HTTP Working GroupB. Campbell
Internet-DraftPing Identity
Intended status: InformationalM. Bishop, Editor
Expires: March 17, 2022Akamai
September 13, 2021

Client-Cert HTTP Header Field: 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.

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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 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 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.

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 field 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 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", "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 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.

2. HTTP Header Field and Processing Rules

2.1. Encoding

The field-values of the HTTP header field 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.X690.1994] 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

2.3. 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 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 field in the original incoming request MUST be removed or overwritten before forwarding the request. An incoming request that has a Client-Cert header field 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 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.

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

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

3. Security Considerations

The header field 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 field 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 field and in relying on its value.

Producing and consuming the Client-Cert header field 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 field thus requiring an "opt-in" to the functionality.

In order to prevent field injection, backend servers MUST only accept the Client-Cert header field 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 field. Otherwise arbitrary clients can control the field value 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 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 field 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.

4. IANA Considerations

The Client-Cert HTTP header field will be added to the registry defined by http-core.

5. References

5.1. Normative References

Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <>.
Leiba, B., “Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words”, BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <>.
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, <>.
Josefsson, S., “The Base16, Base32, and Base64 Data Encodings”, RFC 4648, DOI 10.17487/RFC4648, October 2006, <>.
International Telecommunications Union, “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, 1994.

5.2. Informative References

Rescorla, E., “The Transport Layer Security (TLS) Protocol Version 1.3”, RFC 8446, DOI 10.17487/RFC8446, August 2018, <>.
Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2”, RFC 5246, DOI 10.17487/RFC5246, August 2008, <>.
Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008, <>.
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, <>.
Nottingham, M. and R. Fielding, “Additional HTTP Status Codes”, RFC 6585, DOI 10.17487/RFC6585, April 2012, <>.
Petersson, A. and M. Nilsson, “Forwarded HTTP Extension”, RFC 7239, DOI 10.17487/RFC7239, June 2014, <>.
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, <>.

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 field shown in {#example-header} to the backend. Note that line breaks and whitespace have been added to the field value in Figure 2 for display and formatting purposes only.


Figure 1: Certificate Chain (with client certificate first)


Figure 2: Header Field in HTTP Request to Origin Server

Appendix B. Considerations Considered

B.1. Field Injection

This draft requires that the TTRP sanitize the fields of the incoming request by removing or overwriting any existing instances of the Client-Cert header field before dispatching that request to the backend application. Otherwise, a client could inject its own Client-Cert field that would appear to the backend to have come from the TTRP. Although numerous other methods of detecting/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 standardized mechanism. The potential problem of client field 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 fields is the de facto means of protecting against field injection in practice today. Sanitizing the fields is sufficient when properly implemented and is a 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 field; however, doing so would have had some disadvantages that this draft endeavored to avoid. The Forwarded field syntax allows for information about a full chain of proxied HTTP requests, whereas the Client-Cert field 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 field 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 field 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 field.

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.

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:

Appendix D. Document History








Authors' Addresses

Brian Campbell
Ping Identity
Mike Bishop (editor)