1. RFC 9440
Internet Engineering Task Force (IETF)                       B. Campbell
Request for Comments: 9440                                 Ping Identity
Category: Informational                                   M. Bishop, Ed.
ISSN: 2070-1721                                                   Akamai
                                                               July 2023

                     Client-Cert HTTP Header Field


   This document describes HTTP extension header fields that allow a TLS
   terminating reverse proxy (TTRP) to convey the client certificate
   information of a mutually authenticated TLS connection to the origin
   server in a common and predictable manner.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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

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Table of Contents

   1.  Introduction
     1.1.  Requirements Notation and Conventions
     1.2.  Terminology and Applicability
   2.  HTTP Header Fields and Processing Rules
     2.1.  Encoding
     2.2.  Client-Cert HTTP Header Field
     2.3.  Client-Cert-Chain HTTP Header Field
     2.4.  Processing Rules
   3.  Deployment Considerations
     3.1.  Header Field Compression
     3.2.  Message Header Size
     3.3.  TLS Session Resumption
   4.  Security Considerations
   5.  IANA Considerations
     5.1.  HTTP Field Name Registrations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Example
   Appendix B.  Select Design Considerations
     B.1.  Field Injection
     B.2.  The Forwarded HTTP Extension
     B.3.  The Whole Certificate and Certificate Chain
   Authors' Addresses

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, including the respective validation of such bindings.
   The specific details needed from the certificate also vary with the
   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.  At the time of writing, a common way this information is
   conveyed is by using non-standard fields 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 field 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.

   The scope of this document is to describe existing practice while
   codifying specific details sufficient to facilitate improved and
   lower-touch interoperability.  As such, this document describes two
   HTTP header fields, "Client-Cert" and "Client-Cert-Chain", which a
   TLS terminating reverse proxy (TTRP) adds to requests sent to the
   backend origin servers.  The Client-Cert field value contains the
   end-entity client certificate from the mutually authenticated TLS
   connection between the originating client and the TTRP.  Optionally,
   the Client-Cert-Chain field value contains the certificate chain used
   for validation of the end-entity certificate.  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 field 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 and Applicability

   This document uses the following terminology from Section 3 of
   [STRUCTURED-FIELDS] to specify syntax and parsing: List and Byte

   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 [TLS] [TLS1.2], mutual
   authentication requires the client to send the Certificate and
   CertificateVerify messages during the handshake and the server to
   verify the CertificateVerify and Finished messages.

   HTTP/2 restricts TLS 1.2 renegotiation (Section 9.2.1 of [HTTP/2])
   and prohibits TLS 1.3 post-handshake authentication (Section 9.2.3 of
   [HTTP/2]).  However, they are sometimes used to implement reactive
   client certificate authentication in HTTP/1.1 [HTTP/1.1] where the
   server decides whether to request a client certificate based on the
   HTTP request.  HTTP application data sent on such a connection after
   receipt and verification of the client certificate is also mutually
   authenticated and thus suitable for the mechanisms described in this
   document.  With post-handshake authentication, there is also the
   possibility, though unlikely in practice, of multiple certificates
   and certificate chains from the client on a connection.  In this
   case, only the certificate and chain of the last post-handshake
   authentication are to be utilized for the header fields described

2.  HTTP Header Fields and Processing Rules

   This document designates the following headers, defined further in
   Sections 2.2 and 2.3, respectively, to carry the client certificate
   information of a mutually authenticated TLS connection.  The headers
   convey the information from the reverse proxy to the origin server.

      The end-entity certificate used by the client in the TLS handshake
      with the reverse proxy.

      The certificate chain used for validation of the end-entity
      certificate provided by the client in the TLS handshake with the
      reverse proxy.

2.1.  Encoding

   The headers in this document encode certificates as Byte Sequences
   (Section 3.3.5 of [STRUCTURED-FIELDS]) where the value of the binary
   data is a DER-encoded [ITU.X690] X.509 certificate [RFC5280].  In
   effect, this means that the binary DER certificate is encoded using
   base64 (without line breaks, spaces, or other characters outside the
   base64 alphabet) and delimited with colons on either side.

