated by control or influence over a telecommunications provider operating within the nation.) It may have an ability to attack and take control of routers or management network computers of network operators in other countries. A nation may control a registry (e.g., an RIR) that operates within its territory and might force that registry to act in a rogue capacity. National threat motivations include the desire to control the flow of traffic to/from the nation or to divert traffic destined for other nations (for passive or active wiretapping, including DoS).

4. Attack Characterization

This section describes classes of attacks that may be effected against Internet routing (relative to the context described in Section 1). Attacks are classified based on the target of the attack, an element of the routing system, or the routing security infrastructure on which PATHSEC relies. In general, attacks of interest are ones that attempt to violate the integrity or authenticity of BGP traffic or that violate the authorizations associated with entities participating in the RPKI. Attacks that violate the implied confidentiality of routing traffic, e.g., passive wiretapping attacks, are not considered a requirement for BGP security (see [RFC4272]).

4.1. Active Wiretapping of Sessions between Routers

An adversary may attack the BGP (TCP) session that connects a pair of BGP speakers. An active attack against a BGP (TCP) session can be effected by directing traffic to a BGP speaker from some remote point, or by being positioned as a MITM on the link that carries BGP session traffic. Remote attacks can be effected by any adversary. A MITM attack requires access to the link. Modern transport networks may be as complex as the packet networks that utilize them for inter- AS links. Thus, these transport networks may present significant attack surfaces. Nonetheless, only some classes of adversaries are assumed to be capable of MITM attacks against a BGP session. MITM attacks may be directed against BGP and PATHSEC-protected BGP, or against TCP or IP. Such attacks include replay of selected BGP messages, selective modification of BGP messages, and DoS attacks against BGP routers. [RFC4272] describes several countermeasures for such attacks, and thus this document does not further address such attacks.

4.2. Attacks on a BGP Router

An adversary may attack a BGP router, whether or not it implements PATHSEC. Any adversary that controls routers legitimately, or that can assume control of a router, is assumed to be able to effect the types of attacks described below. Note that any router behavior that can be ascribed to a local routing policy decision is not considered to be an attack. This is because such behavior could be explained as a result of local policy settings and thus is beyond the scope of what PATHSEC can detect as unauthorized behavior. Thus, for example, a router may fail to propagate some or all route withdrawals or effect "route leaks". (These behaviors are not precluded by the specification for BGP and might be the result of a local policy that is not publicly disclosed. As a result, they are not considered attacks. See Section 5 for additional discussion.)

Attacks on a router are equivalent to active wiretapping attacks (in the most general sense) that manipulate (forge, tamper with, or suppress) data contained in BGP updates. The list below illustrates attacks of this type.

AS Insertion: A router might insert one or more ASNs, other than its own ASN, into an update message. This violates the BGP spec and thus is considered an attack.

False (Route) Origination: A router might originate a route for a prefix when the AS that the router represents is not authorized to originate routes for that prefix. This is an attack, but it is addressed by the use of the RPKI [RFC6480].

Secure Path Downgrade: A router might remove AS_PATH data from a PATHSEC-protected update that it receives when forwarding this update to a PATHSEC-enabled neighbor. This behavior violates the PATHSEC security goals and thus is considered an attack.

Invalid AS_PATH Data Insertion: A router might emit a PATHSEC- protected update with "bad" data (such as a signature), i.e., PATHSEC data that cannot be validated by other PATHSEC routers. Such behavior is assumed to violate the PATHSEC goals and thus is considered an attack.

Stale Path Announcement: If PATHSEC-secured announcements can expire, such an announcement may be propagated with PATHSEC data that is "expired". This behavior would violate the PATHSEC goals and is considered a type of replay attack.

Premature Path Announcement Expiration: If a PATHSEC-secured announcement has an associated expiration time, a router might emit a PATHSEC-secured announcement with an expiry time that is very short. Unless the PATHSEC protocol specification mandates a minimum expiry time, this is not an attack. However, if such a time is mandated, this behavior becomes an attack. BGP speakers along a path generally cannot determine if an expiry time is "suspiciously short" since they cannot know how long a route may have been held by an earlier AS, prior to being released.

MITM Attack: A cryptographic key used for point-to-point security (e.g., TCP-AO, TLS, or IPsec) between two BGP routers might be compromised (e.g., by extraction from a router). This would enable an adversary to effect MITM attacks on the link(s) where the key is used. Use of specific security mechanisms to protect inter-router links between ASes is outside the scope of PATHSEC.

