| Internet Engineering Task Force | A. Popov |
| Internet-Draft | M. Nyström |
| Intended status: Standards Track | Microsoft Corp. |
| Expires: April 17, 2016 | D. Balfanz, Ed. |
| A. Langley | |
| Google Inc. | |
| October 15, 2015 |
Token Binding over HTTP
draft-ietf-tokbind-https-02
This document describes a collection of mechanisms that allow HTTP servers to cryptographically bind authentication tokens (such as cookies and OAuth tokens) to a TLS [RFC5246] connection.
We describe both first-party as well as federated scenarios. In a first-party scenario, an HTTP server issues a security token (such as a cookie) to a client, and expects the client to send the security token back to the server at a later time in order to authenticate. Binding the token to the TLS connection between client and server protects the security token from theft, and ensures that the security token can only be used by the client that it was issued to.
Federated token bindings, on the other hand, allow servers to cryptographically bind security tokens to a TLS [RFC5246] connection that the client has with a different server than the one issuing the token.
This Internet-Draft is a companion document to The Token Binding Protocol [TBPROTO]
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 http://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on April 17, 2016.
Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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The Token Binding Protocol [TBPROTO] defines a Token Binding ID for a TLS connection between a client and a server. The Token Binding ID of a TLS connection is related to a private key that the client proves possession of to the server, and is long-lived (i.e., subsequent TLS connections between the same client and server have the same Token Binding ID). When issuing a security token (e.g. an HTTP cookie or an OAuth token) to a client, the server can include the Token Binding ID in the token, thus cryptographically binding the token to TLS connections between that particular client and server, and inoculating the token against theft by attackers.
While the Token Binding Protocol [TBPROTO] defines a message format for establishing a Token Binding ID, it doesn't specify how this message is embedded in higher-level protocols. The purpose of this specification is to define how TokenBindingMessages are embedded in HTTP (both versions 1.1 [RFC2616] and 2 [I-D.ietf-httpbis-http2]). Note that TokenBindingMessages are only defined if the underlying transport uses TLS. This means that Token Binding over HTTP is only defined when the HTTP protocol is layered on top of TLS (commonly referred to as HTTPS).
HTTP clients establish a Token Binding ID with a server by including a special HTTP header in HTTP requests. The HTTP header value is a TokenBindingMessage.
TokenBindingMessages allow clients to establish multiple Token Binding IDs with the server, by including multiple TokenBinding structures in the TokenBindingMessage. By default, a client will establish a provided Token Binding ID with the server, indicating a Token Binding ID that the client will persistently use with the server. Under certain conditions, the client can also include a referred Token Binding ID in the TokenBindingMessage, indicating a Token Binding ID that the client is using with a different server than the one that the TokenBindingMessage is sent to. This is useful in federation scenarios.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
Once a client and server have negotiated the Token Binding Protocol with HTTP/1.1 or HTTP/2 (see The Token Binding Protocol [TBPROTO]), clients MUST include the Token-Binding header in their HTTP requests. The ABNF of the Token-Binding header is:
Token-Binding = "Token-Binding" ":" [CFWS] EncodedTokenBindingMessage
The EncodedTokenBindingMessage is a web-safe Base64-encoding of the TokenBindingMessage as defined in the TokenBindingProtocol [TBPROTO].
The TokenBindingMessage MUST contain a TokenBinding with TokenBindingType provided_token_binding, which MUST be signed with the Token Binding key used by the client for connections between itself and the server that the HTTP request is sent to (clients use different Token Binding keys for different servers). The Token Binding ID established by this TokenBinding is called a Provided Token Binding ID
In HTTP/2, the client SHOULD use Header Compression [I-D.ietf-httpbis-header-compression] to avoid the overhead of repeating the same header in subsequent HTTP requests.