   Note that certificates are often stored in an encoded textual format,
   such as the one described in Section 5.1 of [RFC7468], which is
   already nearly compatible with a Byte Sequence.  If certificates are
   encoded as such, it will be sufficient to replace "---(BEGIN|END)
   CERTIFICATE---" with ":" and remove line breaks in order to generate
   an appropriate item.

2.2.  Client-Cert HTTP Header Field

   In the context of a TLS terminating reverse proxy deployment, the
   proxy makes the TLS client certificate available to the backend
   application with the Client-Cert HTTP header field.  This field
   contains the end-entity certificate used by the client in the TLS

   Client-Cert is a Byte Sequence with the value of the header encoded
   as described in Section 2.1.

   The Client-Cert header field is only for use in HTTP requests and
   MUST NOT be used in HTTP responses.  It is a singleton header field
   value as defined in Section 5.5 of [HTTP], which MUST NOT have a list
   of values or occur multiple times in a request.

   Figure 2 in Appendix A has an example of the Client-Cert header

2.3.  Client-Cert-Chain HTTP Header Field

   In the context of a TLS terminating reverse proxy deployment, the
   proxy MAY make the certificate chain available to the backend
   application with the Client-Cert-Chain HTTP header field.

   Client-Cert-Chain is a List (Section 3.1 of [STRUCTURED-FIELDS]).
   Each item in the List MUST be a Byte Sequence encoded as described in
   Section 2.1.  The order is the same as the ordering in TLS (as
   described in Section 4.4.2 of [TLS]).

   Client-Cert-Chain MUST NOT appear unless Client-Cert is also present,
   and it does not itself include the end-entity certificate that is
   already present in Client-Cert.  The root certificate MAY be omitted
   from Client-Cert-Chain, provided that the target origin server is
   known to possess the omitted trust anchor.

   The Client-Cert-Chain header field is only for use in HTTP requests
   and MUST NOT be used in HTTP responses.  It MAY have a list of values
   or occur multiple times in a request.  For header compression
   purposes, it might be advantageous to split lists into multiple

   Figure 3 in Appendix A has an example of the Client-Cert-Chain header

2.4.  Processing Rules

   This section outlines the applicable processing rules for a TTRP that
   has negotiated a mutually authenticated TLS connection to convey the
   client certificate from that connection to the backend origin
   servers.  This technique is to be used as 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 a mutually authenticated TLS connection
   with the client, such as is described in [TLS] or [TLS1.2], and
   validates the client certificate per its policy and trusted
   certificate authorities.  Each HTTP request on the underlying TLS
   connection is dispatched to the origin server with the following

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

   2.  If so configured, the validation chain of the client certificate
       is placed in the Client-Cert-Chain header field of the request,
       as described in Section 2.3.

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

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

   Backend origin servers may then use the Client-Cert header field 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.  Access control decisions based on the
   client certificate (or lack thereof) can be conveyed by selecting
   response content as appropriate or with an HTTP 403 response, if the
   certificate is deemed unacceptable for the given context.  Note that
   TLS clients that rely on error indications at the TLS layer for an
   unacceptable certificate will not receive those signals.

   When the value of the Client-Cert request header field is used to
   select a response (e.g., the response content is access-controlled),
   the response MUST either be uncacheable (e.g., by sending Cache-
   Control: no-store) or be designated for selective reuse only for
   subsequent requests with the same Client-Cert header field value by
   sending a "Vary: Client-Cert" response header.  If a TTRP encounters
   a response with Client-Cert or Client-Cert-Chain in the Vary header
   field (Section 12.5.5 of [HTTP]), it SHOULD prevent the user agent
   from caching the response by transforming the value of the Vary
   response header field to "*".

