Client-Cert HTTP Header FieldPing Identitybcampbell@pingidentity.comAkamaimbishop@evequefou.be
art
httpbishttpclient certificateThis document describes HTTP extension header fields that allow a TLS
terminating reverse proxy (TTRP) to convey the client certificate
information of a mutually authenticated TLS connection to the origin
server in a common and predictable manner.Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any
errata, and how to provide feedback on it may be obtained at
.
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Table of Contents
. Introduction
. Requirements Notation and Conventions
. Terminology and Applicability
. HTTP Header Fields and Processing Rules
. Encoding
. Client-Cert HTTP Header Field
. Client-Cert-Chain HTTP Header Field
. Processing Rules
. Deployment Considerations
. Header Field Compression
. Message Header Size
. TLS Session Resumption
. Security Considerations
. IANA Considerations
. HTTP Field Name Registrations
. References
. Normative References
. Informative References
. Example
. Select Design Considerations
. Field Injection
. The Forwarded HTTP Extension
. The Whole Certificate and Certificate Chain
Acknowledgements
Authors’ Addresses
IntroductionA fairly common deployment pattern for HTTPS applications is to have the origin
HTTP application servers sit behind a reverse proxy that terminates TLS
connections from clients. The proxy is accessible to the Internet and dispatches
client requests to the appropriate origin server within a private or protected
network. The origin servers are not directly accessible by clients and are only
reachable through the reverse proxy. The backend details of this type of
deployment are typically opaque to clients who make requests to the proxy server
and see responses as though they originated from the proxy server itself.
Although HTTPS is also usually employed between the proxy and the origin server,
the TLS connection that the client establishes for HTTPS is only between itself
and the reverse proxy server.The deployment pattern is found in a number of varieties such as n-tier
architectures, content delivery networks, application load-balancing services,
and ingress controllers.Although not exceedingly prevalent, TLS client certificate
authentication is sometimes employed, and in such cases the origin
server often requires information about the client certificate for its
application logic. Such logic might include access control decisions,
audit logging, and binding issued tokens or cookies to a certificate, including
the respective validation of such bindings. The specific details
needed from the certificate also vary with the application
requirements. In order for these types of application deployments to
work in practice, the reverse proxy needs to convey information about
the client certificate to the origin application server. At the time of
writing, a common way this information is conveyed is by using
non-standard fields to carry the certificate (in some encoding) or
individual parts thereof in the HTTP request that is dispatched to the
origin server. This solution works, but interoperability between
independently developed components can be cumbersome or even impossible
depending on the implementation choices respectively made (like what
field names are used or are configurable, which parts of the certificate
are exposed, or how the certificate is encoded). A well-known
predictable approach to this commonly occurring functionality could
improve and simplify interoperability between independent
implementations.The scope of this document is to describe existing practice while
codifying specific details sufficient to facilitate improved and
lower-touch interoperability. As such, this document describes two HTTP
header fields, “Client-Cert” and “Client-Cert-Chain”,
which a TLS terminating reverse proxy (TTRP) adds to requests sent to
the backend origin servers. The Client-Cert field value
contains the end-entity client certificate from the mutually
authenticated TLS connection between the originating client and the
TTRP. Optionally, the Client-Cert-Chain field value contains
the certificate chain used for validation of the end-entity
certificate. This enables the backend origin server to utilize the
client certificate information in its application logic. While there may
be additional proxies or hops between the TTRP and the origin server
(potentially even with mutually authenticated TLS connections between
them), the scope of the Client-Cert header field is
intentionally limited to exposing to the origin server the certificate
that was presented by the originating client in its connection to the
TTRP.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
when, and only when, they appear in all capitals, as shown here.
Terminology and ApplicabilityThis document uses the following terminology from to specify syntax and parsing: List
and Byte Sequence.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 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 , mutual authentication requires the client to send
the Certificate and CertificateVerify messages during the handshake
and the server to verify the CertificateVerify and Finished
messages.HTTP/2 restricts TLS 1.2 renegotiation () and prohibits TLS 1.3
post-handshake authentication (). However, they are sometimes
used to implement reactive client certificate authentication in
HTTP/1.1 where the server decides whether to
request a client certificate based on the HTTP request. HTTP
application data sent on such a connection after receipt and
verification of the client certificate is also mutually authenticated
and thus suitable for the mechanisms described in this
document. With post-handshake authentication, there is also the
possibility, though unlikely in practice, of multiple certificates and
certificate chains from the client on a connection. In this case, only
the certificate and chain of the last post-handshake authentication
are to be utilized for the header fields described herein.HTTP Header Fields and Processing RulesThis document designates the following headers, defined further in
Sections and , respectively,
to carry the client certificate information of a mutually authenticated
TLS connection. The headers convey the information from the reverse
proxy to the origin server.