For privacy reasons, clients use different private keys to establish Provided Token Binding IDs with different servers. As a result, a server cannot bind a security token (such as an OAuth token or an OpenID Connect identity token) to a TLS connection that the client has with a different server. This is, however, a common requirement in federation scenarios: For example, an Identity Provider may wish to issue an identity token to a client and cryptographically bind that token to the TLS connection between the client and a Relying Party.
In this section we describe mechanisms to achieve this. The common idea among these mechanisms is that a server (called the Token Consumer in this document) gives the client permission to reveal the Provided Token Binding ID that is used between the client and itself, to another server (called the Token Provider in this document). Also common across the mechanisms is how the Token Binding ID is revealed to the Token Provider: The client uses the Token Binding Protocol [TBPROTO], and includes a TokenBinding structure in the Token-Binding HTTP header defined above. What differs between the various mechanisms is how the Token Consumer grants the permission to reveal the Token Binding ID to the Token Provider. Below we specify one such mechanism, which is suitable for redirect-based interactions between Token Consumers and Token Providers.
In a Federated Sign-On protocol, an Identity Provider issues an identity token to a client, which sends the identity token to a Relying Party to authenticate itself. Examples of this include OpenID Connect (where the identity token is called "ID Token") and SAML (where the identity token is a SAML assertion).
To better protect the security of the identity token, the Identity Provider may wish to bind the identity token to the TLS connection between the client and the Relying Party, thus ensuring that only said client can use the identity token: The Relying Party will compare the Token Binding ID in the identity token with the Token Binding ID of the TLS connection between it an the client.
This is an example of a federation scenario, which more generally can be described as follows:
The client will perform this last operation (proving possession of a private key that corresponds to a Token Binding ID between the client and the Token Consumer while delivering the token request to the Token Provider) only if the Token Consumer permits the client to do so.
Below, we specify how Token Consumers can grant this permission. during redirect-based federation protocols.
When a Token Consumer redirects the client to a Token Provider as a means to deliver the token request, it SHOULD include a Include-Referer-Token-Binding-ID HTTP response header in its HTTP response. The ABNF of the Include-Referer-Token-Binding-ID header is:
Include-Referer-Token-Binding-ID = "Include-Referer-Token-Binding-ID" ":"
[CFWS] %x74.72.75.65 ; "true", case-sensitive
Including this response header signals to the client that it should reveal the Token Binding ID used between the client and the Token Consumer to the Token Provider. In the absence of this response header, the client will not disclose any information about the Token Binding used between the client and the Token Consumer to the Token Provider.
This header has only meaning if the HTTP status code is 301, 302, 303, 307 or 308, and MUST be ignored by the client for any other status codes. If the client supports the Token Binding Protocol, and has negotiated the Token Binding Protocol with both the Token Consumer and the Token Provider, it already sends the following header to the Token Provider with each HTTP request (see above):
Token-Binding: EncodedTokenBindingMessage
The TokenBindingMessage SHOULD contain a TokenBinding with TokenBindingType referred_token_binding. If included, this TokenBinding MUST be signed with the Token Binding key used by the client for connections between itself and the Token Consumer (more specifically, the web origin that issued the Include-Referer-Token-Binding-ID response header). The Token Binding ID established by this TokenBinding is called a Referred Token Binding ID.
As described above, the TokenBindingMessage MUST additionally contain a Provided Token Binding ID, i.e., a TokenBinding structure with TokenBindingType provided_token_binding, which MUST be signed with the Token Binding key used by the client for connections between itself and the Token Privider (more specifically, the web origin that the token request sent to).
The Token Binding Protocol [TBPROTO] allows the server and client to negotiate a signature algorithm used in the TokenBindingMessage. It is possible that the Token Binding ID used between the client and the Token Consumer, and the Token Binding ID used between the client and Token Provider, use different signature algorithms. The client MUST use the signature algorithm negotiated with the Token Consumer in the referred_token_binding TokenBinding of the TokenBindingMessage, even if that signature algorithm is different from the one negotiated with the origin that the header is sent to.