Compromised Router Private Key: If PATHSEC mechanisms employ public key cryptography, e.g., to digitally sign data in an update, then a private key associated with a router or an AS might be compromised by an attack against the router. An adversary with access to this key would be able to generate updates that appear to have passed through the AS that this router represents. Such updates might be injected on a link between the compromised router and its neighbors if that link is accessible to the adversary. If the adversary controls another network, it could use this key to forge signatures that appear to come from the AS or router(s) in question, with some constraints. So, for example, an adversary that controls another AS could use a compromised router/AS key to issue PATHSEC-signed data that includes the targeted router/AS. (Neighbors of the adversary's AS ought not accept a route that purports to emanate directly from the targeted AS. So, an adversary could take a legitimate, protected route that passes through the compromised AS, add itself as the next hop, and then forward the resulting route to neighbors.)

Withdrawal Suppression Attack: A PATHSEC-protected update may be signed and announced, and later withdrawn. An adversary controlling intermediate routers could fail to propagate the withdrawal. BGP is already vulnerable to behavior of this sort, so withdrawal suppression is not characterized as an attack under the assumptions upon which this mode is based (i.e., no oracle).

4.3. Attacks on Network Operator Management Computers (Non-CA

Computers)

An adversary may choose to attack computers used by a network operator to manage its network, especially its routers. Such attacks might be effected by an adversary who has compromised the security of these computers. This might be effected via remote attacks, extortion of network operations staff, etc. If an adversary compromises NOC computers, he can execute any management function that authorized network operations the staff would have performed. Thus, the adversary could modify the local routing policy to change preferences, to black-hole certain routes, etc. This type of behavior cannot be externally detected as an attack. Externally, this appears as a form of rogue operator behavior. (Such behavior might be perceived as accidental or malicious by other operators.)

If a network operator participates in the RPKI, an adversary could manipulate the RP tools that extract data from the RPKI, causing the output of these tools to be corrupted in various ways. For example, an attack of this sort could cause the operator to view valid routes as not validated, which could alter its routing behavior.

If an adversary invoked the tool used to manage the repository publication point for this operator, it could delete any objects stored there (certificates, CRLs, manifests, ROAs, or subordinate CA certificates). This could affect the routing status of entities that have allocations/assignments from this network operator (e.g., by deleting their CA certificates).

An adversary could invoke the tool used to request certificate revocation, causing router certificates, ROAs, or subordinate CA certificates to be revoked. An attack of this sort could affect not only this operator but also any operators that receive allocations/ assignments from it, e.g., because their CA certificates were revoked.

If an operator is PATHSEC-enabled, an attack of this sort could cause the affected operator to be viewed as not PATHSEC-enabled, possibly making routes it emits less preferable to other operators.

If an adversary invoked a tool used to request ROAs, it could effectively reallocate some of the prefixes allocated/assigned to the network operator (e.g., by modifying the origin AS in ROAs). This might cause other PATHSEC-enabled networks to view the affected network as no longer originating routes for these prefixes. Multi- homed subscribers of this operator who received an allocation from the operator might find that their traffic was routed via other connections.

If the network operator is PATHSEC-enabled, and makes use of certificates associated with routers/ASes, an adversary could invoke a tool used to request such certificates. The adversary could then replace valid certificates for routers/ASes with ones that might be rejected by PATHSEC-enabled neighbors.

4.4. Attacks on a Repository Publication Point

A critical element of the RPKI is the repository system. An adversary might attack a repository, or a publication point within a repository, to adversely affect routing.

This section considers only those attacks that can be launched by any adversary who controls a computer hosting one or more repository publication points, without access to the cryptographic keys needed to generate valid RPKI-signed products. Such attacks might be effected by an insider or an external threat. Because all repository objects are digitally signed, attacks of this sort translate into DoS attacks against the RPKI RPs. There are a few distinct forms of such attacks, as described below.

Note first that the RPKI calls for RPs to cache the data they acquire and verify from the repository system [RFC6480][RFC6481]. Attacks that delete signed products, insert products with "bad" signatures, tamper with object signatures, or replace newer objects with older (valid) ones, can be detected by RPs (with a few exceptions). RPs are expected to make use of local caches. If repository publication points are unavailable or the retrieved data is corrupted, an RP can revert to using the cached data. This behavior helps insulate RPs from the immediate effects of DoS attacks on publication points.

Each RPKI data object has an associated date on which it expires or is considered stale (certificates expire and CRLs become stale). When an RP uses cached data, how to deal with stale or expired data is a local decision. It is common in PKIs to make use of stale certificate revocation status data when fresher data is not available. Use of expired certificates is less common, although not unknown. Each RP will decide, locally, whether to continue to make use of or ignore cached RPKI objects that are stale or expired.

