Client-Cert HTTP Header: Conveying Client Certificate Information from TLS Terminating Reverse Proxies to Origin Server Applications
draft-bdc-something-something-certificate-00

Aspirational                                                 B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                        January 15, 2020
Expires: July 18, 2020

 Client-Cert HTTP Header: Conveying Client Certificate Information from
     TLS Terminating Reverse Proxies to Origin Server Applications
              draft-bdc-something-something-certificate-00

Abstract

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

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   This Internet-Draft will expire on July 18, 2020.

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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  . . . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
     5.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Appendix A.  Example  . . . . . . . . . . . . . . . . . . . . . .   8
   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 . . . . . . . . . . . . . . . . . .  11
   Appendix D.  Document History . . . . . . . . . . . . . . . . . .  11
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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

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   certificate, and the respective validation of such bindings.  The
   specific details from the certificate needed 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.  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 to some extent
   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 standardized approach to this commonly
   functionality could improve and simplify interoperability between
   implementations.

   This document aspires to standardize an HTTP header field named
   "Client-Cert" that a TLS terminating reverse proxy adds to requests
   that it sends to the origin or backend servers.  The header value
   contains the client certificate from the mutually-authenticated TLS
   connection between the client and reverse proxy, which enables the
   backend origin server to utilize the certificate in its application
   logic.  The usage of the header, both the reverse proxy adding the
   header and the origin server relying on the header for application
   logic, are to be configuration options of the respective systems as
   they will not always be applicable.

1.1.  Requirements Notation and Conventions

   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.

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 session.  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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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.

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
   enabled.

   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

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   connection are dispatched to the origin server with the following
   modifications:

   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.

   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.

   Forward proxies and other intermediaries MUST NOT add the "Client-
   Cert" header to requests.

   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.

3.  Security Considerations

   The header described herein enable a reverse proxy 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 reverse proxy 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 trusted reverse proxies.  And
   reverse proxies 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

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   malicious actions are possible.  As such, extra care is recommended
   in ensuring that proper header sanitation is in place.

   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
   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 reverse proxy
   and the proxy can only make requests to that server.  Other
   deployments that meet the requirements set forth herein are also
   possible.

4.  IANA Considerations

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

5.  References

5.1.  Normative References

   [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,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

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   [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,
              <https://www.rfc-editor.org/info/rfc5280>.

   [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>.

5.2.  Informative References

   [I-D.ietf-oauth-mtls]
              Campbell, B., Bradley, J., Sakimura, N., and T.
              Lodderstedt, "OAuth 2.0 Mutual-TLS Client Authentication
              and Certificate-Bound Access Tokens", draft-ietf-oauth-
              mtls-17 (work in progress), August 2019.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7239]  Petersson, A. and M. Nilsson, "Forwarded HTTP Extension",
              RFC 7239, DOI 10.17487/RFC7239, June 2014,
              <https://www.rfc-editor.org/info/rfc7239>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

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5.3.  URIs

   [1] https://www.iana.org/assignments/message-headers/message-
       headers.xhtml

   [2] https://datatracker.ietf.org/meeting/106/materials/slides-106-
       secdispatch-securing-protocols-between-proxies-and-backend-http-
       servers-00

   [3] https://datatracker.ietf.org/meeting/106/materials/minutes-
       106-secdispatch

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 reverse proxy, 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.

