wdiff draft-ietf-httpbis-client-cert-field-00.txt draft-ietf-httpbis-client-cert-field-01.txt

HTTP B. Campbell
Internet-Draft Ping Identity
Intended status: Informational M. Bishop, Ed.
Expires: 10 December 2021 29 July 2022 Akamai
8 June 2021
25 January 2022

Client-Cert HTTP Header Field: Conveying Client Certificate Information
from TLS Terminating Reverse Proxies to Origin Server Applications
draft-ietf-httpbis-client-cert-field-00 Field
draft-ietf-httpbis-client-cert-field-01

Abstract

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

Note

About This Document

This note is to Readers

_RFC EDITOR: please remove this section be removed before publication_ publishing as an RFC.

Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-httpbis-client-cert-
field/.

Discussion of this draft document takes place on the HTTP working group Working Group
mailing list (ietf-http-wg@w3.org), (mailto:ietf-http-wg@w3.org), which is archived at
https://lists.w3.org/Archives/Public/ietf-http-wg/
(https://lists.w3.org/Archives/Public/ietf-http-wg/).
https://lists.w3.org/Archives/Public/ietf-http-wg/. Working Group
information can be found at http://httpwg.github.io/
(http://httpwg.github.io/); source code and issues list https://httpwg.org/.

Source for this draft and an issue tracker can be found at https://github.com/httpwg/http-
extensions/labels/client-cert-header (https://github.com/httpwg/http-
extensions/labels/client-cert-header).
https://github.com/httpwg/http-extensions/labels/client-cert-field.

Status of This Memo

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 3
1.1. Requirements Notation and Conventions . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. HTTP Header Field Fields and Processing Rules . . . . . . . . . . . 4
2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 4 5
2.2. Client-Cert HTTP Header Field . . . . . . . . . . . . . . 5
2.3. Client-Cert-Chain HTTP Header Field . . . . . . . . . . . 6
2.4. Processing Rules . . . . . . . . . . . . . . . . . . . . 5 6
3. Security Deployment Considerations . . . . . . . . . . . . . . . . . . 7
3.1. Header Field Compression . . 6 . . . . . . . . . . . . . . 8
3.2. Header Block Size . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
5. 9
5.1. HTTP Field Name Registrations . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. 9
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
5.2. 10
6.2. Informative References . . . . . . . . . . . . . . . . . 7 10
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 9 12
Appendix B. Considerations Considered . . . . . . . . . . . . . 10 13
B.1. Header Field Injection . . . . . . . . . . . . . . . . . . . . 10 . 14
B.2. The Forwarded HTTP Extension . . . . . . . . . . . . . . 10 14
B.3. The Whole Certificate and Only the Whole Certificate Chain . . . . 11 . . . 14
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 12 15
Appendix D. Document History . . . . . . . . . . . . . . . . . . 13 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 17

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 headers 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 header 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 two HTTP header field named
"Client-Cert" that fields, Client-
Cert and Client-Cert-Chain, which a TLS terminating reverse proxy
(TTRP) adds to requests that it sends sent to the backend origin servers. The header
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" 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.

TODO: HTTP2 forbids

HTTP/2 restricts TLS 1.2 renegotiation (Section 9.2.1 of [RFC7540])
and prohibits TLS 1.3 post-handshake authentication but it's possible with HTTP1.1 and maybe needs [RFC8740].
However, they are sometimes used to
be discussed explicitly here or somewhere implement reactive client
certificate authentication in this document?
Naively I'd say that the "Client-Cert" header will be sent with
the data of HTTP/1.1 [RFC7230] where the most recent server
decides whether to request a 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 certificate based on the HTTP
request. HTTP application data the cert
messages arrived is hard to impossible so it'll be sent on such a best effort
kind connection after
receipt and verification of thing. the client certificate is also mutually-
authenticated and thus suitable for the mechanisms described in this
document.

2. HTTP Header Field Fields and Processing Rules

2.1. Encoding

The field-values of

This document designates the HTTP header following headers, defined herein utilize further in
Section 2.2 and Section 2.3 respectively, to carry the
following encoded form.

A client
certificate is represented information of a mutually-authenticated TLS connection
from a reverse proxy to origin server.

Client-Cert: Conveys the end-entity certificate used by the client
in text as an "EncodedCertificate",
which is the base64-encoded TLS handshake with the reverse proxy from the reverse proxy
to the origin server.