If an adversary inserts an object into a publication point, and the object has a "bad" signature, the object will not be accepted and used by RPs.

If an adversary modifies any signed product at a publication point, the signature on the product will fail, causing RPs to not accept it. This is equivalent to deleting the object, in many respects.

If an adversary deletes one or more CA certificates, ROAs, or the CRL for a publication point, the manifest for that publication point will allow an RP to detect this attack. An RP can continue to use the last valid instance of the deleted object (as a local policy option), thus minimizing the impact of such an attack.

If an adversary deletes a manifest (and does not replace it with an older instance), RPs are able to detect this action. Such behavior should result in the CA (or publication point maintainer) being notified of the problem. An RP can continue to use the last valid instance of the deleted manifest (a local policy option), thus minimizing the impact of such an attack.

If an adversary deletes newly added CA certificates or ROAs, and replaces the current manifest with the previous manifest, the manifest (and the CRL that it matches) will be "stale" (see [RFC6486]). This alerts an RP that there may be a problem. The RP should use the information from a Ghostbuster Record [RFC6493] to contact the entity responsible for the publication point and request a remedy to the problem (e.g., republish the missing CA certificates and/or ROAs). An RP cannot know the content of the new certificates or ROAs that are not present, but it can continue to use what it has cached. An attack of this sort will, at least temporarily, cause RPs to be unaware of the newly published objects. INRs associated with these objects will be treated as unauthenticated.

If a CA revokes a CA certificate or a ROA (via deleting the corresponding End Entity (EE) certificate), and the adversary tries to reinstate that CA certificate or ROA, the adversary would have to rollback the CRL and the manifest to undo this action by the CA. As above, this would make the CRL and manifest stale, and this is detectable by RPs. An RP cannot know which CA certificates or ROAs were deleted. Depending on local policy, the RP might use the cached instances of the affected objects and thus be tricked into making decisions based on these revoked objects. Here too, the goal is that the CA will be notified of the problem (by RPs) and will remedy the error.

In the attack scenarios above, when a CRL or manifest is described as stale, this means that the next issue date for the CRL or manifest has passed. Until the next issue date, an RP will not detect the attack. Thus, it behooves CAs to select CRL/manifest lifetimes (the two are linked) that represent an acceptable trade-off between risk and operational burdens.

Attacks effected by adversaries that are legitimate managers of publication points can have much greater effects and are discussed below under attacks on or by CAs.

4.5. Attacks on an RPKI CA

Every entity to which INRs have been allocated/assigned is a CA in the RPKI. Each CA is nominally responsible for managing the repository publication point for the set of signed products that it generates. (An INR holder may choose to outsource the operation of the RPKI CA function and the associated publication point. In such cases, the organization operating on behalf of the INR holder becomes the CA from an operational and security perspective. The following discussion does not distinguish such outsourced CA operations.)

Note that attacks attributable to a CA may be the result of malice by the CA (i.e., the CA is the adversary), or they may result from a compromise of the CA.

All of the adversaries listed in Section 2 are presumed to be capable of launching attacks against the computers used to perform CA functions. Some adversaries might effect an attack on a CA by violating personnel or physical security controls as well. The distinction between the CA as an adversary versus the CA as an attack victim is important. Only in the latter case should one expect the CA to remedy problems caused by an attack once the attack has been detected. (If a CA does not take such action, the effects are the same as if the CA is an adversary.)

Note that most of the attacks described below do not require disclosure of a CA's private key to an adversary. If the adversary can gain control of the computer used to issue certificates, it can effect these attacks, even though the private key for the CA remains "secure" (i.e., not disclosed to unauthorized parties). However, if the CA is not the adversary, and if the CA's private key is not compromised, then recovery from these attacks is much easier. This motivates use of hardware security modules to protect CA keys, at least for higher tiers in the RPKI.

An attack by a CA can result in revocation or replacement of any of the certificates that the CA has issued. Revocation of a certificate should cause RPs to delete the (formerly) valid certificate (and associated signed object, in the case of a revoked EE certificate) that they have cached. This would cause repository objects (e.g., CA certificates and ROAs) that are verified under that certificate to be considered invalid, transitively. As a result, RPs would not consider any ROAs or PATHSEC-protected updates to be valid based on these certificates, which would make routes dependent on them less preferred. Because a CA that revokes a certificate is authorized to do so, this sort of attack cannot be detected, intrinsically, by most RPs. However, the entities affected by the revocation or replacement of CA certificates can be expected to detect the attack and contact the CA to effect remediation. If the CA was not the adversary, it should be able to issue new certificates and restore the publication point.