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

3.  Deployment Considerations

3.1.  Header Field Compression

   If the connection between the TTRP and origin is capable of field
   compression (e.g., HPACK [HPACK] or QPACK [QPACK]), and the TTRP
   multiplexes more than one client's requests into that connection, the
   size and variation of Client-Cert and Client-Cert-Chain field values
   can reduce compression efficiency significantly.  An origin could
   mitigate the efficiency loss by increasing the size of the dynamic
   table.  If the TTRP determines that the origin dynamic table is not
   sufficiently large, it may find it beneficial to always send the
   field value as a literal rather than entering it into the table.

3.2.  Message Header Size

   A server in receipt of a larger message header than it is willing to
   handle can send an HTTP 431 (Request Header Fields Too Large) status
   code per Section 5 of [RFC6585].  Due to the typical size of the
   field values containing certificate data, recipients may need to be
   configured to allow for a larger maximum header size.  An
   intermediary generating client certificate header fields on
   connections that allow for advertising the maximum acceptable header
   size (e.g., HTTP/2 [HTTP/2] or HTTP/3 [HTTP/3]) should account for
   the additional size of the header of the requests it sends, versus
   the requests it receives, by advertising a value to its clients that
   is sufficiently smaller so as to allow for the addition of
   certificate data.

3.3.  TLS Session Resumption

   Some TLS implementations do not retain client certificate information
   when resuming.  Providing inconsistent values of Client-Cert and
   Client-Cert-Chain when resuming might lead to errors, so
   implementations that are unable to provide these values SHOULD either
   disable resumption for connections with client certificates or
   initially omit a Client-Cert or Client-Cert-Chain field if it might
   not be available after resuming.

4.  Security Considerations

   The header fields 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.  However, use of
   the header fields outside that intended use case may undermine the
   protections afforded by TLS client certificate authentication.
   Therefore, steps such as those described below need to be taken to
   prevent unintended use, both in sending the header field and in
   relying on its value.

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

   In order to prevent field injection, backend servers MUST only accept
   the Client-Cert and Client-Cert-Chain header fields 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 fields.  Otherwise,
   arbitrary clients can control the field values as seen and used by
   the backend server.  It is important to note that neglecting to
   prevent field 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 field sanitation is in place.

   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 necessary
   functionalities 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 and Client-Cert-Chain
   header fields occurring only on that connection.  Appendix B.3 of
   [HTTPSIG] gives one example of this with an application of HTTP
   Message Signatures.  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.

5.  IANA Considerations

5.1.  HTTP Field Name Registrations

   IANA has registered the following entries in the "Hypertext Transfer
   Protocol (HTTP) Field Name Registry" defined by "HTTP Semantics"

          | Field Name        | Status    | Reference           |
          | Client-Cert       | permanent | RFC 9440, Section 2 |
          | Client-Cert-Chain | permanent | RFC 9440, Section 2 |

             Table 1: Hypertext Transfer Protocol (HTTP) Field
                               Name Registry

6.  References

6.1.  Normative References

   [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,

   [ITU.X690] ITU-T, "Information technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, February 2021,

   [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,

   [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>.

              Nottingham, M. and P-H. Kamp, "Structured Field Values for
              HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,

6.2.  Informative References

   [HPACK]    Peon, R. and H. Ruellan, "HPACK: Header Compression for
              HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,

   [HTTP/1.1] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/info/rfc9112>.

   [HTTP/2]   Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,

   [HTTP/3]   Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/info/rfc9114>.

   [HTTPSIG]  Backman, A., Ed., Richer, J., Ed., and M. Sporny, "HTTP
              Message Signatures", Work in Progress, Internet-Draft,
              draft-ietf-httpbis-message-signatures-17, 2 May 2023,

   [QPACK]    Krasic, C., Bishop, M., and A. Frindell, Ed., "QPACK:
              Field Compression for HTTP/3", RFC 9204,
              DOI 10.17487/RFC9204, June 2022,

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

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

   [RFC7468]  Josefsson, S. and S. Leonard, "Textual Encodings of PKIX,
              PKCS, and CMS Structures", RFC 7468, DOI 10.17487/RFC7468,
              April 2015, <https://www.rfc-editor.org/info/rfc7468>.

