Internet Engineering Task Force (IETF)                        M. Thomson
Request for Comments: 8844                                   E. Rescorla
Updates: 8122                                                    Mozilla
Category: Standards Track                                   January 2021
ISSN: 2070-1721

Unknown Key-Share Attacks on Uses of TLS with the Session Description

Protocol (SDP)

Abstract

This document describes unknown key-share attacks on the use of Datagram Transport Layer Security for the Secure Real-Time Transport Protocol (DTLS-SRTP). Similar attacks are described on the use of DTLS-SRTP with the identity bindings used in Web Real-Time Communications (WebRTC) and SIP identity. These attacks are difficult to mount, but they cause a victim to be misled about the identity of a communicating peer. This document defines mitigation techniques that implementations of RFC 8122 are encouraged to deploy.

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 7841.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8844.

Copyright Notice

Copyright © 2021 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Unknown Key-Share Attack
     2.1.  Limits on Attack Feasibility
     2.2.  Interactions with Key Continuity
     2.3.  Third-Party Call Control
   3.  Unknown Key-Share Attack with Identity Bindings
     3.1.  Example
     3.2.  The "external_id_hash" TLS Extension
       3.2.1.  Calculating "external_id_hash" for WebRTC Identity
       3.2.2.  Calculating external_id_hash for PASSporT
   4.  Unknown Key-Share Attack with Fingerprints
     4.1.  Example
     4.2.  Unique Session Identity Solution
     4.3.  The external_session_id TLS Extension
   5.  Session Concatenation
   6.  Security Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Acknowledgements
   Authors' Addresses

1. Introduction

The use of Transport Layer Security (TLS) [TLS13] with the Session Description Protocol (SDP) [SDP] is defined in [FINGERPRINT]. Further use with Datagram Transport Layer Security (DTLS) [DTLS] and the Secure Real-time Transport Protocol (SRTP) [SRTP] is defined as DTLS-SRTP [DTLS-SRTP].

In these specifications, key agreement is performed using TLS or DTLS, with authentication being tied back to the session description (or SDP) through the use of certificate fingerprints. Communication peers check that a hash, or fingerprint, provided in the SDP matches the certificate that is used in the TLS or DTLS handshake.

WebRTC identity (see Section 7 of [WEBRTC-SEC]) and SIP identity [SIP-ID] both provide a mechanism that binds an external identity to the certificate fingerprints from a session description. However, this binding is not integrity protected and is therefore vulnerable to an identity misbinding attack, also known as an unknown key-share (UKS) attack, where the attacker binds their identity to the fingerprint of another entity. A successful attack leads to the creation of sessions where peers are confused about the identity of the participants.

This document describes a TLS extension that can be used in combination with these identity bindings to prevent this attack.

A similar attack is possible with the use of certificate fingerprints alone. Though attacks in this setting are likely infeasible in existing deployments due to the narrow preconditions (see Section 2.1), this document also describes mitigations for this attack.

The mechanisms defined in this document are intended to strengthen the protocol by preventing the use of unknown key-share attacks in combination with other protocol or implementation vulnerabilities. RFC 8122 [FINGERPRINT] is updated by this document to recommend the use of these mechanisms.

This document assumes that signaling is integrity protected. However, as Section 7 of [FINGERPRINT] explains, many deployments that use SDP do not guarantee integrity of session signaling and so are vulnerable to other attacks. [FINGERPRINT] offers key continuity mechanisms as a potential means of reducing exposure to attack in the absence of integrity protection. Section 2.2 provides some analysis of the effect of key continuity in relation to the described attacks.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Unknown Key-Share Attack

In an unknown key-share attack [UKS], a malicious participant in a protocol claims to control a key that is in reality controlled by some other actor. This arises when the identity associated with a key is not properly bound to the key.