Client-Cert:
The end-entity certificate used by the client in the TLS handshake
with the reverse proxy.
Client-Cert-Chain:
The certificate chain used for validation of the end-entity
certificate provided by the client in the TLS handshake with the
reverse proxy.
EncodingThe headers in this document encode certificates as Byte Sequences
() where
the value of the binary data is a DER-encoded X.509 certificate .
In effect, this means that the binary DER certificate is encoded using
base64 (without line breaks, spaces, or other characters outside the
base64 alphabet) and delimited with colons on either side.Note that certificates are often stored in an encoded textual
format, such as the one described in , which is already nearly
compatible with a Byte Sequence. If certificates are encoded as such, it will be sufficient to
replace “—(BEGIN|END) CERTIFICATE—” with “:” and
remove line breaks in order to generate an appropriate item.Client-Cert HTTP Header FieldIn 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 a Byte Sequence with the value of the header
encoded as described in .The Client-Cert header field is only for use in HTTP
requests and MUST NOT be used in HTTP responses. It is
a singleton header field value as defined in , which MUST NOT
have a list of values or occur multiple times in a request. in has an example of the Client-Cert header field.Client-Cert-Chain HTTP Header FieldIn the context of a TLS terminating reverse proxy deployment, the proxy
MAY make the certificate chain
available to the backend application with the Client-Cert-Chain HTTP header
field.Client-Cert-Chain is a List (). Each item in the
List MUST be a Byte Sequence encoded as described in . The order
is the same as the ordering in TLS (as described in ).Client-Cert-Chain MUST NOT appear unless Client-Cert is also present, and it does
not itself include the end-entity certificate that is already present in Client-Cert.
The root certificate MAY be omitted from Client-Cert-Chain, provided that the target
origin server is known to possess the omitted trust anchor.The Client-Cert-Chain header field is only for use in HTTP requests and MUST NOT be used in HTTP responses. It MAY have a list of values or occur multiple
times in a request. For header compression purposes, it might be advantageous
to split lists into multiple instances. in has an example of the Client-Cert-Chain header field.Processing RulesThis section outlines the applicable processing rules for a TTRP
that has negotiated a mutually authenticated TLS connection to convey
the client certificate from that connection to the backend origin
servers. This technique is to be used as a configuration or deployment
option, and the processing rules described herein are for servers
operating with that option enabled.A TTRP negotiates the use of a mutually authenticated TLS
connection with the client, such as is described in or , and validates the
client certificate per its policy and trusted certificate authorities.
Each HTTP request on the underlying TLS connection is dispatched to
the origin server with the following modifications:
The client certificate is placed in the Client-Cert
header field of the dispatched request, as described in .
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 .
Any occurrence of the Client-Cert or
Client-Cert-Chain header fields in the original incoming
request MUST be removed or overwritten before
forwarding the request. An incoming request that has a
Client-Cert or Client-Cert-Chain header field
MAY be rejected with an HTTP 400 response.
