One document matched: draft-ietf-tokbind-https-02.xml
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<rfc category="std" docName="draft-ietf-tokbind-https-02" ipr="trust200902">
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<!-- ***** FRONT MATTER ***** -->
<front>
<!-- The abbreviated title is used in the page header - it is only necessary if the
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<title>Token Binding over HTTP</title>
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<author fullname="Andrei Popov" initials="A."
surname="Popov">
<organization>Microsoft Corp.</organization>
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<email>andreipo@microsoft.com</email>
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</author>
<author fullname="Magnus Nyström" initials="M."
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<organization>Microsoft Corp.</organization>
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<email>mnystrom@microsoft.com</email>
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<author fullname="Dirk Balfanz" initials="D."
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<email>balfanz@google.com</email>
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<author fullname="Adam Langley" initials="A."
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<organization>Google Inc.</organization>
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<date year="2015" />
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<abstract>
<t>This document describes a collection of mechanisms that allow
HTTP servers to cryptographically bind authentication tokens
(such as cookies and OAuth tokens) to a <xref
target="RFC5246">TLS</xref> connection.</t>
<t>We describe both <spanx style="emph">first-party</spanx> as
well as <spanx style="emph">federated</spanx> 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.</t>
<t>Federated token bindings, on the other hand, allow servers to
cryptographically bind security tokens to a <xref
target="RFC5246">TLS</xref> connection that the client has with
a <spanx style="emph">different</spanx> server than the one
issuing the token.</t>
<t>This Internet-Draft is a companion document to <xref
target="TBPROTO">The Token Binding Protocol</xref></t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t><xref target="TBPROTO">The Token Binding Protocol</xref>
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.</t>
<t>While the <xref target="TBPROTO">Token Binding
Protocol</xref> 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 <xref target="RFC2616">1.1</xref> and
<xref target="I-D.ietf-httpbis-http2">2</xref>). 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).</t>
<t>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.</t>
<t>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 <spanx style="emph">provided</spanx>
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 <spanx
style="emph">referred</spanx> Token Binding ID in the
TokenBindingMessage, indicating a Token Binding ID that the
client is using with a <spanx style="emph">different</spanx>
server than the one that the TokenBindingMessage is sent
to. This is useful in federation scenarios.</t>
<section title="Requirements Language">
<t>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 <xref target="RFC2119" />.</t>
</section>
</section>
<section title="The Token-Binding Header">
<t>Once a client and server have negotiated the Token Binding
Protocol with HTTP/1.1 or HTTP/2 (see <xref
target="TBPROTO">The Token Binding Protocol</xref>), clients
MUST include the Token-Binding header in their HTTP requests. The ABNF
of the Token-Binding header is:
</t>
<figure><artwork><![CDATA[
Token-Binding = "Token-Binding" ":" [CFWS] EncodedTokenBindingMessage
]]></artwork></figure>
<t>The EncodedTokenBindingMessage is a web-safe Base64-encoding
of the TokenBindingMessage as defined in the <xref
target="TBPROTO">TokenBindingProtocol</xref>.</t>
<t>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 <spanx style="emph">Provided Token Binding
ID</spanx></t>
<t>In HTTP/2, the client SHOULD use <xref
target="I-D.ietf-httpbis-header-compression">Header
Compression</xref> to avoid the overhead of repeating the same
header in subsequent HTTP requests.</t>
</section>
<section title="Federation Use Cases">
<section title="Introduction">
<t>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.</t>
<t>In this section we describe mechanisms to achieve this. The
common idea among these mechanisms is that a server (called
the <spanx style="emph">Token Consumer</spanx> 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 <spanx style="emph">Token
Provider</spanx> in this document). Also common across the
mechanisms is how the Token Binding ID is revealed to the
Token Provider: The client uses the <xref
target="TBPROTO">Token Binding Protocol</xref>, and
includes a TokenBinding structure in the Token-Binding HTTP
header defined above. What differs between the various
mechanisms is <spanx style="emph">how</spanx> 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.</t>
</section>
<section title="Overview">
<t>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).</t>
<t>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.</t>
<t>This is an example of a federation scenario, which more
generally can be described as follows:
<list style="symbols">
<t>A Token Consumer causes the client to issue a token request
to the Token Provider. The goal is for the client to obtain a
token and then use it with the Token Consumer.</t>
<t>The client delivers the token request to the Token
Provider.</t>
<t>The Token Provider issues the token. The token is issued
for the specific Token Consumer who requested it (thus
preventing malicious Token Consumers from using tokens with
other Token Consumers). The token is, however, typically a
bearer token, meaning that any client can use it with the
Token Consumer, not just the client to which it was
issued.</t>
<t>Therefore, in the previous step, the Token Provider may
want to include in the token the Token-Binding public key that the client
uses when communicating with the Token Consumer, thus
<spanx style="emph">binding</spanx> the token to client's Token-Binding
keypair. The client
proves possession of the private key when communicating with
the Token Consumer through the <xref target="TBPROTO">Token
Binding Protocol</xref>, and reveals the corresponding public key of this
keypair as part of the Token Binding ID. Comparing the public key from the
token with the public key from the Token Binding ID allows the Token
Consumer to verify that the token was sent to it by the legitimate
client.</t>
<t>To allow the Token Provider to include the Token-Binding public key in
the token, the Token Binding ID (between client and Token Consumer) must
therefore be communicated to the Token Provider along with the token
request. Communicating a Token Binding ID involves proving possession of a
private key and is described in the <xref target="TBPROTO">Token
Binding Protocol</xref>.</t>
</list>
</t>
<t>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.</t>
<t>Below, we specify how Token Consumers can grant this permission. during
redirect-based federation protocols.</t>
</section>
<section title="HTTP Redirects">
<t>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:</t>
<figure><artwork><![CDATA[
Include-Referer-Token-Binding-ID = "Include-Referer-Token-Binding-ID" ":"
[CFWS] %x74.72.75.65 ; "true", case-sensitive
]]></artwork></figure>
<t>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.</t>
<t>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):</t>
<figure><artwork><![CDATA[
Token-Binding: EncodedTokenBindingMessage
]]></artwork></figure>
<t>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 <spanx style="emph">Referred Token
Binding ID</spanx>.</t>
<t>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).
