Aspirational                                                 B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Informational                             23 March 2021
Expires: 24 September 2021

 Client-Cert HTTP Header: Conveying Client Certificate Information from
     TLS Terminating Reverse Proxies to Origin Server Applications


   This document defines the HTTP header field "Client-Cert" that allows
   a TLS terminating reverse proxy to convey the client certificate of a
   mutually-authenticated TLS connection to the origin server in a
   common and predictable manner.

Status of This Memo

   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
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   This Internet-Draft will expire on 24 September 2021.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   provided without warranty as described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  HTTP Header Field and Processing Rules  . . . . . . . . . . .   4
     2.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Client-Cert HTTP Header Field . . . . . . . . . . . . . .   4
     2.3.  Processing Rules  . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   6.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Considerations Considered  . . . . . . . . . . . . .  10
     B.1.  Header Injection  . . . . . . . . . . . . . . . . . . . .  10
     B.2.  The Forwarded HTTP Extension  . . . . . . . . . . . . . .  10
     B.3.  The Whole Certificate and Only the Whole Certificate  . .  11
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .  12
   Appendix D.  Document History . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   A fairly common deployment pattern for HTTPS applications is to have
   the origin 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 origin
   server within a private or protected network.  The origin servers are
   not directly accessible by clients and are only reachable through the
   reverse proxy.  The backend details of this type of deployment 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
   origin server, the TLS connection that the client establishes for
   HTTPS is only between itself and the reverse proxy server.

   The deployment pattern is found in a number of varieties such as
   n-tier architectures, content delivery networks, application load
   balancing services, and ingress controllers.

   Although not exceedingly prevalent, TLS client certificate
   authentication is sometimes employed and in such cases the origin
   server often requires information about the client certificate for
   its application logic.  Such logic might include access control
   decisions, audit logging, and binding issued tokens or cookies to a
   certificate, and the respective validation of such bindings.  The
   specific details from the certificate needed also vary with the

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   application requirements.  In order for these types of application
   deployments to work in practice, the reverse proxy needs to convey
   information about the client certificate to the origin application
   server.  A common way this information is conveyed in practice today
   is by using non-standard headers to carry the certificate (in some
   encoding) or individual parts thereof in the HTTP request that is
   dispatched to the origin server.  This solution works but
   interoperability between independently developed components can be
   cumbersome or even impossible depending on the implementation choices
   respectively made (like what header names are used or are
   configurable, which parts of the certificate are exposed, or how the
   certificate is encoded).  A well-known predictable approach to this
   commonly occurring functionality could improve and simplify
   interoperability between independent implementations.

   This document aspires to standardize an HTTP header field named
   "Client-Cert" that a TLS terminating reverse proxy (TTRP) adds to
   requests that it sends to the backend origin servers.  The header
   value contains the client certificate from the mutually-authenticated
   TLS connection between the originating client and the TTRP.  This
   enables the backend origin server to utilize the client certificate
   information in its application logic.  While there may be additional
   proxies or hops between the TTRP and the origin server (potentially
   even with mutually-authenticated TLS connections between them), the
   scope of the "Client-Cert" header is intentionally limited to
   exposing to the origin server the certificate that was presented by
   the originating client in its connection to the TTRP.

1.1.  Requirements Notation and Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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.

1.2.  Terminology

   Phrases like TLS client certificate authentication or mutually-
   authenticated TLS are used throughout this document to refer to the
   process whereby, in addition to the normal TLS server authentication
   with a certificate, a client presents its X.509 certificate [RFC5280]
   and proves possession of the corresponding private key to a server
   when negotiating a TLS connection or the resumption of such a
   connection.  In contemporary versions of TLS [RFC8446] [RFC5246] this
   requires that the client send the Certificate and CertificateVerify
   messages during the handshake and for the server to verify the
   CertificateVerify and Finished messages.

