| Internet Engineering Task Force | B. Campbell |
| Internet-Draft | Ping Identity |
| Intended status: Standards Track | July 5, 2019 |
| Expires: January 6, 2020 |
HTTPS Token Binding with TLS Terminating Reverse Proxies
draft-ietf-tokbind-ttrp-09
This document defines HTTP header fields that enable a TLS terminating reverse proxy to convey information to a backend server about the validated Token Binding Message received from a client, which enables that backend server to bind, or verify the binding of, cookies and other security tokens to the client's Token Binding key. This facilitates the reverse proxy and backend server functioning together as though they are a single logical server side deployment of HTTPS Token Binding.
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Token Binding over HTTP provides a mechanism that enables HTTP servers to cryptographically bind cookies and other security tokens to a key generated by the client. When the use of Token Binding is negotiated in the TLS handshake [RFC8472] the client sends an encoded Token Binding Message [RFC8471] as a header in each HTTP request, which proves possession of one or more private keys held by the client. The public portion of the keys are represented in the Token Binding IDs of the Token Binding Message and for each one there is a signature over some data, which includes the exported keying material [RFC5705] of the TLS connection. An HTTP server issuing cookies or other security tokens can associate them with the Token Binding ID, which ensures those tokens cannot be used successfully over a different TLS connection or by a different client than the one to which they were issued.
A fairly common deployment architecture for HTTPS applications is to have the backend HTTP application servers sit behind a reverse proxy that terminates TLS connections from clients. The proxy is accessible to the internet and dispatches client requests to the appropriate backend server within a private or protected network. The backend servers are not directly accessible by clients and are only reachable through the reverse proxy. The details of such deployments are typically opaque to clients who make requests to the proxy server and see responses as though they originated from the proxy server itself. Although HTTPS is also usually employed between the proxy and the backend server, the TLS connection that the client establishes for HTTPS is between itself and the reverse proxy server.
Token Binding facilitates a binding of security tokens to a key held by the client by way of the TLS connection between that client and the server. In a deployment where TLS is terminated by a reverse proxy, however, the TLS connection is between the client and the proxy while the backend server is likely the system that will issue and validate cookies or other security tokens. Additional steps are therefore needed to enable the use of Token Binding in such deployment architectures. In the absence of a standardized approach, different implementations will address it differently, which will make interoperability between such implementations difficult or impossible without complex configurations or custom integrations.
This document standardizes HTTP header field names that a TLS terminating reverse proxy (TTRP) adds to requests that it sends to the backend servers. The headers contain information from the validated Token Binding Message sent by the client to the proxy, thus enabling the backend server to bind, or verify the binding of, cookies and other security tokens to the client's Token Binding key. The usage of the headers, both the TTRP adding the headers and the backend application server using the headers to bind cookies or other tokens, are to be configuration options of the respective systems as they will not always be applicable.
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.
The field-values of the HTTP headers defined herein utilize the following encoded forms.
A Token Binding ID is represented as an EncodedTokenBindingID, which is thea base64url encoding of the TokenBindingID byte sequence (see section 3 of [RFC8471]) using the URL and filename safe alphabet described in Section 5 of [RFC4648], with all trailing pad characters '=' omitted and without the inclusion of any line breaks, whitespace, or other additional characters. ABNF [RFC5234] syntax for EncodedTokenBindingID is shown in Figure 1 below.
EncodedTokenBindingID = *( DIGIT / ALPHA / "-" / "_" )
DIGIT = <Defined in Section B.1 of [RFC5234]>
ALPHA = <Defined in Section B.1 of [RFC5234]>
Figure 1: Encoded Token Binding ID ABNF
A Token Binding type value (a single byte) can be represented as an EncodedTokenBindingType, which is a case-insensitive hex encoding (Section 8 of [RFC4648]). The ABNF definition is shown in Figure 2 below.