An endpoint that can acquire the certificate fingerprint of another entity can advertise that fingerprint as their own in SDP. An attacker can use a copy of that fingerprint to cause a victim to communicate with another unaware victim, even though the first victim believes that it is communicating with the attacker.

When the identity of communicating peers is established by higher- layer signaling constructs, such as those in SIP identity [SIP-ID] or WebRTC [WEBRTC-SEC], this allows an attacker to bind their own identity to a session with any other entity.

The attacker obtains an identity assertion for an identity it controls, but binds that to the fingerprint of one peer. The attacker is then able to cause a TLS connection to be established where two victim endpoints communicate. The victim that has its fingerprint copied by the attack correctly believes that it is communicating with the other victim; however, the other victim incorrectly believes that it is communicating with the attacker.

An unknown key-share attack does not result in the attacker having access to any confidential information exchanged between victims. However, the failure in mutual authentication can enable other attacks. A victim might send information to the wrong entity as a result. Where information is interpreted in context, misrepresenting that context could lead to the information being misinterpreted.

A similar attack can be mounted based solely on the SDP "fingerprint" attribute [FINGERPRINT] without compromising the integrity of the signaling channel.

This attack is an aspect of SDP-based protocols upon which the technique known as third-party call control (3PCC) [RFC3725] relies. 3PCC exploits the potential for the identity of a signaling peer to be different than the media peer, allowing the media peer to be selected by the signaling peer. Section 2.3 describes the consequences of the mitigations described here for systems that use 3PCC.

2.1. Limits on Attack Feasibility

The use of TLS with SDP depends on the integrity of session signaling. Assuming signaling integrity limits the capabilities of an attacker in several ways. In particular:

1. An attacker can only modify the parts of the session signaling that they are responsible for producing, namely their own offers and answers.

2. No entity will successfully establish a session with a peer unless they are willing to participate in a session with that peer.

The combination of these two constraints make the spectrum of possible attacks quite limited. An attacker is only able to switch its own certificate fingerprint for a valid certificate that is acceptable to its peer. Attacks therefore rely on joining two separate sessions into a single session. Section 4 describes an attack on SDP signaling under these constraints.

Systems that rely on strong identity bindings, such as those defined in [WEBRTC] or [SIP-ID], have a different threat model, which admits the possibility of attack by an entity with access to the signaling channel. Attacks under these conditions are more feasible as an attacker is assumed to be able both to observe and to modify signaling messages. Section 3 describes an attack that assumes this threat model.

2.2. Interactions with Key Continuity

Systems that use key continuity (as defined in Section 15.1 of [ZRTP] or as recommended in Section 7 of [FINGERPRINT]) might be able to detect an unknown key-share attack if a session with either the attacker or the genuine peer (i.e., the victim whose fingerprint was copied by an attacker) was established in the past. Whether this is possible depends on how key continuity is implemented.

Implementations that maintain a single database of identities with an index of peer keys could discover that the identity saved for the peer key does not match the claimed identity. Such an implementation could notice the disparity between the actual keys (those copied from a victim) and the expected keys (those of the attacker).

In comparison, implementations that first match based on peer identity could treat an unknown key-share attack as though their peer had used a newly configured device. The apparent addition of a new device could generate user-visible notices (e.g., "Mallory appears to have a new device"). However, such an event is not always considered alarming; some implementations might silently save a new key.

2.3. Third-Party Call Control

Third-party call control (3PCC) [RFC3725] is a technique where a signaling peer establishes a call that is terminated by a different entity. An unknown key-share attack is very similar in effect to some 3PCC practices, so use of 3PCC could appear to be an attack. However, 3PCC that follows RFC 3725 guidance is unaffected, and peers that are aware of changes made by a 3PCC controller can correctly distinguish actions of a 3PCC controller from an attack.

3PCC as described in RFC 3725 is incompatible with SIP identity [SIP-ID], as SIP Identity relies on creating a binding between SIP requests and SDP. The controller is the only entity that generates SIP requests in RFC 3725. Therefore, in a 3PCC context, only the use of the "fingerprint" attribute without additional bindings or WebRTC identity [WEBRTC-SEC] is possible.