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   [[ HTTP2 forbids TLS renegotiation and post-handshake authentication
   but it's possible with HTTP1.1 and maybe needs to be discussed
   explicitly here or somewhere in this document?  Naively I'd say that
   the "Client-Cert" header will be sent with the data of the most
   recent client cert anytime after renegotiation or post-handshake
   auth.  And only for requests that are fully covered by the cert but
   that in practice making the determination of where exactly in the
   application data the cert messages arrived is hard to impossible so
   it'll be a best effort kind of thing. ]]

2.  HTTP Header Field and Processing Rules

2.1.  Encoding

   The field-values of the HTTP header defined herein utilize the
   following encoded form.

   A certificate is represented in text as an "EncodedCertificate",
   which is the base64-encoded (Section 4 of [RFC4648]) DER [ITU.X690]
   PKIX certificate.  The encoded value MUST NOT include any line
   breaks, whitespace, or other additional characters.  ABNF [RFC5234]
   syntax for "EncodedCertificate" is shown in the figure below.

    EncodedCertificate = 1*( DIGIT / ALPHA / "+" / "/" ) 0*2"="

    DIGIT = <Defined in Section B.1 of [RFC5234]>  ; A-Z / a-z
    ALPHA = <Defined in Section B.1 of [RFC5234]>  ; 0-9

2.2.  Client-Cert HTTP Header Field

   In the context of a TLS terminating reverse proxy (TTRP) deployment,
   the TTRP makes the TLS client certificate available to the backend
   application with the following header field.

   Client-Cert  The end-entity client certificate as an
      "EncodedCertificate" value.

   The "Client-Cert" header field defined herein is only for use in HTTP
   requests and MUST NOT be used in HTTP responses.  It is a single HTTP
   header field-value as defined in Section 3.2 of [RFC7230], which MUST
   NOT have a list of values or occur multiple times in a request.

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2.3.  Processing Rules

   This section outlines the applicable processing rules for a TLS
   terminating reverse proxy (TTRP) that has negotiated a mutually-
   authenticated TLS connection to convey the client certificate from
   that connection to the backend origin servers.  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

   A TTRP negotiates the use of a mutually-authenticated TLS connection
   with the client, such as is described in [RFC8446] or [RFC5246], and
   validates the client certificate per its policy and trusted
   certificate authorities.  Each HTTP request on the underlying TLS
   connection are dispatched to the origin server with the following

   1.  The client certificate is be placed in the "Client-Cert" header
       field of the dispatched request as defined in Section 2.2.

   2.  Any occurrence of the "Client-Cert" header in the original
       incoming request MUST be removed or overwritten before forwarding
       the request.  An incoming request that has a "Client-Cert" header
       MAY be rejected with an HTTP 400 response.

   Requests made over a TLS connection where the use of client
   certificate authentication was not negotiated MUST be sanitized by
   removing any and all occurrences "Client-Cert" header field prior to
   dispatching the request to the backend server.

   Backend origin servers may then use the "Client-Cert" header of the
   request to determine if the connection from the client to the TTRP
   was mutually-authenticated and, if so, the certificate thereby
   presented by the client.

   Forward proxies and other intermediaries MUST NOT add the "Client-
   Cert" header to requests, or modify an existing "Client-Cert" header.
   Similarly, clients MUST NOT employ the "Client-Cert" header in

   A server that receives a request with a "Client-Cert" header value
   that it considers to be too large can respond with an HTTP 431 status
   code per Section 5 of [RFC6585].

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3.  Security Considerations

   The header described herein enable a TTRP and backend or origin
   server to function together as though, from the client's perspective,
   they are a single logical server side deployment of HTTPS over a
   mutually-authenticated TLS connection.  Use of the "Client-Cert"
   header outside that intended use case, however, may undermine the
   protections afforded by TLS client certificate authentication.
   Therefore steps MUST be taken to prevent unintended use, both in
   sending the header and in relying on its value.

   Producing and consuming the "Client-Cert" header SHOULD be a
   configurable option, respectively, in a TTRP and backend server (or
   individual application in that server).  The default configuration
   for both should be to not use the "Client-Cert" header thus requiring
   an "opt-in" to the functionality.