EncodedTokenBindingType = 1*2HEXDIG
HEXDIG = <Defined in Section B.1 of [RFC5234]>
Figure 2: Encoded Token Binding Type ABNF
The Token Binding Protocol recommends that implementations make Token Binding IDs available to the application as opaque byte sequences, enabling those applications to use the Token Binding IDs when generating and verifying bound tokens. In the context of a TLS terminating reverse proxy (TTRP) deployment, the TTRP makes the Token Binding ID(s) available to the backend application with the following header fields.
Both Sec-Provided-Token-Binding-ID and Sec-Referred-Token-Binding-ID are single HTTP header field-valued as defined in Section 3.2 of [RFC7230], which MUST NOT have a list of values or occur multiple times in a request.
All header fields defined herein are only for use in HTTP requests and MUST NOT to be used in HTTP responses.
This section defines the applicable processing rules for a TLS terminating reverse proxy (TTRP) and backend server(s) to provide server side support of Token Binding over HTTP using the HTTP headers described in Section 2.2. Use of the technique is to be a configuration or deployment option and the processing rules described herein are for servers operating with that option enabled.
A TTRP negotiates the use of Token Binding with the client, such as is described in [RFC8472] and validates the Token Binding Message as defined in The Token Binding Protocol and Token Binding over HTTP for each HTTP request on the underlying TLS connection. Requests with a valid Token Binding Message (and meeting any other authorization or policy requirements of the TTRP) are dispatched to the backend server with the following modifications.
Requests made over a connection where the use of Token Binding was not negotiated MUST be sanitized by removing any occurrences of the Sec-Provided-Token-Binding-ID, Sec-Referred-Token-Binding-ID, and Sec-Other-Token-Binding-ID header fields prior to dispatching the request to the backend server.
Forward proxies and other intermediaries MUST NOT add the Sec-Provided-Token-Binding-ID Sec-Referred-Token-Binding-ID, or Sec-Other-Token-Binding-ID header to requests.
Extra line breaks and whitespace have been added to the following examples for display and formatting purposes only.
The following Sec-Token-Binding header is from an HTTP request made over a TLS connection between the client and the TTRP where the use of Token Binding has been negotiated. The base64url-encoded representation of the exported keying material for that connection is AYVUayPTP9RmELNpGjFl6Ykm2CUx7pUMxe35yb11dgU, which can be used to validate the Token Binding Message. The encoded Token Binding Message has the provided Token Binding that the client uses with the server.
Sec-Token-Binding: AIkAAgBBQKzyIrmcY_YCtHVoSHBut69vrGfFdy1_YKTZfFJv 6BjrZsKD9b9FRzSBxDs1twTqnAS71M1RBumuihhI9xqxXKkAQEtxe4jeUJU0WezxlQ XWVSBFeHxFMdXRBIH_LKOSAuSMOJ0XEw1Q8DE248qkOiRKzw3KdSNYukYEPmO21bQi 3YYAAA
Figure 3: Header in HTTP Request to TTRP
After validating the Token Binding Message, the TTRP removes the Sec-Token-Binding header and adds the following Sec-Provided-Token-Binding-ID header with the provided Token Binding ID to the request that is dispatched to the backend server.
Sec-Provided-Token-Binding-ID: AgBBQKzyIrmcY_YCtHVoSHBut69vrGfFdy1_ YKTZfFJv6BjrZsKD9b9FRzSBxDs1twTqnAS71M1RBumuihhI9xqxXKk
Figure 4: Header in HTTP Request to Backend Server
The following Sec-Token-Binding header is from an HTTP request made over a TLS connection between the client and the TTRP where the use of Token Binding has been negotiated. The base64url-encoded representation of the exported keying material for that connection is wEWWCP1KPxfq-QL4NxYII_P4ti_9YYqrTpGs28BZEqE, which can be used to validate the Token Binding Message. The encoded Token Binding Message has the provided Token Binding that the client uses with the server as well as the referred Token Binding that it uses with a different server.