Client-Cert-Chain: Conveys the certificate chain used for validation
of the end-entity certificate used by the client in the TLS
handshake from the reverse proxy to the origin server.

2.1. Encoding

The headers in this document encode certificates as Structured Field
Byte Sequences (Section 4 3.3.5 of [RFC4648]) [RFC8941]) where the value of the
binary data is a DER encoded [ITU.X690.1994] PKIX certificate. The X.509 certificate
[RFC5280]. In effect, this means that the binary DER certificate is
encoded value MUST NOT include
any using base64 (without line breaks, whitespace, spaces, or other additional characters. ABNF
[RFC5234] syntax for "EncodedCertificate" is shown
characters outside the base64 alphabet) and delimited with colons on
either side.

Note that certificates are often stored encoded in a textual format,
such as the figure
below.

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

DIGIT = <Defined one described in Section B.1 5.1 of [RFC5234]> ; A-Z / a-z
ALPHA = <Defined [RFC7468], which is
already nearly compatible with a Structured Field Byte Sequence; if
so, it will be sufficient to replace ---(BEGIN|END) CERTIFICATE---
with : and remove line breaks in Section B.1 of [RFC5234]> ; 0-9 order to generate an appropriate
item.

2.2. Client-Cert HTTP Header Field

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

Client-Cert: The This field
contains the end-entity client certificate as used by the client in the TLS
handshake.

Client-Cert is an
"EncodedCertificate" value. Item Structured Header [RFC8941]. Its value MUST
be a Byte Sequence (Section 3.3.5 of [RFC8941]). Its ABNF is:

Client-Cert = sf-binary

The value of the header is encoded as described in Section 2.1.

The "Client-Cert" 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 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. request.

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 used for validation of the end-
entity certificate available to the backend application with the
Client-Cert-Chain HTTP header field. This field contains
certificates used by the proxy to validate the certificate used by
the client in the TLS handshake. These certificates might or might
not have been provided by the client during the TLS handshake.

Client-Cert-Chain is a List Structured Header [RFC8941]. Each item
in the list MUST be a Byte Sequence (Section 3.3.5 of [RFC8941])
encoded as described in Section 2.1.

The header's ABNF is:

Client-Cert-Chain = sf-list

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
instances.

The first certificate in the list SHOULD directly certify the end-
entity certificate provided in the Client-Cert header; each following
certificate SHOULD directly certify the one immediately preceding it.
Because certificate validation requires that trust anchors be
distributed independently, a certificate that specifies a trust
anchor MAY be omitted from the chain, provided that the server is
known to possess any omitted certificates.

However, for maximum compatibility, servers SHOULD be prepared to
handle potentially extraneous certificates and arbitrary orderings.

2.4. 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" Client-Cert header field
of the dispatched request request, as defined 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" 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" Client-Cert or Client-Cert-Chain 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" of the Client-Cert and Client-Cert-
Chain header field fields prior to dispatching the request to the backend
server.

Backend origin servers may then use the "Client-Cert" 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" Client-Cert
or Client-Cert-Chain header fields to requests, or modify an existing "Client-Cert" header.
Client-Cert or Client-Cert-Chain header field. Similarly, clients
MUST NOT employ the "Client-Cert" Client-Cert or Client-Cert-Chain header field in
requests.

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 value by sending
a Vary: Client-Cert response header. If a TTRP encounters a response
with a client-cert field name in the Vary header field, it SHOULD
prevent the user agent from caching the response by transforming the
value of the Vary response header field to *.

3. Deployment Considerations
3.1. Header Field Compression

If the client certificate header field is generated by an
intermediary on a connection that compresses fields (e.g., using
HPACK [RFC7541] or QPACK [I-D.ietf-quic-qpack]) and more than one
client's requests are multiplexed into that connection, it can reduce
compression efficiency significantly, due to the typical size of the
field value and its variation between clients. Recipients that
anticipate connections with these characteristics can mitigate the
efficiency loss by increasing the size of the dynamic table. If a
recipient does not do so, senders may find it beneficial to always
send the field value as a literal, rather than entering it into the
dynamic table.

3.2. Header Block Size

A server in receipt of a larger header block 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 block size. An
intermediary generating client certificate header fields on
connections that allow for advertising the maximum acceptable header
block size (e.g. HTTP/2 [RFC7540] or HTTP/3 [I-D.ietf-quic-http])
should account for the additional size of header block of the
requests it sends vs. requests it receives by advertising a request with a "Client-Cert" header value to
its clients that it considers is sufficiently smaller so as to be too large can respond with an HTTP 431 status
code per Section 5 allow for the
addition of [RFC6585].