Requests to the TTRP made over a TLS connection where the use of client certificate
authentication was not negotiated MUST be sanitized by removing any and all
occurrences of the Client-Cert and Client-Cert-Chain header fields prior to
dispatching the request to the backend server.Backend origin servers may then use the Client-Cert header
field of the request to determine if the connection from the client to
the TTRP was mutually authenticated and, if so, the certificate
thereby presented by the client. Access control decisions based on
the client certificate (or lack thereof) can be conveyed by selecting
response content as appropriate or with an HTTP 403 response, if the
certificate is deemed unacceptable for the given context. Note that
TLS clients that rely on error indications at the TLS layer for an
unacceptable certificate will not receive those signals.When the value of the Client-Cert request header field is
used to select a response (e.g., the response content is
access-controlled), the response MUST either be
uncacheable (e.g., by sending Cache-Control: no-store) or be
designated for selective reuse only for subsequent requests with the
same Client-Cert header field value by sending a “Vary:
Client-Cert” response header. If a TTRP encounters a response
with Client-Cert or Client-Cert-Chain in the Vary header
field (), it SHOULD prevent the user agent from caching
the response by transforming the value of the Vary response
header field to “*”.Forward proxies and other intermediaries MUST NOT
add the Client-Cert or Client-Cert-Chain header
fields to requests or modify an existing Client-Cert or
Client-Cert-Chain header field. Similarly, clients
MUST NOT employ the Client-Cert or
Client-Cert-Chain header field in requests.Deployment ConsiderationsHeader Field CompressionIf the connection between the TTRP and origin is capable of field
compression (e.g., HPACK
or QPACK ), and the TTRP
multiplexes more than one client’s requests into that connection, the
size and variation of Client-Cert and
Client-Cert-Chain field values can reduce compression
efficiency significantly. An origin could mitigate the efficiency
loss by increasing the size of the dynamic table. If the TTRP
determines that the origin dynamic table is not sufficiently large, it
may find it beneficial to always send the field value as a literal
rather than entering it into the table.Message Header SizeA server in receipt of a larger message header than it is willing
to handle can send an HTTP 431 (Request Header Fields Too Large)
status code per . Due to the typical size of the field values
containing certificate data, recipients may need to be configured to
allow for a larger maximum header size. An intermediary generating
client certificate header fields on connections that allow for
advertising the maximum acceptable header size (e.g., HTTP/2 or HTTP/3 ) should account for the additional size of
the header of the requests it sends, versus the requests it receives,
by advertising a value to its clients that is sufficiently smaller so
as to allow for the addition of certificate data.TLS Session ResumptionSome 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.Security ConsiderationsThe header fields described herein enable a TTRP and backend or
origin server to function together as though, from the client’s
perspective, they are a single logical server-side deployment of HTTPS
over a mutually authenticated TLS connection. However, use of the header
fields outside that intended use case may undermine the protections
afforded by TLS client certificate authentication. Therefore, steps such
as those described below need to be taken to prevent unintended use,
both in sending the header field and in relying on its value.Producing and consuming the Client-Cert and Client-Cert-Chain header
fields SHOULD be configurable
options, respectively, in a TTRP and backend server (or in an individual application in
that server). The default configuration for both should be to not use the
header fields, thus requiring an “opt-in” to the functionality.In order to prevent field injection, backend servers MUST only accept the
Client-Cert and Client-Cert-Chain header fields from a trusted
TTRP (or other proxy in a trusted path
from the TTRP). A TTRP MUST sanitize the incoming request before forwarding it
on by removing or overwriting any existing instances of the fields. Otherwise,
arbitrary clients can control the field values as seen and used by the backend
server. It is important to note that neglecting to prevent field injection does
not “fail safe” in that the nominal functionality will still work as expected
even when malicious actions are possible. As such, extra care is recommended in
ensuring that proper field sanitation is in place.The communication between a TTRP and backend server needs to be secured against
eavesdropping and modification by unintended parties.The configuration options and request sanitization are necessary functionalities
of the respective servers. The other requirements can be met in a number of
ways, which will vary based on specific deployments. The communication between a
TTRP and backend or origin server, for example, might be authenticated in some
way with the insertion and consumption of the Client-Cert
and Client-Cert-Chain header fields occurring
only on that connection.
gives one example of
this with an application of HTTP Message Signatures.
Alternatively, the network topology might dictate a
private network such that the backend application is only able to accept
requests from the TTRP and the proxy can only make requests to that server.