An adversary that controls the CA for a publication point can publish signed products that create more subtle types of DoS attacks against RPs. For example, such an attacker could create subordinate CA certificates with Subject Information Access (SIA) pointers that lead RPs on a "wild goose chase" looking for additional publication points and signed products. An attacker could publish certificates with very brief validity intervals or CRLs and manifests that become "stale" very quickly. This sort of attack would cause RPs to access repositories more frequently, and that might interfere with legitimate accesses by other RPs.

An attacker with this capability could create very large numbers of ROAs to be processed (with prefixes that are consistent with the allocation for the CA) and correspondingly large manifests. An attacker could create very deep subtrees with many ROAs per publication point, etc. All of these types of DoS attacks against RPs are feasible within the syntactic and semantic constraints established for RPKI certificates, CRLs, and signed objects.

An attack that results in revocation and replacement (e.g., key rollover or certificate renewal) of a CA certificate would cause RPs to replace the old, valid certificate with the new one. This new certificate might contain a public key that does not correspond to the private key held by the certificate subject. That would cause objects signed by that subject to be rejected as invalid, and prevent the affected subject from being able to sign new objects. As above, RPs would not consider any ROAs issued under the affected CA certificate to be valid, and updates based on router certificates issued by the affected CA would be rejected. This would make routes dependent on these signed products less preferred. However, the constraints imposed by the use of extensions detailed in [RFC3779] prevent a compromised CA from issuing (valid) certificates with INRs outside the scope of the CA, thus limiting the impact of the attack.

An adversary that controls a CA could issue CA certificates with overlapping INRs to different entities when no transfer of INRs is intended. This could cause confusion for RPs as conflicting ROAs could be issued by the distinct (subordinate) CAs.

An adversary could replace a CA certificate, use the corresponding private key to issue new signed products, and then publish them at a publication point controlled by the attacker. This would effectively transfer the affected INRs to the adversary or to a third party of his choosing. The result would be to cause RPs to view the entity that controls the private key in question as the legitimate INR holder. Again, the constraints imposed by the use of the extensions in RFC 3779 prevent a compromised CA from issuing (valid) certificates with INRs outside the scope of the CA, thus limiting the impact of the attack.

Finally, an entity that manages a repository publication point can inadvertently act as an attacker (an example of Walt Kelly's most famous "Pogo" quote [Kelly70]). For example, a CA might fail to replace its own certificate in a timely fashion (well before it expires). It might fail to issue its CRL and manifest prior to expiration, creating stale instances of these products that cause concern for RPs. A CA with many subordinate CAs (e.g., an RIR or NIR) might fail to distribute the expiration times for the CA certificates that it issues. A network with many ROAs might do the same for the EE certificates associated with the ROAs it generates. A CA could rollover its key but fail to reissue subordinate CA certificates under its new key. Poor planning with regard to rekey intervals for managed CAs could impose undue burdens for RPs, despite a lack of malicious intent. All of these examples of mismanagement could adversely affect RPs, despite the absence of malicious intent.

5. Residual Vulnerabilities

The RPKI, upon which PATHSEC relies, has several residual vulnerabilities that were discussed in the preceding text (Sections 4.4 and 4.5). These vulnerabilities are of two principle forms:

• The RPKI repository system may be attacked in ways that make its contents unavailable, not current, or inconsistent. The principle defense against most forms of DoS attacks is the use of a local cache by each RP. The local cache ensures availability of previously acquired RPKI data in the event that a repository is inaccessible or if the repository contents are deleted (maliciously). Nonetheless, the system cannot ensure that every RP will always have access to up-to-date RPKI data. An RP, when it detects a problem with acquired repository data, has two options:

1. The RP may choose to make use of its local cache, employing local configuration settings that tolerate expired or stale objects. (Such behavior is, nominally, always within the purview of an RP in PKI.) Using cached, expired, or stale data subjects the RP to attacks that take advantage of the RP's ignorance of changes to this data.

2. The RP may chose to purge expired objects. Purging expired objects removes the security information associated with the real-world INRs to which the objects refer. This is equivalent to the affected INRs not having been afforded protection via the RPKI. Since use of the RPKI (and PATHSEC) is voluntary, there may always be a set of INRs that are not protected by these mechanisms. Thus, purging moves the affected INRs to the set of non-participating INR holders. This more conservative response enables an attacker to move INRs from the protected set to the unprotected set.

