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Updates:
Keywords: Unknown Key-Share Attack, SDP, DTLS-SRTP, WebRTC, SIP identity

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 (c) 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