Other deployments that meet the requirements set forth herein are also possible.IANA ConsiderationsHTTP Field Name RegistrationsIANA has registered the following entries in the “Hypertext Transfer
Protocol (HTTP) Field Name Registry” defined by “HTTP Semantics” :
Hypertext Transfer Protocol (HTTP) Field Name Registry
Field Name
Status
Reference
Client-Cert
permanent
RFC 9440,
Client-Cert-Chain
permanent
RFC 9440,
ReferencesNormative ReferencesHTTP SemanticsThe Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document describes the overall architecture of HTTP, establishes common terminology, and defines aspects of the protocol that are shared by all versions. In this definition are core protocol elements, extensibility mechanisms, and the “http” and “https” Uniform Resource Identifier (URI) schemes.This document updates RFC 3864 and obsoletes RFCs 2818, 7231, 7232, 7233, 7235, 7538, 7615, 7694, and portions of 7230.Information technology – ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)ITU-TKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) ProfileThis memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK]Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.Structured Field Values for HTTPInformative ReferencesHPACK: Header Compression for HTTP/2This specification defines HPACK, a compression format for efficiently representing HTTP header fields, to be used in HTTP/2.HTTP/1.1The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns.This document obsoletes portions of RFC 7230.HTTP/2This specification describes an optimized expression of the semantics of the Hypertext Transfer Protocol (HTTP), referred to as HTTP version 2 (HTTP/2). HTTP/2 enables a more efficient use of network resources and a reduced latency by introducing field compression and allowing multiple concurrent exchanges on the same connection.This document obsoletes RFCs 7540 and 8740.HTTP/3The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC and describes how HTTP/2 extensions can be ported to HTTP/3.HTTP Message SignaturesAmazonBespoke EngineeringDigital BazaarWork in ProgressQPACK: Field Compression for HTTP/3This specification defines QPACK: a compression format for efficiently representing HTTP fields that is to be used in HTTP/3. This is a variation of HPACK compression that seeks to reduce head-of-line blocking.Additional HTTP Status CodesThis document specifies additional HyperText Transfer Protocol (HTTP) status codes for a variety of common situations. [STANDARDS-TRACK]Forwarded HTTP ExtensionThis document defines an HTTP extension header field that allows proxy components to disclose information lost in the proxying process, for example, the originating IP address of a request or IP address of the proxy on the user-agent-facing interface. In a path of proxying components, this makes it possible to arrange it so that each subsequent component will have access to, for example, all IP addresses used in the chain of proxied HTTP requests.This document also specifies guidelines for a proxy administrator to anonymize the origin of a request.Textual Encodings of PKIX, PKCS, and CMS StructuresThis document describes and discusses the textual encodings of the Public-Key Infrastructure X.509 (PKIX), Public-Key Cryptography Standards (PKCS), and Cryptographic Message Syntax (CMS). The textual encodings are well-known, are implemented by several applications and libraries, and are widely deployed. This document articulates the de facto rules by which existing implementations operate and defines them so that future implementations can interoperate.OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access TokensThis document describes OAuth client authentication and certificate-bound access and refresh tokens using mutual Transport Layer Security (TLS) authentication with X.509 certificates. OAuth clients are provided a mechanism for authentication to the authorization server using mutual TLS, based on either self-signed certificates or public key infrastructure (PKI). OAuth authorization servers are provided a mechanism for binding access tokens to a client’s mutual-TLS certificate, and OAuth protected resources are provided a method for ensuring that such an access token presented to it was issued to the client presenting the token.The Transport Layer Security (TLS) Protocol Version 1.3This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery.This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations.The Transport Layer Security (TLS) Protocol Version 1.2This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK]ExampleIn a hypothetical example where a TLS client
would present the client and intermediate certificate from when establishing a mutually authenticated TLS
connection with the TTRP, the proxy would send the Client-Cert
field shown in to the backend. Note that
line breaks and extra spaces have been added to the field value in Figures and for
display and formatting purposes only.Certificate Chain (with Client Certificate First)
—–BEGIN CERTIFICATE—–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—–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—–
Header Field in HTTP Request to Origin Server
