HTTP Working Group B. Campbell
Internet-Draft Ping Identity
Intended status: Informational M. Bishop, Ed.
Expires: November 26, 2022 Akamai
May 25, 2022
Client-Cert HTTP Header Field
draft-ietf-httpbis-client-cert-field-02
Abstract
This document defines HTTP extension header fields that 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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation and Conventions . . . . . . . . . . 4
1.2. Terminology and Applicability . . . . . . . . . . . . . . 4
2. HTTP Header Fields and Processing Rules . . . . . . . . . . . 5
2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Client-Cert HTTP Header Field . . . . . . . . . . . . . . 5
2.3. Client-Cert-Chain HTTP Header Field . . . . . . . . . . . 6
2.4. Processing Rules . . . . . . . . . . . . . . . . . . . . 6
3. Deployment Considerations . . . . . . . . . . . . . . . . . . 8
3.1. Header Field Compression . . . . . . . . . . . . . . . . 8
3.2. Header Block Size . . . . . . . . . . . . . . . . . . . . 8
3.3. TLS Session Resumption . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. HTTP Field Name Registrations . . . . . . . . . . . . . . 10
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 12
Appendix B. Considerations Considered . . . . . . . . . . . . . 14
B.1. Field Injection . . . . . . . . . . . . . . . . . . . . . 14
B.2. The Forwarded HTTP Extension . . . . . . . . . . . . . . 15
B.3. The Whole Certificate and Certificate Chain . . . . . . . 15
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 16
Appendix D. Document History . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
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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 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
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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",
"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 and Applicability
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.
HTTP/2 restricts TLS 1.2 renegotiation (Section 9.2.1 of [RFC7540])
and prohibits TLS 1.3 post-handshake authentication [RFC8740].
However, they are sometimes used to implement reactive client
certificate authentication in HTTP/1.1 [RFC7230] 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 which case
only the certificate and chain of the last post-handshake
authentication are to be utilized for the header fields described
herein.
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2. HTTP Header Fields and Processing Rules
This document designates the following headers, defined further in
Section 2.2 and Section 2.3 respectively, to carry the client
certificate 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 the 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 3.3.5 of [RFC8941]) where the value of the
binary data is a DER encoded [ITU.X690.1994] 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 encoded in a textual format,
such as the one described in Section 5.1 of [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 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
handshake.
Client-Cert is an 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.
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The "Client-Cert" header field 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. 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
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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 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 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.
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 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
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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 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.
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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
header fields 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" and "Client-Cert-Chain"
header fields SHOULD be configurable 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 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 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" 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.
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5. IANA Considerations
5.1. HTTP Field Name Registrations
Please register the following entries in the "Hypertext Transfer
Protocol (HTTP) Field Name Registry" defined by
[I-D.ietf-httpbis-semantics]:
o Field name: Client-Cert
o Status: permanent
o Specification document: Section 2 of [this document]
o Field name: Client-Cert-Chain
o Status: permanent
o Specification document: Section 2 of [this document]
6. References
6.1. Normative References
[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.
[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, .
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[RFC8941] Nottingham, M. and P-H. Kamp, "Structured Field Values for
HTTP", RFC 8941, DOI 10.17487/RFC8941, February 2021,
.
6.2. Informative References
[I-D.ietf-httpbis-semantics]
Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
Semantics", draft-ietf-httpbis-semantics-19 (work in
progress), September 2021.
[I-D.ietf-quic-http]
Bishop, M., "Hypertext Transfer Protocol Version 3
(HTTP/3)", draft-ietf-quic-http-34 (work in progress),
February 2021.
[I-D.ietf-quic-qpack]
Krasic, C. '., Bishop, M., and A. Frindell, "QPACK: Header
Compression for HTTP/3", draft-ietf-quic-qpack-21 (work in
progress), February 2021.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
.
[RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status
Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012,
.
[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,
.
[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, .
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
.
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[RFC7541] Peon, R. and H. Ruellan, "HPACK: Header Compression for
HTTP/2", RFC 7541, DOI 10.17487/RFC7541, May 2015,
.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
.
[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,
.
[RFC8740] Benjamin, D., "Using TLS 1.3 with HTTP/2", RFC 8740,
DOI 10.17487/RFC8740, February 2020,
.
6.3. URIs
[1] https://datatracker.ietf.org/meeting/106/materials/slides-106-
secdispatch-securing-protocols-between-proxies-and-backend-http-
servers-00
[2] 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 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.
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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-----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-----END CERTIFICATE-----
Figure 1: Certificate Chain (with client certificate first)
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Client-Cert: :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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:
Figure 3: Certificate Chain 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" and "Client-Cert-Chain" header fields before
dispatching that request to the backend application. Otherwise, a
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client could inject its own values 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 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 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 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 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" 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, make properly sanitizing
its content as required by Section 4 to prevent field injection
considerably more difficult and error-prone. Thus, this draft 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 [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
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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 deployments that require such information.
Appendix C. Acknowledgements
The 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:
o Evan Anderson
o Annabelle Backman
o Alan Frindell
o Rory Hewitt
o Fredrik Jeansson
o Benjamin Kaduk
o Torsten Lodderstedt
o Kathleen Moriarty
o Mark Nottingham
o Erik Nygren
o Mike Ounsworth
o Matt Peterson
o Eric Rescorla
o Justin Richer
o Michael Richardson
o Joe Salowey
o Rich Salz
o Mohit Sethi
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o Rifaat Shekh-Yusef
o Travis Spencer
o Nick Sullivan
o Willy Tarreau
o Martin Thomson
o Peter Wu
o Hans Zandbelt
Appendix D. Document History
To be removed by the RFC Editor before publication as an RFC
draft-ietf-httpbis-client-cert-field-02
o Add a note about cert retention on TLS session resumption
o Say to use only the last one in the case of multiple post-
handshake client cert authentications
draft-ietf-httpbis-client-cert-field-01
o Use RFC 8941 Structured Field Values for HTTP
o Introduce a separate header that can convey the certificate chain
o Add considerations on header compression and size
o Describe interaction with caching
o Fill out IANA Considerations with HTTP field name registrations
o Discuss renegotiation
draft-ietf-httpbis-client-cert-field-00
o Initial WG revision
o Mike Bishop added as co-editor
draft-bdc-something-something-certificate-05
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o Change intended status of the draft to Informational
o Editorial updates and (hopefully) clarifications
draft-bdc-something-something-certificate-04
o Update reference from draft-ietf-oauth-mtls to RFC8705
draft-bdc-something-something-certificate-03
o 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
o Editorial tweaks + further discussion notes
draft-bdc-something-something-certificate-01
o Use the RFC v3 Format or die trying
draft-bdc-something-something-certificate-00
o Initial draft after a time constrained and rushed secdispatch
presentation [1] at IETF 106 in Singapore with the recommendation
to write up a draft (at the end of the minutes [2]) 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
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