</t>
</section>
<section title="Negotiated Key Parameters">
<t>The <xref target="TBPROTO">Token Binding Protocol</xref>
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.</t>
<t>Token Providers SHOULD support all the
SignatureAndHashAlgorithms specified in the <xref
target="TBPROTO">Token Binding Protocol</xref>. 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.</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<section title="Security Token Replay">
<t>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.</t>
</section>
<section title="Privacy Considerations">
<t>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.</t>
</section>
<section title="Triple Handshake Vulnerability in TLS">
<t>The Token Binding protocol relies on the exported key material (EKM)
value <xref target="RFC5705" /> to associate a TLS connection with a
TLS Token Binding. The triple handshake attack <xref
target="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 <xref
target="I-D.ietf-tls-session-hash"/> has also been negotiated.</t>
</section>
<section title="Sensitivity of the Token-Binding Header">
<t>
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.
</t>
<t>
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.
</t>
<t>
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).
</t>
<t>
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).
</t>
<t>
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.
</t>
</section>
<section title="Securing Federated Sign-On Protocols">
<t>
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.
</t>
<t>
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:
<list style="symbols">
<t>
The client has an authentication token with the Token Provider
that is bound to the client's Token Binding key.
</t>
<t>
The client wants to establish a secure (i.e., free of
men-in-the-middle) authenticated session with the Token Consumer,
but hasn't done so yet (in other words, we're about to run the
federated sign-on protocol).
</t>
<t>
A man-in-the-middle is allowed to intercept the connection
between client and Token Consumer or between Client and Token
Provider (or both).
</t>
</list>
The goal is to detect the presence of the man-in-the-middle in these
scenarios.
</t>
<t>
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).
</t>
<t>
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 <spanx style="emph">its</spanx> 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.
</t>
<t>
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.
</t>
</section>
</section>
</middle>
<!-- *****BACK MATTER ***** -->
<back>
<!-- References split into informative and normative -->
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<references title="Normative References">
&RFC2119;
&RFC5246;
&RFC7301;
&RFC2616;
&RFC5705;
&RFC5929;
&RFC4492;
&RFC5226;
<reference anchor="TBPROTO">
<front>
<title>The Token Binding Protocol Version 1.0</title>
<author initials="A." surname="Popov">
<organization>Microsoft</organization>
</author>
<date year="2014" />
</front>
</reference>
<?rfc include="reference.I-D.ietf-httpbis-header-compression.xml"?>
</references>
<references title="Informative References">
<?rfc include="reference.I-D.ietf-httpbis-http2.xml"?>
<?rfc include="reference.I-D.ietf-tls-session-hash.xml"?>
<reference anchor="TRIPLE-HS">
<front>
<title>Triple Handshakes and Cookie Cutters: Breaking and Fixing Authentication over
TLS. IEEE Symposium on Security and Privacy</title>
<author initials="K." surname="Bhargavan">
<organization>Inria Paris-Rocquencourt</organization>
</author>
<author initials="A." surname="Delignat-Lavaud">
<organization>Inria Paris-Rocquencourt</organization>
</author>
<author initials="C." surname="Fournet">
<organization>Inria Paris-Rocquencourt</organization>
</author>
<author initials="A." surname="Pironti">
<organization>Inria Paris-Rocquencourt</organization>
</author>
<author initials="P." surname="Strub">
<organization>Inria Paris-Rocquencourt</organization>
</author>
<date year="2014" />
</front>
</reference>
</references>
<!-- Change Log
v00 2014-08-21 Andrei Popov Initial version
v00 2015-03-27 Andrei Popov Renamed as tokbind WG draft
v01 2015-06-30 Dirk Balfanz Added Sec- prefix to header
v02 2015-06-30 Dirk Balfanz Removed Sec- prefix, added design rationalization in Security Considerations section
-->
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 05:22:02 |