Sec-Token-Binding: ARIAAgBBQCfsI1D1sTq5mvT_2H_dihNIvuHJCHGjHPJchPav NbGrOo26-2JgT_IsbvZd4daDFbirYBIwJ-TK1rh8FzrC-psAQMyYIqXj7djGPev1dk jV9XxLYGCyqOrBVEtBHrMUCeo22ymLg3OiFcl_fmOPxJbjxI6lKcF0lyfy-dSQmPIe zQ0AAAECAEFArPIiuZxj9gK0dWhIcG63r2-sZ8V3LX9gpNl8Um_oGOtmwoP1v0VHNI HEOzW3BOqcBLvUzVEG6a6KGEj3GrFcqQBAHQm0pzgUTXKLRamuKE1pmmP9I3UBVpoe 1DBCe9H2l1VPpsImakUa6crAqZ-0CGBmji7bYzQogpKcyxTTFk5zdwAA
Figure 5: Header in HTTP Request to TTRP
After validating the Token Binding Message, the TTRP removes the Sec-Token-Binding header and adds the following Sec-Provided-Token-Binding-ID and Sec-Referred-Token-Binding-ID headers, with the provided and referred Token Binding IDs respectively, to the request that is dispatched to the backend server.
Sec-Provided-Token-Binding-ID: AgBBQCfsI1D1sTq5mvT_2H_dihNIvuHJCHGj HPJchPavNbGrOo26-2JgT_IsbvZd4daDFbirYBIwJ-TK1rh8FzrC-ps Sec-Referred-Token-Binding-ID: AgBBQKzyIrmcY_YCtHVoSHBut69vrGfFdy1_ YKTZfFJv6BjrZsKD9b9FRzSBxDs1twTqnAS71M1RBumuihhI9xqxXKk
Figure 6: Headers in HTTP Request to Backend Server
The following Sec-Token-Binding header is from an HTTP request made over a TLS connection between the client and the TTRP where the use of Token Binding has been negotiated. The base64url-encoded representation of the exported keying material for that connection is Zr_1DESCcDoaltcZCK613UrEWHRf2B3w9i3bwcxpacc, which can be used to validate the Token Binding Message. The encoded Token Binding Message has the provided Token Binding and two other Token Bindings.
Sec-Token-Binding: AZsAAgBBQA35hcCjI5GEHLLAZ0i2l2ZvQe-bSPAP7jovkZJM 4wYHgmmXNd1aRpnQmXK9ghUmrdtS6p_e2uSlMXIVKOIwgysAQJ-TKyVGF37XUXMy79 ybwJyPpfCG9Iq6fdIxLX_yJn-L__Z3p_WIL3g17K0OH3XZmJS3qZNNEVu_8HmPN-d9 hGMAAE0CAEFAR68GbdIQyrHqkorJF0sekYJvf8iV03obGxbaWbqAEJetsYxprB6c3M x5KDHBGZjsFbeFW5Xec_EaxX0Hw3RmJwBA-Fu22kokRbB7G0D0g6_sdCHTbczSCmnm 6rqP1x7kRIIj_kJNCCWcwMMFzbsBTXcm5fJrRdBTcsqiiqYD6aJ1SgAACwIAQUCDqt 6m63By8b1lvhN-n9OsQThoLomzKpMicSZGwR166jplhbkjrFsHzdNqzLFFEhCT9s0p XrcbpOHsZnpRSkmhAEBfOwxjK3Y9EOeMrqjo0IUhmurW2EgtSRBjDwc0r-rDT231Zv _f1oePB8Pkd1kgAtgKX5EDiemfo1YER3_I2cv3AAA
Figure 7: Header in HTTP Request to TTRP
After validating the Token Binding Message, the TTRP removes the Sec-Token-Binding header and adds the following Sec-Provided-Token-Binding-ID and Sec-Other-Token-Binding-ID headers to the request that is dispatched to the backend server.