Token Providers SHOULD support all the SignatureAndHashAlgorithms specified in the Token Binding Protocol [TBPROTO]. If a token provider does not support the SignatureAndHashAlgorithm specified in the referred_token_binding TokenBinding in the TokenBindingMessage, it MUST issue an unbound token.
The goal of the Federated Token Binding mechanisms is to prevent attackers from exporting and replaying tokens used in protocols between the client and Token Consumer, thereby impersonating legitimate users and gaining access to protected resources. Bound tokens can still be replayed by malware present in the client. In order to export the token to another machine and successfully replay it, the attacker also needs to export the corresponding private key. The Token Binding private key is therefore a high-value asset and MUST be strongly protected, ideally by generating it in a hardware security module that prevents key export.
The Token Binding protocol uses persistent, long-lived TLS Token Binding IDs. To protect privacy, TLS Token Binding IDs are never transmitted in clear text and can be reset by the user at any time, e.g. when clearing browser cookies. Unique Token Binding IDs MUST be generated for connections to different origins, so they cannot be used by cooperating servers to link user identities.
The Token Binding protocol relies on the exported key material (EKM) value [RFC5705] to associate a TLS connection with a TLS Token Binding. The triple handshake attack [TRIPLE-HS] is a known TLS protocol vulnerability allowing the attacker to synchronize keying manterial between TLS connections. The attacker can then successfully replay bound tokens. For this reason, the Token Binding protocol MUST NOT be negotiated unless the Extended Master Secret TLS extension [I-D.ietf-tls-session-hash] has also been negotiated.
The purpose of the Token Binding protocol is to convince the server that the client that initiated the TLS connection controls a certain key pair. For the server to correctly draw this conclusion after processing the Token-Binding header, certain secrecy and integrity requirements must be met.
For example, the client's private Token Binding key must be kept secret by the client. If the private key is not secret, then another actor in the system could create a valid Token Binding header, impersonating the client. This can render the main purpose of the protocol - to bind bearer tokens to certain clients - moot: Consider, for example, an attacker who obtained (perhaps through a network intrusion) an authentication cookie that a client uses with a certain server. Consider further that the server bound that cookie to the client's Token Binding ID precisely to thwart cookie theft. If the attacker were to come into possession of the client's private key, he could then establish a TLS connection with the server and craft a Token-Binding header that matches the binding present in the cookie, thus successfully authenticating as the client, and gaining access to the client's data at the server. The Token Binding protocol, in this case, didn't successfully bind the cookie to the client.
Likewise, we need integrity protection of the Token-Binding header: A client shouldn't be tricked into sending a Token-Binding header to a server that contains Token Binding messages about key pairs that the client doesn't control. Consider an attacker A that somehow has knowledge of the exported keying material (EKM) for a TLS connection between a client C and a server S. (While that is somewhat unlikely, it's also not entirely out of the question, since the client might not treat the EKM as a secret - after all, a pre-image-resistant hash function has been applied to the TLS master secret, making it impossible for someone knowing the EKM to recover the TLS master secret. Such considerations might lead some clients to not treat the EKM as a secret.) Such an attacker A could craft a Token-Binding header with A's key pair over C's EKM. If the attacker could now trick C to send such a header to S, it would appear to S as if C controls a certain key pair when in fact it doesn't (the attacker A controls the key pair).
If A has a pre-existing relationship with S (perhaps has an account on S), it now appears to the server S as if A is connecting to it, even though it is really C. (If the server S doesn't simply use Token Binding keys to identify clients, but also uses bound authentication cookies, then A would also have to trick C into sending one of A's cookies to S, which it can do through a variety of means - inserting cookies through Javascript APIs, setting cookies through related-domain attacks, etc.) In other words, A tricked C into logging into A's account on S. This could lead to a loss of privacy for C, since A presumably has some other way to also access the account, and can thus indirectly observe A's behavior (for example, if S has a feature that lets account holders see their activity history on S).