3. certificate data.

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. Use of the "Client-Cert"
header fields outside that intended use case, however, may undermine
the protections afforded by TLS client certificate authentication.
Therefore
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" Client-Cert and Client-Cert-Chain header
fields SHOULD be a configurable option, options, 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 fields
thus requiring an "opt-in" to the functionality.

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

5. IANA Considerations

TODO:

5.1. HTTP Field Name Registrations

Please register the "Client-Cert" HTTP header field following entries in the registry "Hypertext Transfer
Protocol (HTTP) Field Name Registry" defined by http-core.

5.
[I-D.ietf-httpbis-semantics]:

* Field name: Client-Cert

* Status: permanent

* Specification document: Section 2 of [this document]

* Field name: Client-Cert-Chain

* Status: permanent

* Specification document: Section 2 of [this document]

6. References

5.1.
6.1. Normative References

[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/rfc/rfc2119>.

[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/rfc/rfc8174>.

[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/rfc/rfc5280>.

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

[ITU.X690.1994]
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.

[RFC8941] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.

6.2. Informative References

[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/rfc/rfc8446>.

[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/rfc/rfc5246>.

[RFC5234] Crocker, D., Ed.

[RFC7540] Belshe, M., Peon, R., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 5234, 7540,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/rfc/rfc5234>. 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/rfc/rfc7540>.

[RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740,
DOI 10.17487/RFC8740, February 2020,
<https://www.rfc-editor.org/rfc/rfc8740>.

[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/rfc/rfc7230>.

[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/rfc/rfc7468>.

[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
<https://www.rfc-editor.org/rfc/rfc7541>.

[I-D.ietf-quic-qpack]
Krasic, C. '., Bishop, M., and A. Frindell, "QPACK: Header
Compression for HTTP/3", Work in Progress, Internet-Draft,
draft-ietf-quic-qpack-21, 2 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
qpack-21>.

[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
<https://www.rfc-editor.org/rfc/rfc6585>.

[I-D.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
quic-http-34, 2 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-quic-
http-34>.

[I-D.ietf-httpbis-semantics]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", Work in Progress, Internet-Draft, draft-ietf-
httpbis-semantics-19, 12 September 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-
semantics-19>.

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

[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,
<https://www.rfc-editor.org/rfc/rfc8705>.

[RFC8941] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
<https://www.rfc-editor.org/rfc/rfc8941>.

[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, <https://www.rfc-editor.org/rfc/rfc7250>.

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

Figure 1: Certificate Chain (with client certificate first)

Client-Cert: MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJM
ZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0y
MDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZI
zj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p5Be5
F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIwADAfBgNV
HSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMCBsAwEwYDVR0l
BAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1wbGUuY29tMAoGCCqG
SM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMjeSkC3dFCOOB8TAiEAx/kH
SB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk= :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJ
MZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0
yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZ
Izj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p5Be
5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIwADAfBgN
VHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMCBsAwEwYDVR0
lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1wbGUuY29tMAoGCCq
GSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMjeSkC3dFCOOB8TAiEAx/k
HSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:

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 this header:

Client-Cert-Chain: :MIIB5jCCAYugAwIBAgIBFjAKBggqhkjOPQQDAjBWMQsw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:, :MIICBjCCAaygAw
IBAgIJAKS0yiqKtlhoMAoGCCqGSM49BAMCMFYxCzAJBgNVBAYTAlVTMRswGQYDV
QQKDBJMZXQncyBBdXRoZW50aWNhdGUxKjAoBgNVBAMMIUxldCdzIEF1dGhlbnRp
Y2F0ZSBSb290IEF1dGhvcml0eTAeFw0yMDAxMTQyMTI1NDVaFw00MDAxMDkyMTI
1NDVaMFYxCzAJBgNVBAYTAlVTMRswGQYDVQQKDBJMZXQncyBBdXRoZW50aWNhdG
UxKjAoBgNVBAMMIUxldCdzIEF1dGhlbnRpY2F0ZSBSb290IEF1dGhvcml0eTBZM
BMGByqGSM49AgEGCCqGSM49AwEHA0IABFoaHU+Z5bPKmGzlYXtCf+E6HYj62fOR
aHDOrt+yyh3H/rTcs7ynFfGn+gyFsrSP3Ez88rajv+U2NfD0o0uZ4PmjYzBhMB0
GA1UdDgQWBBTEA2Q6eecKu9g9yb5glbkhhVINGDAfBgNVHSMEGDAWgBTEA2Q6ee
cKu9g9yb5glbkhhVINGDAPBgNVHRMBAf8EBTADAQH/MA4GA1UdDwEB/wQEAwIBh
jAKBggqhkjOPQQDAgNIADBFAiEAmAeg1ycKHriqHnaD4M/UDBpQRpkmdcRFYGMg
1Qyrkx4CIB4ivz3wQcQkGhcsUZ1SOImd/lq1Q0FLf09rGfLQPWDc:

