| Network Working Group | M. Thomson |
| Internet-Draft | E. Rescorla |
| Updates: 8122 (if approved) | Mozilla |
| Intended status: Standards Track | November 12, 2018 |
| Expires: May 16, 2019 |
Unknown Key Share Attacks on uses of Transport Layer Security with the Session Description Protocol (SDP)
draft-ietf-mmusic-sdp-uks-latest
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 mislead about the identity of a communicating peer. Simple mitigation techniques are defined for each.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 16, 2019.
Copyright (c) 2018 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.
The use of Transport Layer Security (TLS) [RFC5246] with the Session Description Protocol (SDP) [RFC4566] is defined in [RFC8122]. Further use with Datagram Transport Layer Security (DTLS) [RFC6347] and the Secure Real-time Transport Protocol (SRTP) [RFC3711] is defined as DTLS-SRTP [RFC5763].
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. This is defined in [RFC8122].
The design in RFC 8122 relies on the integrity of the signaling channel. Certificate fingerprints are assumed to be provided by the communicating peers and carried by the signaling channel without being subject to modification.
The use of certificate fingerprints as described in RFC 8122 is vulnerable to a potential unknown key-share (UKS) attack where a misbehaving endpoint is able to advertise a key that it does not control. This could lead to the creation of sessions where peers are confused about the identify of the participants.
The theorized attacks described here are largely hypotetical; the conditions for mounting an attack in practice are challenging (see Section 2.2). The mechanisms defined in this document are intended to strengthen the protocol by preventing the use of unknown key shares in combination with other protocol or implementation vulnerabilities. An extension to TLS is defined that can be used to mitigate unknown key share attacks in a setting where fingerprints are used.
Similar attacks and mitigations are described in Section 4 for sessions that use an external identity bound to fingerprints, like that defined in WebRTC identity (see Section 7 of [WEBRTC-SEC]) or SIP identity [SIP-ID].
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.
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.
In usages of TLS that use SDP for negotiation, an endpoint is able to acquire the certificate fingerprint of another entity. By advertising that fingerprint in place of one of its own, the malicious endpoint can cause its peer to communicate with a different peer, even though it believes that it is communicating with the malicious endpoint.
When the identity of communicating peers is established by higher-layer signaling constructs, such as those in SIP identity [RFC8224] or WebRTC [WEBRTC-SEC], this allows an attacker to bind their own identity to a session with any other entity.
By substituting the fingerprint of one peer for its own, an attacker is able to cause a TLS connection to be established where one endpoint might make an incorrect assumption about the identity of its peer. The TLS peer is not the same as the signaling peer.
The peer does not suffer any such confusion, resulting in each peer involved in the session having a different view of the nature of the session.
This attack applies to any communications established based on the SDP fingerprint attribute [RFC8122].
This attack is an aspect of SDP-based protocols that the technique known as third-party call control (3PCC) [_3PCC] relies on. 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 2.5 describes the consequences of the mitigations described here for systems that use 3PCC.
This vulnerability might be used by an attacker to create a session where there is confusion about the communicating endpoints.
A SIP endpoint or WebRTC 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 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.
The same technique can be used to cause two victims to both believe they are talking to the attacker when they are talking to each other.
In a related attack, a single call using WebRTC or SIP identity can be attacked so that it produces the same outcome. This attack does not depend on concurrent call initiation.
This document assumes that signaling is integrity protected. However, as Section 7 of [RFC8122] explains, many deployments that use SDP do not guarantee integrity of session signaling and so are vulnerable to other attacks. [RFC8122] offers key continuity mechanisms as a potential means of reducing exposure to attack in the absence of integrity protection. Section 2.4 provides some analysis of the effect of key continuity in relation to the described attacks.
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:
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.
The second condition is not necessary with WebRTC identity if the victim has or is configured with a target peer identity (as defined in [WEBRTC]). Furthermore, any identity displayed by a browser could be different to the identity used by the application, since the attack affects the browser’s understanding of the peer’s identity.
Many contemporary deployments do not provide signaling integrity. Without signaling integrity or explicit peer authentication, such as is defined in WebRTC [WEBRTC-SEC] or SIP identity [SIP-ID], no assurances can be made about communication peers.
In this example, two sessions are created with the same endpoint at 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 endpoint that their session has completed, and that the session with the other endpoint has succeeded.
In addition to the constraints described in Section 2.2, the attacker in this example also needs to the ability to view and drop packets between victims. That is, the attacker is on-path.
This example 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 1: Example Attack Scenario
In Figure 1, 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 session that is ostensibly between Mallory and Norma is complete. Mallory begins forwarding DTLS and media packets sent to her by Norma to Patsy. These packets 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’, 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.
Though Patsy needs to believe that the second signaling session has been successfully established, Mallory has no real interest in seeing that session complete. Mallory only needs to ensure that Patsy does not abandon the session prematurely. For this reason, it might be necessary to permit the signaling from Patsy to reach Norma to allow Patsy to receive a call completion signal, such as a SIP ACK. Once the second session completes, Mallory might cause DTLS packets sent by Norma to Patsy to be dropped, though these will likely be discarded by Patsy.
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 is 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 concurrently, if the attacker (Mallory) is on the network path between the victims, and if the same certificate - and therefore SDP fingerprint attribute value - is used in both sessions.
Where ICE [ICE] is used, Mallory also needs to ensure that connectivity between Patsy and Norma succeed, either by forwarding checks or answering and generating the necessary messages.
Systems that use key continuity might be able to detect an unknown key-share attack if a session with the actual peer (i.e., Patsy in the example) 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 on 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 (Patsy) and the expected keys (Mallory).
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.