Therefore, we need to protect the integrity of the Token-Binding header. One origin should not be able to set the Token-Binding header (through a DOM API or otherwise) that the User Agent uses with another origin.
As explained above, in a federated sign-in scenario a client will prove possession of two different key pairs to a Token Provider: One key pair is the "provided" Token Binding key pair (which the client normally uses with the Token Provider), and the other is the "referred" Token Binding key pair (which the client normally uses with the Token Consumer). The Token Provider is expected to issue a token that is bound to the referred Token Binding key.
Both proofs (that of the provided Token Binding key and that of the referred Token Binding key) are necessary. To show this, consider the following scenario:
The goal is to detect the presence of the man-in-the-middle in these scenarios.
First, consider a man-in-the-middle between the client and the Token Provider. Recall that we assume that the client possesses a bound authentication token (e.g., cookie) for the Token Provider. The man-in-the-middle can intercept and modify any message sent by the client to the Token Provider, and any message sent by the Token Provider to the client. (This means, among other things, that the man-in-the-middle controls the Javascript running at the client in the origin of the Token Provider.) It is not, however, in possession of the client's Token Binding key. Therefore, it can either choose to replace the Token Binding key in requests from the client to the Token Provider, and create a Token-Binding header that matches the TLS connection between the man-in-the-middle and the Token Provider; or it can choose to leave the Token-Binding header unchanged. If it chooses the latter, the signature in the Token Binding message (created by the original client on the exported keying material (EKM) for the connection between client and man-in-the-middle) will not match the EKM between man-in-the-middle and the Token Provider. If it chooses the former (and creates its own signature, with its own Token Binding key, over the EKM for the connection between man-in-the-middle and Token Provider), then the Token Binding message will match the connection between man-in-the-middle and Token Provider, but the Token Binding key in the message will not match the Token Binding key that the client's authentication token is bound to. Either way, the man-in-the-middle is detected by the Token Provider, but only if the proof of key possession of the provided Token Binding key is required in the protocol (as we do above).
Next, consider the presence of a man-in-the-middle between client and Token Consumer. That man-in-the-middle can intercept and modify any message sent by the client to the Token Consumer, and any message sent by the Token Consumer to the client. The Token Consumer is the party that redirects the client to the Token Provider. In this case, the man-in-the-middle controls the redirect URL, and can tamper with any redirect URL issued by the Token Consumer (as well as with any Javascript running in the origin of the Token Consumer). The goal of the man-in-the-middle is to trick the Token Issuer to issue a token bound to its Token Binding key, not to the Token Binding key of the legitimate client. To thwart this goal of the man-in-the-middle, the client's referred Token Binding key must be communicated to the Token Producer in a manner that can not be affected by the man-in-the-middle (who, as we recall, can modify redirect URLs and Javascript at the client). Including the referred Token Binding message in the Token-Binding header (as opposed to, say, including the referred Token Binding key in an application-level message as part of the redirect URL) is one way to assure that the man-in-the-middle between client and Token Consumer cannot affect the communication of the referred Token Binding key to the Token Provider.
Therefore, the Token-Binding header in the federated sign-on use case contains both, a proof of possession of the provided Token Binding key, as well as a proof of possession of the referred Token Binding key.
| [I-D.ietf-httpbis-http2] | Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol version 2", Internet-Draft draft-ietf-httpbis-http2-17, February 2015. |
| [I-D.ietf-tls-session-hash] | Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley, A. and M. Ray, "Transport Layer Security (TLS) Session Hash and Extended Master Secret Extension", Internet-Draft draft-ietf-tls-session-hash-06, July 2015. |
| [TRIPLE-HS] | Bhargavan, K., Delignat-Lavaud, A., Fournet, C., Pironti, A. and P. Strub, "Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over TLS. IEEE Symposium on Security and Privacy", 2014. |