The attack mitigation mechanisms described in this document will prevent the use of 3PCC if peers have different views of the involved identities or the value of SDP "tls-id" attributes.

For 3PCC to work with the proposed mechanisms, TLS peers need to be aware of the signaling so that they can correctly generate and check the TLS extensions. For a connection to be successfully established, a 3PCC controller needs either to forward SDP without modification or to avoid modifications to "fingerprint", "tls-id", and "identity" attributes. A controller that follows the best practices in RFC 3725 is expected to forward SDP without modification, thus ensuring the integrity of these attributes.

3. Unknown Key-Share Attack with Identity Bindings

The identity assertions used for WebRTC (Section 7 of [WEBRTC-SEC]) and the Personal Assertion Token (PASSporT) used in SIP identity ([SIP-ID], [PASSPORT]) are bound to the certificate fingerprint of an endpoint. An attacker can cause an identity binding to be created that binds an identity they control to the fingerprint of a first victim.

An attacker can thereby cause a second victim to believe that they are communicating with an attacker-controlled identity, when they are really talking to the first victim. The attacker does this by creating an identity assertion that covers a certificate fingerprint of the first victim.

A variation on the same technique can be used to cause both victims to believe they are talking to the attacker when they are talking to each other. In this case, the attacker performs the identity misbinding once for each victim.

The authority certifying the identity binding is not required to verify that the entity requesting the binding actually controls the keys associated with the fingerprints, and this might appear to be the cause of the problem. SIP and WebRTC identity providers are not required to perform this validation.

A simple solution to this problem is suggested by [SIGMA]. The identity of endpoints is included under a message authentication code (MAC) during the cryptographic handshake. Endpoints then validate that their peer has provided an identity that matches their expectations. In TLS, the Finished message provides a MAC over the entire handshake, so that including the identity in a TLS extension is sufficient to implement this solution.

Rather than include a complete identity binding, which could be sizable, a collision- and preimage-resistant hash of the binding is included in a TLS extension as described in Section 3.2. Endpoints then need only validate that the extension contains a hash of the identity binding they received in signaling. If the identity binding is successfully validated, the identity of a peer is verified and bound to the session.

This form of unknown key-share attack is possible without compromising signaling integrity, unless the defenses described in Section 4 are used. In order to prevent both forms of attack, endpoints MUST use the "external_session_id" extension (see Section 4.3) in addition to the "external_id_hash" (Section 3.2) so that two calls between the same parties can't be altered by an attacker.

3.1. Example

In the example shown in Figure 1, it is assumed that the attacker also controls the signaling channel.

Mallory (the attacker) presents two victims, Norma and Patsy, with two separate sessions. In the first session, Norma is presented with the option to communicate with Mallory; a second session with Norma is presented to Patsy.

     Norma                   Mallory                   Patsy
     (fp=N)                   -----                    (fp=P)
       |                        |                        |
       |<---- Signaling1 ------>|                        |
       |   Norma=N Mallory=P    |                        |
       |                        |<---- Signaling2 ------>|
       |                        |   Norma=N Patsy=P      |
       |                                                 |
       |<=================DTLS (fp=N,P)=================>|
       |                                                 |
     (peer = Mallory!)                         (peer = Norma)

Figure 1: Example Attack on Identity Bindings

The attack requires that Mallory obtain an identity binding for her own identity with the fingerprints presented by Patsy (P), which Mallory might have obtained previously. This false binding is then presented to Norma ('Signaling1' in Figure 1).

Patsy could be similarly duped, but in this example, a correct binding between Norma's identity and fingerprint (N) is faithfully presented by Mallory. This session ('Signaling2' in Figure 1) can be entirely legitimate.