   [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,

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

   [TLS1.2]   Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

Appendix A.  Example

   In a hypothetical example where a TLS client would present 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 field shown in Figure 2 to the backend.  Note
   that line breaks and extra spaces have been added to the field value
   in Figures 2 and 3 for display and formatting purposes only.

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

        Figure 1: Certificate Chain (with Client Certificate First)


          Figure 2: Header Field in HTTP Request to Origin Server

   If the proxy were configured to also include the certificate chain,
   it would also include the Client-Cert-Chain header field.  Note that
   while the following example does illustrate the TTRP inserting the
   root certificate, many deployments will opt to omit the trust anchor.

   Client-Cert-Chain: :MIIB5jCCAYugAwIBAgIBFjAKBggqhkjOPQQDAjBWMQsw

        Figure 3: Certificate Chain in HTTP Request to Origin Server

Appendix B.  Select Design Considerations

B.1.  Field Injection

   This document requires that the TTRP sanitize the fields of the
   incoming request by removing or overwriting any existing instances of
   the Client-Cert and Client-Cert-Chain header fields before
   dispatching that request to the backend application.  Otherwise, a
   client could inject its own values that would appear to the backend
   to have come from the TTRP.  Although numerous other methods of
   detecting and preventing field injection are possible, such as the
   use of a unique secret value as part of the field name or value or
   the application of a signature, HMAC, or AEAD, there is no common
   general mechanism.  The potential problem of client field injection
   is not at all unique to the functionality of this document;
   therefore, it would be inappropriate for this document to define a
   one-off solution.  Since a generic common solution does not currently
   exist, stripping and sanitizing the fields is the de facto means of
   protecting against field injection in practice.  Sanitizing the
   fields is sufficient when properly implemented and is a normative
   requirement of Section 4.

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 document could
   have been communicated with an extension parameter to the Forwarded
   field; however, doing so would have had some disadvantages that this
   document endeavored to avoid.  The Forwarded field syntax allows for
   information about a full chain of proxied HTTP requests, whereas the
   Client-Cert and Client-Cert-Chain header fields of this document are
   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 field is
   expressive but also more complicated, which would make processing it
   more cumbersome and, more importantly, would make properly sanitizing
   its content, as required by Section 4 to prevent field injection,
   considerably more difficult and error-prone.  Thus, this document
   opted for a flatter and more straightforward structure.

B.3.  The Whole Certificate and Certificate Chain

   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 that described in [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 document opted to pass the
   full, encoded certificate as the value of the Client-Cert field.

   The validation of the client certificate and chain of the mutually
   authenticated TLS connection is typically performed by the TTRP
   during the handshake.  With the responsibility of certificate
   validation falling on the TTRP, the end-entity certificate is
   oftentimes sufficient for the needs of the origin server.  The
   separate Client-Cert-Chain field can convey the certificate chain for
   origin server deployments that require this additional information.


   The authors would like to thank the following individuals who have
   contributed to this document in various ways, ranging from just being
   generally supportive of bringing forth the document to providing
   specific feedback or content:

   *  Evan Anderson

   *  Annabelle Backman

   *  Alan Frindell

   *  Rory Hewitt

   *  Fredrik Jeansson

   *  Benjamin Kaduk

   *  Torsten Lodderstedt

   *  Kathleen Moriarty

   *  Mark Nottingham

   *  Erik Nygren

   *  Mike Ounsworth

   *  Lucas Pardue

   *  Matt Peterson

   *  Eric Rescorla

   *  Justin Richer

   *  Michael Richardson

   *  Joe Salowey

   *  Rich Salz

   *  Mohit Sethi

   *  Rifaat Shekh-Yusef

   *  Travis Spencer

   *  Nick Sullivan

   *  Willy Tarreau

   *  Martin Thomson

   *  Peter Wu

   *  Hans Zandbelt

Authors' Addresses

   Brian Campbell
   Ping Identity
   Email: bcampbell@pingidentity.com

   Mike Bishop (editor)
   Email: mbishop@evequefou.be
  1. RFC 9440