Figure 3: Certificate Chain in HTTP Request to Origin Server

Appendix B. Considerations Considered
B.1. Header Field Injection

This draft requires that the TTRP sanitize the headers fields of the incoming
request by removing or overwriting any existing instances of the "Client-Cert"
Client-Cert and Client-Cert-Chain header fields before dispatching
that request to the backend application. Otherwise, a client could
inject its own
"Client-Cert" header values that would appear to the backend to have come
from the TTRP. Although numerous other methods of detecting/
preventing header field injection are possible; such as the use of a unique
secret value as part of the header 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 header field injection is not at
all unique to the functionality of this draft draft, and it would therefor therefore
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 fields is the de facto means of protecting
against header field injection in practice today. Sanitizing the headers fields is
sufficient when properly implemented and is a normative requirement
of Section 3. 4.

B.2. The Forwarded HTTP Extension

The "Forwarded" 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, Forwarded
field; however, doing so would have had some disadvantages that this
draft endeavored to avoid. The "Forwarded" header Forwarded field syntax allows for
information about a full chain of proxied HTTP requests, whereas the "Client-Cert"
Client-Cert and Client-Cert-Chain header fields of this document is 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" header 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 4 to prevent header field injection
considerably more difficult and error prone. error-prone. Thus, this draft opted
for the a flatter and more straightforward structure of a single
"Client-Cert" header. structure.

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

TODO: 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 oftentimes sufficient 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 needs of [RFC8941] 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
origin server. The separate Client-Cert-Chain field can convey 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 chain for further
discussion should this draft progress.

TODO: It has also been suggested deployments 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. require such information.

Appendix C. Acknowledgements

The author authors 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 Alan Frindell

* Rory Hewitt

* Fredrik Jeansson

* Benjamin Kaduk

* Torsten Lodderstedt

* Kathleen Moriarty

* Mark Nottingham

* Erik Nygren

* Mike Ounsworth

* Matt Peterson

* Eric Rescorla

* Justin Richer

* Michael Richardson
* Joe Salowey

* Rich Salz

* Mohit Sethi

* Rifaat Shekh-Yusef

* Travis Spencer

* Nick Sullivan

* Martin Thomson

* Peter Wu

* Hans Zandbelt

Appendix D. Document History

To be removed by the RFC Editor before publication as an RFC

draft-ietf-httpbis-client-cert-field-01

* Use RFC 8941 Structured Field Values for HTTP

* Introduce a separate header that can convey the certificate chain

* Add considerations on header compression and size

* Describe interaction with caching

* Fill out IANA Considerations with HTTP field name registrations

* Discuss renegotiation

draft-ietf-httpbis-client-cert-field-00

* Initial WG revision

* Mike Bishop added as co-editor

draft-bdc-something-something-certificate-05

* Change intended status of the draft to Informational

* Editorial updates and (hopefully) clarifications

draft-bdc-something-something-certificate-04
* Update reference from draft-ietf-oauth-mtls to RFC8705

draft-bdc-something-something-certificate-03

* 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

draft-bdc-something-something-certificate-02

* Editorial tweaks + further discussion notes

draft-bdc-something-something-certificate-01

* Use the RFC v3 Format or die trying

draft-bdc-something-something-certificate-00

* Initial draft after a time constrained and rushed secdispatch
presentation (https://datatracker.ietf.org/meeting/106/materials/
slides-106-secdispatch-securing-protocols-between-proxies-and-
backend-http-servers-00) at IETF 106 in Singapore with the
recommendation to write up a draft (at the end of the minutes
(https://datatracker.ietf.org/meeting/106/materials/minutes-
106-secdispatch)) and some folks expressing interest despite the
rather poor presentation

Authors' Addresses

Brian Campbell
Ping Identity

Email: bcampbell@pingidentity.com

Mike Bishop (editor)
Akamai

Email: mbishop@evequefou.be