   In order to prevent header injection, backend servers MUST only
   accept the "Client-Cert" header from a trusted TTRP (or other proxy
   in a trusted path from the TTRP).  A TTRP MUST sanitize the incoming
   request before forwarding it on by removing or overwriting any
   existing instances of the header.  Otherwise arbitrary clients can
   control the header value as seen and used by the backend server.  It
   is important to note that neglecting to prevent header injection does
   not "fail safe" in that the nominal functionality will still work as
   expected even when malicious actions are possible.  As such, extra
   care is recommended in ensuring that proper header sanitation is in

   The communication between a TTRP 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 TTRP and backend or origin
   server, for example, might be authenticated in some way with the
   insertion and consumption of the "Client-Cert" header 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 TTRP and the proxy can only make requests to
   that server.  Other deployments that meet the requirements set forth
   herein are also possible.

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4.  IANA Considerations

   [[ TBD if this draft progresses, register the "Client-Cert" HTTP
   header field in the "Permanent Message Header Field Names" registry
   headers.xhtml) defined in [RFC3864] ]]

5.  Normative References

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

   [ITU.X690] International Telecommunications Union, "Information
              Technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", August 2015.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

6.  Informative References

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,

   [RFC6585]  Nottingham, M. and R. Fielding, "Additional HTTP Status
              Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,

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   [RFC8705]  Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", RFC 8705,
              DOI 10.17487/RFC8705, February 2020,

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

              Nottingham, M. and P. Kamp, "Structured Field Values for
              HTTP", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-header-structure-19, 3 June 2020,

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

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Appendix A.  Example

   In a hypothetical example where a TLS client presents the client and
   intermediate certificate from Figure 1 when establishing a mutually-
   authenticated TLS connection with the TTRP, the proxy would send the
   "Client-Cert" header shown in {#example-header} to the backend.  Note
   that line breaks and whitespace have been added to the value of the
   header field in Figure 2 for display and formatting purposes only.

   -----END CERTIFICATE-----
   -----END CERTIFICATE-----
   -----END CERTIFICATE-----

        Figure 1: Certificate Chain (with client certificate first)

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             Figure 2: Header in HTTP Request to Origin Server

Appendix B.  Considerations Considered

B.1.  Header Injection

   This draft requires that the TTRP sanitize the headers of the
   incoming request by removing or overwriting any existing instances of
   the "Client-Cert" header before dispatching that request to the
   backend application.  Otherwise, a client could inject its own
   "Client-Cert" header that would appear to the backend to have come
   from the TTRP.  Although numerous other methods of detecting/
   preventing header injection are possible; such as the use of a unique
   secret value as part of the header name or value or the application
   of a signature, HMAC, or AEAD, there is no common general
   standardized mechanism.  The potential problem of client header
   injection is not at all unique to the functionality of this draft and
   it would therefor be inappropriate for this draft to define a one-off
   solution.  In the absence of a generic standardized solution existing
   currently, stripping/sanitizing the headers is the de facto means of
   protecting against header injection in practice today.  Sanitizing
   the headers is sufficient when properly implemented and is normative
   requirement of Section 3.

B.2.  The Forwarded HTTP Extension

   The "Forwarded" HTTP header field defined in [RFC7239] allows proxy
   components to disclose information lost in the proxying process.  The
   TLS client certificate information of concern to this draft could
   have been communicated with an extension parameter to the "Forwarded"
   header field, however, doing so would have had some disadvantages
   that this draft endeavored to avoid.  The "Forwarded" header syntax
   allows for information about a full chain of proxied HTTP requests,
   whereas the "Client-Cert" header of this document is concerned only
   with conveying information about the certificate presented by the
   originating client on the TLS connection to the TTRP (which appears
   as the server from that client's perspective) to backend
   applications.  The multi-hop syntax of the "Forwarded" header is

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   expressive but also more complicated, which would make processing it
   more cumbersome, and more importantly, make properly sanitizing its
   content as required by Section 3 to prevent header injection
   considerably more difficult and error prone.  Thus, this draft opted
   for the flatter and more straightforward structure of a single
   "Client-Cert" header.