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   -----BEGIN CERTIFICATE-----
   MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJMZXQncyBB
   dXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0yMDAx
   MTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZI
   zj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p
   5Be5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIw
   ADAfBgNVHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMC
   BsAwEwYDVR0lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1w
   bGUuY29tMAoGCCqGSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMje
   SkC3dFCOOB8TAiEAx/kHSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   MIIB5jCCAYugAwIBAgIBFjAKBggqhkjOPQQDAjBWMQswCQYDVQQGEwJVUzEbMBkG
   A1UECgwSTGV0J3MgQXV0aGVudGljYXRlMSowKAYDVQQDDCFMZXQncyBBdXRoZW50
   aWNhdGUgUm9vdCBBdXRob3JpdHkwHhcNMjAwMTE0MjEzMjMwWhcNMzAwMTExMjEz
   MjMwWjA6MRswGQYDVQQKDBJMZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxB
   IEludGVybWVkaWF0ZSBDQTBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABJf+aA54
   RC5pyLAR5yfXVYmNpgd+CGUTDp2KOGhc0gK91zxhHesEYkdXkpS2UN8Kati+yHtW
   CV3kkhCngGyv7RqjZjBkMB0GA1UdDgQWBBRm3WjLa38lbEYCuiCPct0ZaSED2DAf
   BgNVHSMEGDAWgBTEA2Q6eecKu9g9yb5glbkhhVINGDASBgNVHRMBAf8ECDAGAQH/
   AgEAMA4GA1UdDwEB/wQEAwIBhjAKBggqhkjOPQQDAgNJADBGAiEA5pLvaFwRRkxo
   mIAtDIwg9D7gC1xzxBl4r28EzmSO1pcCIQCJUShpSXO9HDIQMUgH69fNDEMHXD3R
   RX5gP7kuu2KGMg==
   -----END CERTIFICATE-----
   -----BEGIN CERTIFICATE-----
   MIICBjCCAaygAwIBAgIJAKS0yiqKtlhoMAoGCCqGSM49BAMCMFYxCzAJBgNVBAYT
   AlVTMRswGQYDVQQKDBJMZXQncyBBdXRoZW50aWNhdGUxKjAoBgNVBAMMIUxldCdz
   IEF1dGhlbnRpY2F0ZSBSb290IEF1dGhvcml0eTAeFw0yMDAxMTQyMTI1NDVaFw00
   MDAxMDkyMTI1NDVaMFYxCzAJBgNVBAYTAlVTMRswGQYDVQQKDBJMZXQncyBBdXRo
   ZW50aWNhdGUxKjAoBgNVBAMMIUxldCdzIEF1dGhlbnRpY2F0ZSBSb290IEF1dGhv
   cml0eTBZMBMGByqGSM49AgEGCCqGSM49AwEHA0IABFoaHU+Z5bPKmGzlYXtCf+E6
   HYj62fORaHDOrt+yyh3H/rTcs7ynFfGn+gyFsrSP3Ez88rajv+U2NfD0o0uZ4Pmj
   YzBhMB0GA1UdDgQWBBTEA2Q6eecKu9g9yb5glbkhhVINGDAfBgNVHSMEGDAWgBTE
   A2Q6eecKu9g9yb5glbkhhVINGDAPBgNVHRMBAf8EBTADAQH/MA4GA1UdDwEB/wQE
   AwIBhjAKBggqhkjOPQQDAgNIADBFAiEAmAeg1ycKHriqHnaD4M/UDBpQRpkmdcRF
   YGMg1Qyrkx4CIB4ivz3wQcQkGhcsUZ1SOImd/lq1Q0FLf09rGfLQPWDc
   -----END CERTIFICATE-----

        Figure 1: Certificate Chain (with client certificate first)

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   Client-Cert: MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJM
    ZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0y
    MDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZI
    zj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p5Be5
    F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIwADAfBgNV
    HSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMCBsAwEwYDVR0l
    BAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1wbGUuY29tMAoGCCqG
    SM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMjeSkC3dFCOOB8TAiEAx/kH
    SB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=

             Figure 2: Header in HTTP Request to Origin Server

Appendix B.  Considerations Considered

B.1.  Header Injection

   This draft requires that the reverse proxy 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 reverse proxy.  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 the 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
   reverse proxy (which appears as the server from that client's
   perspective) to backend applications.  The multi-hop syntax of the

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   "Forwarded" header is 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 like [I-D.ietf-oauth-mtls] 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 reverse proxy.
   With the responsibility of certificate validation falling on the
   proxy, only the end-entity certificate is passed to the backend - the
   root Certificate Authority is not included nor are any intermediates.

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: Annabelle Backman, Benjamin Kaduk, Torsten Lodderstedt,
   Kathleen Moriarty, Mike Ounsworth, Matt Peterson, Justin Richer, Rich
   Salz, Rifaat Shekh-Yusef, Travis Spencer, 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) ]]

   draft-bdc-something-something-certificate-00

   o  Initial draft after a time constrained and rushed secdispatch
      presentation [2] at IETF 106 in Singapore with the recommendation
      to write up a draft (at the end of the minutes [3]) and some folks
      expressing interest despite the rather poor presentation

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Internet-Draft             Client-Cert Header               January 2020

Author's Address

   Brian Campbell
   Ping Identity

   Email: bcampbell@pingidentity.com

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