A DTLS session is established directly between Norma and Patsy. In order for this to happen, Mallory can substitute transport-level information in both sessions, though this is not necessary if Mallory is on the network path between Norma and Patsy.

As a result, Patsy correctly believes that she is communicating with Norma. However, Norma incorrectly believes that she is talking to Mallory. As stated in Section 2, Mallory cannot access media, but Norma might send information to Patsy that Norma might not intend or that Patsy might misinterpret.

3.2. The "external_id_hash" TLS Extension

The "external_id_hash" TLS extension carries a hash of the identity assertion that the endpoint sending the extension has asserted to its peer. Both peers include a hash of their own identity assertion.

The "extension_data" for the "external_id_hash" extension contains a "ExternalIdentityHash" struct, described below using the syntax defined in Section 3 of [TLS13]:

      struct {
         opaque binding_hash<0..32>;
      } ExternalIdentityHash;

Where an identity assertion has been asserted by a peer, this extension includes a SHA-256 hash of the assertion. An empty value is used to indicate support for the extension.

   Note:  For both types of identity assertion, if SHA-256 should prove
      to be inadequate in the future (see [AGILITY]), a new TLS
      extension that uses a different hash function can be defined.

Identity bindings might be provided by only one peer. An endpoint that does not produce an identity binding MUST generate an empty "external_id_hash" extension in its ClientHello or -- if a client provides the extension -- in ServerHello or EncryptedExtensions. An empty extension has a zero-length "binding_hash" field.

A peer that receives an "external_id_hash" extension that does not match the value of the identity binding from its peer MUST immediately fail the TLS handshake with an "illegal_parameter" alert. The absence of an identity binding does not relax this requirement; if a peer provided no identity binding, a zero-length extension MUST be present to be considered valid.

Implementations written prior to the definition of the extensions in this document will not support this extension for some time. A peer that receives an identity binding but does not receive an "external_id_hash" extension MAY accept a TLS connection rather than fail a connection where the extension is absent.

The endpoint performs the validation of the "external_id_hash" extension in addition to the validation required by [FINGERPRINT] and any verification of the identity assertion [WEBRTC-SEC] [SIP-ID]. An endpoint MUST validate any external_session_id value that is present; see Section 4.3.

An "external_id_hash" extension with a "binding_hash" field that is any length other than 0 or 32 is invalid and MUST cause the receiving endpoint to generate a fatal "decode_error" alert.

In TLS 1.3, an "external_id_hash" extension sent by a server MUST be sent in the EncryptedExtensions message.

3.2.1. Calculating "external_id_hash" for WebRTC Identity

A WebRTC identity assertion (Section 7 of [WEBRTC-SEC]) is provided as a JSON [JSON] object that is encoded into a JSON text. The JSON text is encoded using UTF-8 [UTF8] as described by Section 8.1 of [JSON]. The content of the "external_id_hash" extension is produced by hashing the resulting octets with SHA-256 [SHA]. This produces the 32 octets of the "binding_hash" parameter, which is the sole contents of the extension.

The SDP "identity" attribute includes the base64 [BASE64] encoding of the UTF-8 encoding of the same JSON text. The "external_id_hash" extension is validated by performing base64 decoding on the value of the SDP "identity" attribute, hashing the resulting octets using SHA-256, and comparing the results with the content of the extension. In pseudocode form, using the "identity-assertion-value" field from the SDP "identity" attribute grammar as defined in [WEBRTC-SEC]:

   external_id_hash = SHA-256(b64decode(identity-assertion-value))
   
   Note:  The base64 of the SDP "identity" attribute is decoded to avoid
      capturing variations in padding.  The base64-decoded identity
      assertion could include leading or trailing whitespace octets.
      WebRTC identity assertions are not canonicalized; all octets are
      hashed.