Sec-Provided-Token-Binding-ID: AgBBQA35hcCjI5GEHLLAZ0i2l2ZvQe-bSPAP 7jovkZJM4wYHgmmXNd1aRpnQmXK9ghUmrdtS6p_e2uSlMXIVKOIwgys Sec-Other-Token-Binding-ID: 4d.AgBBQEevBm3SEMqx6pKKyRdLHpGCb3_IldN6 GxsW2lm6gBCXrbGMaawenNzMeSgxwRmY7BW3hVuV3nPxGsV9B8N0Zic,B.AgBBQIO q3qbrcHLxvWW-E36f06xBOGguibMqkyJxJkbBHXrqOmWFuSOsWwfN02rMsUUSEJP2 zSletxuk4exmelFKSaE
Figure 8: Headers in HTTP Request to Backend Server
TLS 1.2 is cited in this document because, at the time of writing, it is the latest version that is widely deployed. However, this document is applicable with other TLS versions that allow for negotiating the use of Token Binding. Token Binding for Transport Layer Security (TLS) Version 1.3 Connections, for example, describes Token Binding with TLS 1.3. Implementation security considerations for TLS, including version recommendations, can be found in Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS).
The headers described herein enable a reverse proxy and backend server to function together as though they are a single logical server side deployment of HTTPS Token Binding. Use of the headers outside that intended use case, however, may undermine the protections afforded by Token Binding. Therefore steps MUST be taken to prevent unintended use, both in sending the headers and in relying on their value.
Producing and consuming the headers SHOULD be a configurable option, respectively, in a reverse proxy and backend server (or individual application in that server). The default configuration for both should be to not use the headers thus requiring an "opt-in" to the functionality.
Backend servers MUST only accept the headers from trusted reverse proxies. And reverse proxies MUST sanitize the incoming request before forwarding it on by removing or overwriting any existing instances of the headers. Otherwise arbitrary clients can control the header values as seen and used by the backend server.
The communication between a reverse proxy and backend server needs to be secured against eavesdropping and modification by unintended parties.
The configuration options and request sanitization are necessarily functionally of the respective servers. The other requirements can be met in a number of ways, which will vary based on specific deployments. The communication between a reverse proxy and backend server, for example, might be over a mutually authenticated TLS with the insertion and consumption headers occurring only on that connection. Alternatively the network topology might dictate a private network such that the backend application is only able to accept requests from the reverse proxy and the proxy can only make requests to that server. Other deployments that meet the requirements set forth herein are also possible.
Employing the Sec- header field prefix for the headers defined herein denotes them as forbidden header names (see [fetch-spec]), which means they cannot be set or modified programmatically by script running in-browser.
This document specifies the following new HTTP header fields, registration of which is requested in the "Permanent Message Header Field Names" registry defined in [RFC3864].
| [fetch-spec] | WhatWG, "Fetch", Living Standard |
| [I-D.ietf-tokbind-tls13] | Harper, N., "Token Binding for Transport Layer Security (TLS) Version 1.3 Connections", Internet-Draft draft-ietf-tokbind-tls13-01, May 2018. |
| [RFC3864] | Klyne, G., Nottingham, M. and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004. |
| [RFC8446] | Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018. |
The author would like to thank the following people for their various contributions to the specification: Vinod Anupam, Dirk Balfanz, John Bradley, William Denniss, Nick Harper, Jeff Hodges, Subodh Iyengar, Leif Johansson, Michael B. Jones, Yoav Nir, James Manger, Andrei Popov, Eric Rescorla, Piotr Sikora, Martin Thomson, and Hans Zandbelt
[[ to be removed by the RFC Editor before publication as an RFC ]]
draft-ietf-tokbind-ttrp-09
draft-ietf-tokbind-ttrp-08
draft-ietf-tokbind-ttrp-07
draft-ietf-tokbind-ttrp-06
draft-ietf-tokbind-ttrp-05
draft-ietf-tokbind-ttrp-04
draft-ietf-tokbind-ttrp-03
draft-ietf-tokbind-ttrp-02
draft-ietf-tokbind-ttrp-01
draft-ietf-tokbind-ttrp-00
draft-campbell-tokbind-ttrp-01
draft-campbell-tokbind-ttrp-00