Third-party call control (3PCC) [_3PCC] is a technique where a signaling peer establishes a call that is terminated by a different entity. This attack is very similar to the 3PCC technique, except where the TLS peers are aware of the use of 3PCC.
For 3PCC to work with the proposed defense, TLS peers need to be aware of the signaling so that they can correctly generate (and check) the extension. It is understood that this technique will prevent the use of 3PCC if peers are not able to access signaling.
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 attacks of the sort described.
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]). Validating that peers use the correct identifier then means that the session is 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.
The external_session_id TLS extension carries the unique identifier that an endpoint selects. When used with SDP, the value includes the tls-id attribute from the SDP that the endpoint generated when negotiating the session. This document only defines use of this extensions 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 a ExternalSessionId struct, described below using the syntax defined in [RFC5246]:
struct {
opaque id<20..255>;
} ExternalSessionId;
For SDP, the 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 [RFC0020].
Where RTP and RTCP [RFC3550] 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 usually distinguishable. RTP/RTCP multiplexing is advised to address this problem.
The external_session_id extension is included in a ClientHello and either ServerHello (for TLS and DTLS versions less than 1.3) or EncryptedExtensions (for TLS 1.3). In TLS 1.3, the external_session_id extension MUST NOT be included in a ServerHello.
Endpoints MUST check that the 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 compare the encoded TLS extension octets with the encoded SDP attribute value. An endpoint that receives a external_session_id extension that is not identical to the value that it expects MUST abort the connection with a fatal handshake_failure alert.
An endpoint that is communicating with a peer that does not support this extension will receive a ClientHello, ServerHello or EncryptedExtensions 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, the external_session_id extension MUST be sent in the EncryptedExtensions message.
The identity assertions used for WebRTC (Section 7 of [WEBRTC-SEC]) and the SIP PASSPoRT using in SIP identity ([SIP-ID], [PASSPoRT]) are bound only to the certificate fingerprint of an endpoint and can therefore be copied by an attacker along with any SDP fingerprint attributes. An attacker can exploit this by causing a victim to believe that they are communicating with an attacker-controlled identity, when they are really talking to another entity of the attacker’s choice. The attacker only needs to create an identity assertion that covers a certificate fingerprint of their choosing.
The problem might appear to be caused by the fact that an identity provider is not required to verify that the entity requesting an identity assertion controls the keys associated with the certificate that is used. A WebRTC identity provider is not required, or even able, to perform validation. This is not an issue because verification is not a necessary condition for a secure protocol, nor would it be sufficient to prevent attack [SIGMA].
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 assertion - which could be sizeable - a collision- and pre-image-resistant hash of the identity assertion is included in a TLS extension. Peers then need only validate that the extension contains a hash of the identity assertion they received in signaling in addition to validating the identity assertion.
Endpoints can also use the external_session_id extension in addition to this so that two calls between the same parties can’t be altered by an attacker.
The id_hash TLS extension carries a hash of the identity assertion that communicating peers have exchanged.
The extension_data for the ext_id_hash extension contains a ExternalIdentityHash struct, described below using the syntax defined in [RFC5246]:
struct {
opaque assertion_hash<0..32>;
} ExternalIdentityHash;
A WebRTC identity assertion is provided as a JSON [RFC7159] object that is encoded into a JSON text. The resulting string is then encoded using UTF-8 [RFC3629]. The content of the ext_id_hash extension are produced by hashing the resulting octets with SHA-256 [SHA]. This produces the 32 octets of the assertion_hash parameter, which is the sole contents of the extension.
The SDP identity attribute includes the base64 [RFC4648] encoding of the same octets that were input to the hash. The ext_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.
Where a PASSPoRT is used, the compact form of the PASSPoRT MUST be expanded into the full form. The base64 encoding used in the Identity (or ‘y’) header field MUST be decoded then used as input to SHA-256. This produces the 32 octet assertion_hash value used for creating or validating the extension.
Identity assertions in either form might be provided by only one peer. An endpoint that does not produce an identity assertion MUST generate an empty ext_id_hash extension in its ClientHello. This allows its peer to include a hash of its identity assertion. An endpoint without an identity assertion MUST omit the ext_id_hash extension from its ServerHello or EncryptedExtensions message.
A peer that receives an ext_id_hash extension that is not equal to the value of the identity assertion from its peer MUST immediately fail the TLS handshake with an error. This includes cases where the assertion is absent.
An ext_id_hash extension that is any length other than 0 or 32 is invalid and MUST cause the receiving endpoint to generate a fatal decode_error alert.
A peer that receives an identity assertion, but does not receive an ext_id_hash extension MAY choose to fail the connection, though it is expected that implementations written prior to the definition of the extensions in this document will not support both for some time.
In TLS 1.3, the ext_id_hash extension MUST be sent in the EncryptedExtensions message.
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 creates a situation where both peers believe that they are talking to the attacker when they are talking to each other.
This kind of attack is prevented by systems that enable peer authentication such as WebRTC identity [WEBRTC-SEC] or SIP identity [SIP-ID]. However, session concatention remains possible at higher layers: an attacker can establish two independent sessions and simply forward any data it receives from one into the other.
In the absence of any higher-level concept of peer identity, the use of session identifiers does not prevent session concatenation. The value to an attacker is limited unless information from the TLS connection is extracted and used with the signaling. For instance, a key exporter [EXPORTER] might be used to create a shared secret or unique identifier that is used in a secondary protocol.
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 unless they also validate the identity of peers at both layers. 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.
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.
Information extracted from a TLS connection therefore MUST NOT be used in a secondary protocol outside of that connection if that protocol relies on the signaling protocol having the same peers. Similarly, data 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.
This entire document contains security considerations.
This document registers two extensions in the TLS “ExtensionType Values” registry established in [RFC5246]:
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 useful review and input.