3.2.2. Calculating external_id_hash for PASSporT

Where the compact form of PASSporT [PASSPORT] is used, it MUST be expanded into the full form. The base64 encoding used in the SIP Identity (or 'y') header field MUST be decoded then used as input to SHA-256. This produces the 32-octet "binding_hash" value used for creating or validating the extension. In pseudocode, using the "signed-identity-digest" parameter from the "Identity" header field grammar defined [SIP-ID]:

   external_id_hash = SHA-256(b64decode(signed-identity-digest))

4. Unknown Key-Share Attack with Fingerprints

An attack on DTLS-SRTP is possible because the identity of peers involved is not established prior to establishing the call. Endpoints use certificate fingerprints as a proxy for authentication, but as long as fingerprints are used in multiple calls, they are vulnerable to attack.

Even if the integrity of session signaling can be relied upon, an attacker might still be able to create a session where there is confusion about the communicating endpoints by substituting the fingerprint of a communicating endpoint.

An endpoint that is configured to reuse a certificate can be attacked if it is willing to initiate two calls at the same time, one of which is with an attacker. The attacker can arrange for the victim to incorrectly believe that it is calling the attacker when it is in fact calling a second party. The second party correctly believes that it is talking to the victim.

As with the attack on identity bindings, this can be used to cause two victims to both believe they are talking to the attacker when they are talking to each other.

4.1. Example

To mount this attack, two sessions need to be created with the same endpoint at almost precisely the same time. One of those sessions is initiated with the attacker, the second session is created toward another honest endpoint. The attacker convinces the first endpoint that their session with the attacker has been successfully established, but media is exchanged with the other honest endpoint. The attacker permits the session with the other honest endpoint to complete only to the extent necessary to convince the other honest endpoint to participate in the attacked session.

In addition to the constraints described in Section 2.1, the attacker in this example also needs the ability to view and drop packets between victims. That is, the attacker needs to be on path for media.

The attack shown in Figure 2 depends on a somewhat implausible set of conditions. It is intended to demonstrate what sort of attack is possible and what conditions are necessary to exploit this weakness in the protocol.

     Norma                   Mallory                 Patsy
     (fp=N)                   -----                  (fp=P)
       |                        |                      |
       +---Signaling1 (fp=N)--->|                      |
       +-----Signaling2 (fp=N)------------------------>|
       |<-------------------------Signaling2 (fp=P)----+
       |<---Signaling1 (fp=P)---+                      |
       |                        |                      |
       |=======DTLS1=======>(Forward)======DTLS1======>|
       |<======DTLS2========(Forward)<=====DTLS2=======|
       |=======Media1======>(Forward)======Media1=====>|
       |<======Media2=======(Forward)<=====Media2======|
       |                       |                       |
       |=======DTLS2========>(Drop)                    |
       |                       |                       |

Figure 2: Example Attack Scenario Using Fingerprints

In this scenario, there are two sessions initiated at the same time by Norma. Signaling is shown with single lines ('-'), DTLS and media with double lines ('=').

The first session is established with Mallory, who falsely uses Patsy's certificate fingerprint (denoted with 'fp=P'). A second session is initiated between Norma and Patsy. Signaling for both sessions is permitted to complete.

Once signaling is complete on the first session, a DTLS connection is established. Ostensibly, this connection is between Mallory and Norma, but Mallory forwards DTLS and media packets sent to her by Norma to Patsy. These packets are denoted 'DTLS1' because Norma associates these with the first signaling session ('Signaling1').

Mallory also intercepts packets from Patsy and forwards those to Norma at the transport address that Norma associates with Mallory. These packets are denoted 'DTLS2' to indicate that Patsy associates these with the second signaling session ('Signaling2'); however, Norma will interpret these as being associated with the first signaling session ('Signaling1').

The second signaling exchange ('Signaling2'), which is between Norma and Patsy, is permitted to continue to the point where Patsy believes that it has succeeded. This ensures that Patsy believes that she is communicating with Norma. In the end, Norma believes that she is communicating with Mallory, when she is really communicating with Patsy. Just like the example in Section 3.1, Mallory cannot access media, but Norma might send information to Patsy that Norma might not intend or that Patsy might misinterpret.

Though Patsy needs to believe that the second signaling session has been successfully established, Mallory has no real interest in seeing that session also be established. Mallory only needs to ensure that Patsy maintains the active session and does not abandon the session prematurely. For this reason, it might be necessary to permit the signaling from Patsy to reach Norma in order to allow Patsy to receive a call setup completion signal, such as a SIP ACK. Once the second session is established, Mallory might cause DTLS packets sent by Norma to Patsy to be dropped. However, if Mallory allows DTLS packets to pass, it is likely that Patsy will discard them as Patsy will already have a successful DTLS connection established.

For the attacked session to be sustained beyond the point that Norma detects errors in the second session, Mallory also needs to block any signaling that Norma might send to Patsy asking for the call to be abandoned. Otherwise, Patsy might receive a notice that the call has failed and thereby abort the call.

This attack creates an asymmetry in the beliefs about the identity of peers. However, this attack is only possible if the victim (Norma) is willing to conduct two sessions nearly simultaneously; if the attacker (Mallory) is on the network path between the victims; and if the same certificate -- and therefore the SDP "fingerprint" attribute value -- is used by Norma for both sessions.

Where Interactive Connectivity Establishment (ICE) [ICE] is used, Mallory also needs to ensure that connectivity checks between Patsy and Norma succeed, either by forwarding checks or by answering and generating the necessary messages.

4.2. Unique Session Identity Solution

The solution to this problem is to assign a new identifier to communicating peers. Each endpoint assigns their peer a unique identifier during call signaling. The peer echoes that identifier in the TLS handshake, binding that identity into the session. Including this new identity in the TLS handshake means that it will be covered by the TLS Finished message, which is necessary to authenticate it (see [SIGMA]).

Successfully validating that the identifier matches the expected value means that the connection corresponds to the signaled session and is therefore established between the correct two endpoints.

This solution relies on the unique identifier given to DTLS sessions using the SDP "tls-id" attribute [DTLS-SDP]. This field is already required to be unique. Thus, no two offers or answers from the same client will have the same value.

A new "external_session_id" extension is added to the TLS or DTLS handshake for connections that are established as part of the same call or real-time session. This carries the value of the "tls-id" attribute and provides integrity protection for its exchange as part of the TLS or DTLS handshake.

4.3. The external_session_id TLS Extension

The "external_session_id" TLS extension carries the unique identifier that an endpoint selects. When used with SDP, the value MUST include the "tls-id" attribute from the SDP that the endpoint generated when negotiating the session. This document only defines use of this extension for SDP; other methods of external session negotiation can use this extension to include a unique session identifier.

The "extension_data" for the "external_session_id" extension contains an ExternalSessionId struct, described below using the syntax defined in [TLS13]:

      struct {
         opaque session_id<20..255>;
      } ExternalSessionId;

For SDP, the "session_id" field of the extension includes the value of the "tls-id" SDP attribute as defined in [DTLS-SDP] (that is, the "tls-id-value" ABNF production). The value of the "tls-id" attribute is encoded using ASCII [ASCII].

Where RTP and RTCP [RTP] are not multiplexed, it is possible that the two separate DTLS connections carrying RTP and RTCP can be switched. This is considered benign since these protocols are designed to be distinguishable as SRTP [SRTP] provides key separation. Using RTP/ RTCP multiplexing [RTCP-MUX] further avoids this problem.

The "external_session_id" extension is included in a ClientHello, and if the extension is present in the ClientHello, either ServerHello (for TLS and DTLS versions older than 1.3) or EncryptedExtensions (for TLS 1.3).