• Any CA in the RPKI may misbehave within the bounds of the INRs allocated to it, e.g., it may issue certificates with duplicate resource allocations or revoke certificates inappropriately. This vulnerability is intrinsic in any PKI, but its impact is limited in the RPKI because of the use of extensions in RFC 3779. It is anticipated that RPs will deal with such misbehavior through administrative means once it is detected.

PATHSEC has a separate set of residual vulnerabilities:

• It has been stated that "route leaks" are viewed as a routing security problem by many operators. However, BGP itself does not include semantics that preclude what many perceive as route leaks, and there is no definition of the term in any RFC. This makes it inappropriate to address route leaks in this document. Additionally, route leaks are outside the scope of PATHSEC, consistent with the security context noted in Section 1 of this document. If, at a later time, the SIDR security context is revised to include route leaks, and an appropriate definition exists, this document should be revised.

• PATHSEC is not required to protect all attributes associated with an AS_PATH, even though some of these attributes may be employed as inputs to routing decisions. Thus, attacks that modify (or strip) these other attributes are not prevented/detected by PATHSEC. As noted in Section 1, the SIDR security context calls for protecting only the information needed to verify that a received route traversed the ASes in question, and that the NLRI in the route is what was advertised. (The AS_PATH data also may have traversed ASes within a confederation that are not represented. However, these ASes are not externally visible and thus do not influence route selection, so their omission in this context is not a security concern.) Thus, protection of other attributes is outside the scope of this document, as described in Section 1. If, at a later time, the SIDR security context is revised to include protection of additional BGP attributes, this document should be revised.

• PATHSEC cannot ensure that an AS will withdraw a route when the AS no longer has a route for a prefix, as noted in Section 4.2. PATHSEC may incorporate features to limit the lifetime of an advertisement. Such lifetime limits provide an upper bound on the time that the failure to withdraw a route will remain effective.

6. Security Considerations

A threat model is, by definition, a security-centric document. Unlike a protocol description, a threat model does not create security problems nor does it purport to address security problems. This model postulates a set of threats (i.e., motivated, capable adversaries) and examines classes of attacks that these threats are capable of effecting, based on the motivations ascribed to the threats. It describes the impact of these types of attacks on PATHSEC, including the RPKI on which PATHSEC relies. It describes how the design of the RPKI (and the PATHSEC design goals) address classes of attacks, where applicable. It also notes residual vulnerabilities.

7. Acknowledgements

The authors with to thank the members of the SIDR working group for the extensive feedback provided during the development of this document.

8. Informative References

   [Kelly70]  Kelly, W., "We Have Met The Enemy and He Is Us: Pogo Earth
              Day Poster", April 1970.

[Kent2000]

              Kent, S., Lynn, C., and K. Seo, "Design and Analysis of
              the Secure Border Gateway Protocol (S-BGP)", IEEE DISCEX
              Conference, June 2000.
   
   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779, June 2004.
   
   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.
   
   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis", RFC
              4272, January 2006.
   
   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.
   
   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.
   
   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, February 2012.
   
   [RFC6481]  Huston, G., Loomans, R., and G. Michaelson, "A Profile for
              Resource Certificate Repository Structure", RFC 6481,
              February 2012.
   
   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482, February 2012.
   
   [RFC6486]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 6486, February 2012.
   
   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487, February
              2012.
   
   [RFC6488]  Lepinski, M., Chi, A., and S. Kent, "Signed Object
              Template for the Resource Public Key Infrastructure
              (RPKI)", RFC 6488, February 2012.
   
   [RFC6493]  Bush, R., "The Resource Public Key Infrastructure (RPKI)
              Ghostbusters Record", RFC 6493, February 2012.
   
   [RFC6810]  Bush, R. and R. Austein, "The Resource Public Key
              Infrastructure (RPKI) to Router Protocol", RFC 6810,
              January 2013.
   
   [SIDR-CH]  "Secure Inter-Domain Routing: Charter for Working Group",
              September 2013, <http://tools.ietf.org/wg/sidr/
              charters?item=charter-sidr-2013-09-20.txt>.
   
   [Sam04]    Samuel, A., "Hacktivism and the Future of Political
              Participation", Ph.D. dissertation, Harvard University,
              September 2004, <http://www.alexandrasamuel.com/
              dissertation/pdfs/Samuel-Hacktivism-entire.pdf>.

Authors' Addresses

   Stephen Kent
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138
   USA

EMail:

          kent@bbn.com
   
   Andrew Chi
   University of North Carolina - Chapel Hill
   c/o Department of Computer Science
   CB 3175, Sitterson Hall
   Chapel Hill, NC  27599
   USA
   
   EMail: achi@cs.unc.edu