B.3.  The Whole Certificate and Only the Whole Certificate

   Different applications will have varying requirements about what
   information from the client certificate is needed, such as the
   subject and/or issuer distinguished name, subject alternative
   name(s), serial number, subject public key info, fingerprint, etc..
   Furthermore some applications, such as "OAuth 2.0 Mutual-TLS Client
   Authentication and Certificate-Bound Access Tokens" [RFC8705], make
   use of the entire certificate.  In order to accommodate the latter
   and ensure wide applicability by not trying to cherry-pick particular
   certificate information, this draft opted to pass the full encoded
   certificate as the value of the "Client-Cert" header.

   The handshake and validation of the client certificate (chain) of the
   mutually-authenticated TLS connection is performed by the TTRP.  With
   the responsibility of certificate validation falling on the TTRP,
   only the end-entity certificate is passed to the backend - the root
   Certificate Authority is not included nor are any intermediates.

   [[ It has been suggested that more information about the certificate
   chain might be needed/wanted by the backend application (to
   independently evaluate the cert chain, for example, although that
   seems like it would be terribly inefficient) and that any
   intermediates as well as the root should also be somehow conveyed,
   which is an area for further discussion should this draft progress.
   One potential approach suggested by a few folks is to allow some
   configurability in what is sent along with maybe a prefix token to
   indicate what's being sent - something like "Client-Cert: FULL <cert>
   <intermediate> <anchor>" or "Client-Cert: EE <cert>" as the strawman.
   Or a perhaps a parameter or other construct of
   [I-D.ietf-httpbis-header-structure] to indicate what's being sent.
   It's also been suggested that the end-entity certificate by itself
   might sometimes be too big (esp. e.g., with some post-quantum
   signature schemes).  Hard to account for it both being too much data
   and not enough data at the same time.  But potentially opening up
   configuration options to send only specific attribute(s) from the
   client certificate is a possibility for that.  In the author's humble
   opinion the end-entity certificate by itself strikes a good balance
   for the vast majority of needs and avoids optionality.  But, again,
   this is an area for further discussion should this draft progress. ]]

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   [[ It has also been suggested that maybe considerations for [RFC7250]
   Raw Public Keys is maybe worth considering.  This too is this is an
   area for further discussion and consideration should this draft
   progress. ]]

Appendix C.  Acknowledgements

   The author would like to thank the following individuals who've
   contributed in various ways ranging from just being generally
   supportive of bringing forth the draft to providing specific feedback
   or content: Evan Anderson, Annabelle Backman, Mike Bishop, Rory
   Hewitt, Fredrik Jeansson, Benjamin Kaduk, Torsten Lodderstedt,
   Kathleen Moriarty, Mark Nottingham, Mike Ounsworth, Matt Peterson,
   Eric Rescorla, Justin Richer, Michael Richardson, Joe Salowey, Rich
   Salz, Mohit Sethi, Rifaat Shekh-Yusef, Travis Spencer, Nick Sullivan,
   Peter Wu, and Hans Zandbelt.

   [[ Please let me know if you've been erroneously omitted or if you
   prefer not to be named ]]

Appendix D.  Document History

   [[ To be removed by the RFC Editor before publication as an RFC
   (should that come to pass) ]]


   *  Change intended status of the draft to Informational

   *  Editorial updates and (hopefully) clarifications


   *  Update reference from draft-ietf-oauth-mtls to RFC8705


   *  Expanded [[further discussion notes]] to capture some of the
      feedback in and around the presentation of the draft in
      SECDISPATCH at IETF 107 and add those who've provided such
      feedback to the acknowledgements


   *  Editorial tweaks + [[further discussion notes]]


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   *  Use the RFC v3 Format or die trying


   *  Initial draft after a time constrained and rushed secdispatch
      presentation (
      backend-http-servers-00) at IETF 106 in Singapore with the
      recommendation to write up a draft (at the end of the minutes
      106-secdispatch)) and some folks expressing interest despite the
      rather poor presentation

Author's Address

   Brian Campbell
   Ping Identity


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