Endpoints MUST check that the "session_id" parameter in the extension that they receive includes the "tls-id" attribute value that they received in their peer's session description. Endpoints can perform string comparison by ASCII decoding the TLS extension value and comparing it to the SDP attribute value or by comparing the encoded TLS extension octets with the encoded SDP attribute value. An endpoint that receives an "external_session_id" extension that is not identical to the value that it expects MUST abort the connection with a fatal "illegal_parameter" alert.

The endpoint performs the validation of the "external_id_hash" extension in addition to the validation required by [FINGERPRINT].

If an endpoint communicates with a peer that does not support this extension, it will receive a ClientHello, ServerHello, or EncryptedExtensions message that does not include this extension. An endpoint MAY choose to continue a session without this extension in order to interoperate with peers that do not implement this specification.

In TLS 1.3, an "external_session_id" extension sent by a server MUST be sent in the EncryptedExtensions message.

This defense is not effective if an attacker can rewrite "tls-id" values in signaling. Only the mechanism in "external_id_hash" is able to defend against an attacker that can compromise session integrity.

5. Session Concatenation

Use of session identifiers does not prevent an attacker from establishing two concurrent sessions with different peers and forwarding signaling from those peers to each other. Concatenating two signaling sessions in this way creates two signaling sessions, with two session identifiers, but only the TLS connections from a single session are established as a result. In doing so, the attacker creates a situation where both peers believe that they are talking to the attacker when they are talking to each other.

In the absence of any higher-level concept of peer identity, an attacker who is able to copy the session identifier from one signaling session to another can cause the peers to establish a direct TLS connection even while they think that they are connecting to the attacker. This differs from the attack described in the previous section in that there is only one TLS connection rather than two. This kind of attack is prevented by systems that enable peer authentication, such as WebRTC identity [WEBRTC-SEC] or SIP identity [SIP-ID]; however, these systems do not prevent establishing two back-to-back connections as described in the previous paragraph.

Use of the "external_session_id" does not guarantee that the identity of the peer at the TLS layer is the same as the identity of the signaling peer. The advantage that an attacker gains by concatenating sessions is limited unless data is exchanged based on the assumption that signaling and TLS peers are the same. If a secondary protocol uses the signaling channel with the assumption that the signaling and TLS peers are the same, then that protocol is vulnerable to attack. While out of scope for this document, a signaling system that can defend against session concatenation requires that the signaling layer is authenticated and bound to any TLS connections.

It is important to note that multiple connections can be created within the same signaling session. An attacker might concatenate only part of a session, choosing to terminate some connections (and optionally forward data) while arranging to have peers interact directly for other connections. It is even possible to have different peers interact for each connection. This means that the actual identity of the peer for one connection might differ from the peer on another connection.

Critically, information about the identity of TLS peers provides no assurances about the identity of signaling peers and does not transfer between TLS connections in the same session. Information extracted from a TLS connection therefore MUST NOT be used in a secondary protocol outside of that connection if that protocol assumes that the signaling protocol has the same peers. Similarly, security-sensitive information from one TLS connection MUST NOT be used in other TLS connections even if they are established as a result of the same signaling session.

6. Security Considerations

When combined with identity assertions, the mitigations in this document ensure that there is no opportunity to misrepresent the identity of TLS peers. This assurance is provided even if an attacker can modify signaling messages.

Without identity assertions, the mitigations in this document prevent the session splicing attack described in Section 4. Defense against session concatenation (Section 5) additionally requires that protocol peers are not able to claim the certificate fingerprints of other entities.

7. IANA Considerations

This document registers two extensions in the "TLS ExtensionType Values" registry established in [TLS13]:

• The "external_id_hash" extension defined in Section 3.2 has been assigned a code point of 55; it is recommended and is marked as "CH, EE" in TLS 1.3.

• The "external_session_id" extension defined in Section 4.3 has been assigned a code point of 56; it is recommended and is marked as "CH, EE" in TLS 1.3.

8. References

8.1. Normative References

   [ASCII]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <https://www.rfc-editor.org/info/rfc20>.
   
   [BASE64]   Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.
   
   [DTLS]     Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.
   
   [DTLS-SDP] Holmberg, C. and R. Shpount, "Session Description Protocol
              (SDP) Offer/Answer Considerations for Datagram Transport
              Layer Security (DTLS) and Transport Layer Security (TLS)",
              RFC 8842, DOI 10.17487/RFC8842, January 2021,
              <https://www.rfc-editor.org/info/rfc8842>.

[DTLS-SRTP]

              Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <https://www.rfc-editor.org/info/rfc5763>.

[FINGERPRINT]

              Lennox, J. and C. Holmberg, "Connection-Oriented Media
              Transport over the Transport Layer Security (TLS) Protocol
              in the Session Description Protocol (SDP)", RFC 8122,
              DOI 10.17487/RFC8122, March 2017,
              <https://www.rfc-editor.org/info/rfc8122>.
   
   [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.
   
   [PASSPORT] Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.
   
   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
   
   [SDP]      Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.
   
   [SHA]      Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.
   
   [SIP-ID]   Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.
   
   [SRTP]     Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.
   
   [TLS13]    Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.
   
   [UTF8]     Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

[WEBRTC-SEC]

              Rescorla, E., "WebRTC Security Architecture", RFC 8827,
              DOI 10.17487/RFC8827, January 2021,
              <https://www.rfc-editor.org/info/rfc8827>.

8.2. Informative References

   [AGILITY]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.
   
   [ICE]      Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", RFC 8445,
              DOI 10.17487/RFC8445, July 2018,
              <https://www.rfc-editor.org/info/rfc8445>.
   
   [RFC3725]  Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
              Camarillo, "Best Current Practices for Third Party Call
              Control (3pcc) in the Session Initiation Protocol (SIP)",
              BCP 85, RFC 3725, DOI 10.17487/RFC3725, April 2004,
              <https://www.rfc-editor.org/info/rfc3725>.
   
   [RTCP-MUX] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
              Control Packets on a Single Port", RFC 5761,
              DOI 10.17487/RFC5761, April 2010,
              <https://www.rfc-editor.org/info/rfc5761>.
   
   [RTP]      Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.
   
   [SIGMA]    Krawczyk, H., "SIGMA: The 'SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE
              Protocols", Advances in Cryptology -- CRYPTO 2003, Lecture
              Notes in Computer Science, Vol. 2729,
              DOI 10.1007/978-3-540-45146-4_24, August 2003,
              <https://doi.org/10.1007/978-3-540-45146-4_24>.
   
   [UKS]      Blake-Wilson, S. and A. Menezes, "Unknown Key-Share
              Attacks on the Station-to-Station (STS) Protocol", Public
              Key Cryptography, Lecture Notes in Computer Science, Vol.
              1560, DOI 10.1007/3-540-49162-7_12, March 1999,
              <https://doi.org/10.1007/3-540-49162-7_12>.
   
   [WEBRTC]   Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0:
              Real-time Communication Between Browsers", W3C Proposed
              Recommendation, <https://www.w3.org/TR/webrtc/>.
   
   [ZRTP]     Zimmermann, P., Johnston, A., Ed., and J. Callas, "ZRTP:
              Media Path Key Agreement for Unicast Secure RTP",
              RFC 6189, DOI 10.17487/RFC6189, April 2011,
              <https://www.rfc-editor.org/info/rfc6189>.

Acknowledgements

This problem would not have been discovered if it weren't for discussions with Sam Scott, Hugo Krawczyk, and Richard Barnes. A solution similar to the one presented here was first proposed by Karthik Bhargavan, who provided valuable input on this document. Thyla van der Merwe assisted with a formal model of the solution. Adam Roach and Paul E. Jones provided significant review and input.

Authors' Addresses

Martin Thomson
Mozilla

Email:

          mt@lowentropy.net

Eric Rescorla
Mozilla

Email:

          ekr@rtfm.com