Client-Cert: :MIIBqDCCAU6gAwIBAgIBBzAKBggqhkjOPQQDAjA6MRswGQYDVQQKDBJ
MZXQncyBBdXRoZW50aWNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTAeFw0
yMDAxMTQyMjU1MzNaFw0yMTAxMjMyMjU1MzNaMA0xCzAJBgNVBAMMAkJDMFkwEwYHKoZ
Izj0CAQYIKoZIzj0DAQcDQgAE8YnXXfaUgmnMtOXU/IncWalRhebrXmckC8vdgJ1p5Be
5F/3YC8OthxM4+k1M6aEAEFcGzkJiNy6J84y7uzo9M6NyMHAwCQYDVR0TBAIwADAfBgN
VHSMEGDAWgBRm3WjLa38lbEYCuiCPct0ZaSED2DAOBgNVHQ8BAf8EBAMCBsAwEwYDVR0
lBAwwCgYIKwYBBQUHAwIwHQYDVR0RAQH/BBMwEYEPYmRjQGV4YW1wbGUuY29tMAoGCCq
GSM49BAMCA0gAMEUCIBHda/r1vaL6G3VliL4/Di6YK0Q6bMjeSkC3dFCOOB8TAiEAx/k
HSB4urmiZ0NX5r5XarmPk0wmuydBVoU4hBVZ1yhk=:
If the proxy were configured to also include the certificate chain, it would
also include the Client-Cert-Chain header field. Note that while
the following example does illustrate the TTRP inserting the root certificate,
many deployments will opt to omit the trust anchor.Certificate Chain in HTTP Request to Origin Server
Client-Cert-Chain: :MIIB5jCCAYugAwIBAgIBFjAKBggqhkjOPQQDAjBWMQsw
CQYDVQQGEwJVUzEbMBkGA1UECgwSTGV0J3MgQXV0aGVudGljYXRlMSowKAYDVQQ
DDCFMZXQncyBBdXRoZW50aWNhdGUgUm9vdCBBdXRob3JpdHkwHhcNMjAwMTE0Mj
EzMjMwWhcNMzAwMTExMjEzMjMwWjA6MRswGQYDVQQKDBJMZXQncyBBdXRoZW50a
WNhdGUxGzAZBgNVBAMMEkxBIEludGVybWVkaWF0ZSBDQTBZMBMGByqGSM49AgEG
CCqGSM49AwEHA0IABJf+aA54RC5pyLAR5yfXVYmNpgd+CGUTDp2KOGhc0gK91zx
hHesEYkdXkpS2UN8Kati+yHtWCV3kkhCngGyv7RqjZjBkMB0GA1UdDgQWBBRm3W
jLa38lbEYCuiCPct0ZaSED2DAfBgNVHSMEGDAWgBTEA2Q6eecKu9g9yb5glbkhh
VINGDASBgNVHRMBAf8ECDAGAQH/AgEAMA4GA1UdDwEB/wQEAwIBhjAKBggqhkjO
PQQDAgNJADBGAiEA5pLvaFwRRkxomIAtDIwg9D7gC1xzxBl4r28EzmSO1pcCIQC
JUShpSXO9HDIQMUgH69fNDEMHXD3RRX5gP7kuu2KGMg==:, :MIICBjCCAaygAw
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Select Design ConsiderationsField InjectionThis document requires that the TTRP sanitize the fields of the incoming request by
removing or overwriting any existing instances of the Client-Cert
and Client-Cert-Chain header fields
before dispatching that request to the backend application. Otherwise, a client
could inject its own values that would appear to the backend to
have come from the TTRP. Although numerous other methods of detecting and preventing
field injection are possible, such as the use of a unique secret value as part
of the field name or value or the application of a signature, HMAC, or AEAD,
there is no common general mechanism. The potential problem of
client field injection is not at all unique to the functionality of this document;
therefore, it would be inappropriate for this document to define a one-off
solution. Since a generic common solution does not currently exist,
stripping and sanitizing the fields is the de facto means of protecting against
field injection in practice. Sanitizing the fields is sufficient when
properly implemented and is a normative requirement of .The Forwarded HTTP ExtensionThe Forwarded HTTP header field defined in allows proxy components to disclose information
lost in the proxying process. The TLS client certificate information
of concern to this document could have been communicated with an
extension parameter to the Forwarded field; however, doing so
would have had some disadvantages that this document endeavored to
avoid. The Forwarded field syntax allows for information
about a full chain of proxied HTTP requests, whereas the
Client-Cert and Client-Cert-Chain header fields of
this document are concerned only with conveying information about the
certificate presented by the originating client on the TLS connection
to the TTRP (which appears as the server from that client’s
perspective) to backend applications. The multi-hop syntax of the
Forwarded field is expressive but also more complicated,
which would make processing it more cumbersome and, more importantly,
would make properly sanitizing its content, as required by to prevent field injection, considerably more difficult
and error-prone. Thus, this document opted for a flatter and more
straightforward structure.The Whole Certificate and Certificate ChainDifferent applications will have varying requirements about what
information from the client certificate is needed, such as the subject
and/or issuer distinguished name, subject alternative name(s), serial
number, subject public key info, fingerprint, etc. Furthermore, some
applications, such as that described in , make
use of the entire certificate. In order to accommodate the latter and
ensure wide applicability by not trying to cherry-pick particular
certificate information, this document opted to pass the full, encoded
certificate as the value of the Client-Cert field.The validation of the client certificate and chain of the mutually
authenticated TLS connection is typically performed by the TTRP during
the handshake. With the responsibility of certificate validation
falling on the TTRP, the end-entity certificate is oftentimes
sufficient for the needs of the origin server. The separate
Client-Cert-Chain field can convey the certificate chain for
origin server deployments that require this additional
information.AcknowledgementsThe authors would like to thank the following individuals who have contributed to this document in various ways, ranging from just being generally supportive of bringing forth the document to providing specific feedback or content: