Internet Engineering Task Force (IETF) R. Fielding, Ed.
Request for Comments: 7235 Adobe
Obsoletes: 2616 J. Reschke, Ed.
Updates: 2617 greenbytes
Category: Standards Track June 2014
Hypertext Transfer Protocol (HTTP/1.1): Authentication
The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypermedia information
systems. This document defines the HTTP Authentication framework.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
Table of Contents
1. Introduction ....................................................31.1. Conformance and Error Handling .............................31.2. Syntax Notation ............................................32. Access Authentication Framework .................................32.1. Challenge and Response .....................................32.2. Protection Space (Realm) ...................................53. Status Code Definitions .........................................63.1. 401 Unauthorized ...........................................63.2. 407 Proxy Authentication Required ..........................64. Header Field Definitions ........................................74.1. WWW-Authenticate ...........................................74.2. Authorization ..............................................84.3. Proxy-Authenticate .........................................84.4. Proxy-Authorization ........................................95. IANA Considerations .............................................95.1. Authentication Scheme Registry .............................95.1.1. Procedure ...........................................95.1.2. Considerations for New Authentication Schemes ......105.2. Status Code Registration ..................................115.3. Header Field Registration .................................116. Security Considerations ........................................126.1. Confidentiality of Credentials ............................126.2. Authentication Credentials and Idle Clients ...............126.3. Protection Spaces .........................................137. Acknowledgments ................................................148. References .....................................................148.1. Normative References ......................................148.2. Informative References ....................................14Appendix A. Changes from RFCs 2616 and 2617 .......................16Appendix B. Imported ABNF .........................................16Appendix C. Collected ABNF ........................................17Index .............................................................18
HTTP provides a general framework for access control and
authentication, via an extensible set of challenge-response
authentication schemes, which can be used by a server to challenge a
client request and by a client to provide authentication information.
This document defines HTTP/1.1 authentication in terms of the
architecture defined in "Hypertext Transfer Protocol (HTTP/1.1):
Message Syntax and Routing" [RFC7230], including the general
framework previously described in "HTTP Authentication: Basic and
Digest Access Authentication" [RFC2617] and the related fields and
status codes previously defined in "Hypertext Transfer Protocol --
The IANA Authentication Scheme Registry (Section 5.1) lists
registered authentication schemes and their corresponding
specifications, including the "basic" and "digest" authentication
schemes previously defined by RFC 2617.
1.1. Conformance and Error Handling
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Conformance criteria and considerations regarding error handling are
defined in Section 2.5 of [RFC7230].
1.2. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] with a list extension, defined in Section 7 of
[RFC7230], that allows for compact definition of comma-separated
lists using a '#' operator (similar to how the '*' operator indicates
repetition). Appendix B describes rules imported from other
documents. Appendix C shows the collected grammar with all list
operators expanded to standard ABNF notation.
2. Access Authentication Framework
2.1. Challenge and Response
HTTP provides a simple challenge-response authentication framework
that can be used by a server to challenge a client request and by a
client to provide authentication information. It uses a case-
insensitive token as a means to identify the authentication scheme,
followed by additional information necessary for achieving
authentication via that scheme. The latter can be either a comma-
separated list of parameters or a single sequence of characters
capable of holding base64-encoded information.
Authentication parameters are name=value pairs, where the name token
is matched case-insensitively, and each parameter name MUST only
occur once per challenge.
auth-scheme = token
auth-param = token BWS "=" BWS ( token / quoted-string )
token68 = 1*( ALPHA / DIGIT /
"-" / "." / "_" / "~" / "+" / "/" ) *"="
The token68 syntax allows the 66 unreserved URI characters
([RFC3986]), plus a few others, so that it can hold a base64,
base64url (URL and filename safe alphabet), base32, or base16 (hex)
encoding, with or without padding, but excluding whitespace
A 401 (Unauthorized) response message is used by an origin server to
challenge the authorization of a user agent, including a
WWW-Authenticate header field containing at least one challenge
applicable to the requested resource.
A 407 (Proxy Authentication Required) response message is used by a
proxy to challenge the authorization of a client, including a
Proxy-Authenticate header field containing at least one challenge
applicable to the proxy for the requested resource.
challenge = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
Note: Many clients fail to parse a challenge that contains an
unknown scheme. A workaround for this problem is to list well-
supported schemes (such as "basic") first.
A user agent that wishes to authenticate itself with an origin server
-- usually, but not necessarily, after receiving a 401 (Unauthorized)
-- can do so by including an Authorization header field with the
A client that wishes to authenticate itself with a proxy -- usually,
but not necessarily, after receiving a 407 (Proxy Authentication
Required) -- can do so by including a Proxy-Authorization header
field with the request.
Both the Authorization field value and the Proxy-Authorization field
value contain the client's credentials for the realm of the resource
being requested, based upon a challenge received in a response
(possibly at some point in the past). When creating their values,
the user agent ought to do so by selecting the challenge with what it
considers to be the most secure auth-scheme that it understands,
obtaining credentials from the user as appropriate. Transmission of
credentials within header field values implies significant security
considerations regarding the confidentiality of the underlying
connection, as described in Section 6.1.
credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]
Upon receipt of a request for a protected resource that omits
credentials, contains invalid credentials (e.g., a bad password) or
partial credentials (e.g., when the authentication scheme requires
more than one round trip), an origin server SHOULD send a 401
(Unauthorized) response that contains a WWW-Authenticate header field
with at least one (possibly new) challenge applicable to the
Likewise, upon receipt of a request that omits proxy credentials or
contains invalid or partial proxy credentials, a proxy that requires
authentication SHOULD generate a 407 (Proxy Authentication Required)
response that contains a Proxy-Authenticate header field with at
least one (possibly new) challenge applicable to the proxy.
A server that receives valid credentials that are not adequate to
gain access ought to respond with the 403 (Forbidden) status code
(Section 6.5.3 of [RFC7231]).
HTTP does not restrict applications to this simple challenge-response
framework for access authentication. Additional mechanisms can be
used, such as authentication at the transport level or via message
encapsulation, and with additional header fields specifying
authentication information. However, such additional mechanisms are
not defined by this specification.
2.2. Protection Space (Realm)
The "realm" authentication parameter is reserved for use by
authentication schemes that wish to indicate a scope of protection.
A protection space is defined by the canonical root URI (the scheme
and authority components of the effective request URI; see Section
5.5 of [RFC7230]) of the server being accessed, in combination with
the realm value if present. These realms allow the protected
resources on a server to be partitioned into a set of protection
spaces, each with its own authentication scheme and/or authorization
database. The realm value is a string, generally assigned by the
origin server, that can have additional semantics specific to the
authentication scheme. Note that a response can have multiple
challenges with the same auth-scheme but with different realms.
The protection space determines the domain over which credentials can
be automatically applied. If a prior request has been authorized,
the user agent MAY reuse the same credentials for all other requests
within that protection space for a period of time determined by the
authentication scheme, parameters, and/or user preferences (such as a
configurable inactivity timeout). Unless specifically allowed by the
authentication scheme, a single protection space cannot extend
outside the scope of its server.
For historical reasons, a sender MUST only generate the quoted-string
syntax. Recipients might have to support both token and
quoted-string syntax for maximum interoperability with existing
clients that have been accepting both notations for a long time.
3. Status Code Definitions
3.1. 401 Unauthorized
The 401 (Unauthorized) status code indicates that the request has not
been applied because it lacks valid authentication credentials for
the target resource. The server generating a 401 response MUST send
a WWW-Authenticate header field (Section 4.1) containing at least one
challenge applicable to the target resource.
If the request included authentication credentials, then the 401
response indicates that authorization has been refused for those
credentials. The user agent MAY repeat the request with a new or
replaced Authorization header field (Section 4.2). If the 401
response contains the same challenge as the prior response, and the
user agent has already attempted authentication at least once, then
the user agent SHOULD present the enclosed representation to the
user, since it usually contains relevant diagnostic information.
3.2. 407 Proxy Authentication Required
The 407 (Proxy Authentication Required) status code is similar to 401
(Unauthorized), but it indicates that the client needs to
authenticate itself in order to use a proxy. The proxy MUST send a
Proxy-Authenticate header field (Section 4.3) containing a challenge
applicable to that proxy for the target resource. The client MAY
repeat the request with a new or replaced Proxy-Authorization header
field (Section 4.4).
4. Header Field Definitions
This section defines the syntax and semantics of header fields
related to the HTTP authentication framework.
The "WWW-Authenticate" header field indicates the authentication
scheme(s) and parameters applicable to the target resource.
WWW-Authenticate = 1#challenge
A server generating a 401 (Unauthorized) response MUST send a
WWW-Authenticate header field containing at least one challenge. A
server MAY generate a WWW-Authenticate header field in other response
messages to indicate that supplying credentials (or different
credentials) might affect the response.
A proxy forwarding a response MUST NOT modify any WWW-Authenticate
fields in that response.
User agents are advised to take special care in parsing the field
value, as it might contain more than one challenge, and each
challenge can contain a comma-separated list of authentication
parameters. Furthermore, the header field itself can occur multiple
WWW-Authenticate: Newauth realm="apps", type=1,
title="Login to \"apps\"", Basic realm="simple"
This header field contains two challenges; one for the "Newauth"
scheme with a realm value of "apps", and two additional parameters
"type" and "title", and another one for the "Basic" scheme with a
realm value of "simple".
Note: The challenge grammar production uses the list syntax as
well. Therefore, a sequence of comma, whitespace, and comma can
be considered either as applying to the preceding challenge, or to
be an empty entry in the list of challenges. In practice, this
ambiguity does not affect the semantics of the header field value
and thus is harmless.
The "Authorization" header field allows a user agent to authenticate
itself with an origin server -- usually, but not necessarily, after
receiving a 401 (Unauthorized) response. Its value consists of
credentials containing the authentication information of the user
agent for the realm of the resource being requested.
Authorization = credentials
If a request is authenticated and a realm specified, the same
credentials are presumed to be valid for all other requests within
this realm (assuming that the authentication scheme itself does not
require otherwise, such as credentials that vary according to a
challenge value or using synchronized clocks).
A proxy forwarding a request MUST NOT modify any Authorization fields
in that request. See Section 3.2 of [RFC7234] for details of and
requirements pertaining to handling of the Authorization field by
The "Proxy-Authenticate" header field consists of at least one
challenge that indicates the authentication scheme(s) and parameters
applicable to the proxy for this effective request URI (Section 5.5
of [RFC7230]). A proxy MUST send at least one Proxy-Authenticate
header field in each 407 (Proxy Authentication Required) response
that it generates.
Proxy-Authenticate = 1#challenge
Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
only to the next outbound client on the response chain. This is
because only the client that chose a given proxy is likely to have
the credentials necessary for authentication. However, when multiple
proxies are used within the same administrative domain, such as
office and regional caching proxies within a large corporate network,
it is common for credentials to be generated by the user agent and
passed through the hierarchy until consumed. Hence, in such a
configuration, it will appear as if Proxy-Authenticate is being
forwarded because each proxy will send the same challenge set.
Note that the parsing considerations for WWW-Authenticate apply to
this header field as well; see Section 4.1 for details.
The "Proxy-Authorization" header field allows the client to identify
itself (or its user) to a proxy that requires authentication. Its
value consists of credentials containing the authentication
information of the client for the proxy and/or realm of the resource
Proxy-Authorization = credentials
Unlike Authorization, the Proxy-Authorization header field applies
only to the next inbound proxy that demanded authentication using the
Proxy-Authenticate field. When multiple proxies are used in a chain,
the Proxy-Authorization header field is consumed by the first inbound
proxy that was expecting to receive credentials. A proxy MAY relay
the credentials from the client request to the next proxy if that is
the mechanism by which the proxies cooperatively authenticate a given
5. IANA Considerations
5.1. Authentication Scheme Registry
The "Hypertext Transfer Protocol (HTTP) Authentication Scheme
Registry" defines the namespace for the authentication schemes in
challenges and credentials. It has been created and is now
maintained at <http://www.iana.org/assignments/http-authschemes>.
Registrations MUST include the following fields:
o Authentication Scheme Name
o Pointer to specification text
o Notes (optional)
Values to be added to this namespace require IETF Review (see
[RFC5226], Section 4.1).
5.1.2. Considerations for New Authentication Schemes
There are certain aspects of the HTTP Authentication Framework that
put constraints on how new authentication schemes can work:
o HTTP authentication is presumed to be stateless: all of the
information necessary to authenticate a request MUST be provided
in the request, rather than be dependent on the server remembering
prior requests. Authentication based on, or bound to, the
underlying connection is outside the scope of this specification
and inherently flawed unless steps are taken to ensure that the
connection cannot be used by any party other than the
authenticated user (see Section 2.3 of [RFC7230]).
o The authentication parameter "realm" is reserved for defining
protection spaces as described in Section 2.2. New schemes MUST
NOT use it in a way incompatible with that definition.
o The "token68" notation was introduced for compatibility with
existing authentication schemes and can only be used once per
challenge or credential. Thus, new schemes ought to use the
auth-param syntax instead, because otherwise future extensions
will be impossible.
o The parsing of challenges and credentials is defined by this
specification and cannot be modified by new authentication
schemes. When the auth-param syntax is used, all parameters ought
to support both token and quoted-string syntax, and syntactical
constraints ought to be defined on the field value after parsing
(i.e., quoted-string processing). This is necessary so that
recipients can use a generic parser that applies to all
Note: The fact that the value syntax for the "realm" parameter is
restricted to quoted-string was a bad design choice not to be
repeated for new parameters.
o Definitions of new schemes ought to define the treatment of
unknown extension parameters. In general, a "must-ignore" rule is
preferable to a "must-understand" rule, because otherwise it will
be hard to introduce new parameters in the presence of legacy
recipients. Furthermore, it's good to describe the policy for
defining new parameters (such as "update the specification" or
"use this registry").
o Authentication schemes need to document whether they are usable in
origin-server authentication (i.e., using WWW-Authenticate),
and/or proxy authentication (i.e., using Proxy-Authenticate).
o The credentials carried in an Authorization header field are
specific to the user agent and, therefore, have the same effect on
HTTP caches as the "private" Cache-Control response directive
(Section 22.214.171.124 of [RFC7234]), within the scope of the request in
which they appear.
Therefore, new authentication schemes that choose not to carry
credentials in the Authorization header field (e.g., using a newly
defined header field) will need to explicitly disallow caching, by
mandating the use of either Cache-Control request directives
(e.g., "no-store", Section 126.96.36.199 of [RFC7234]) or response
directives (e.g., "private").
5.2. Status Code Registration
The "Hypertext Transfer Protocol (HTTP) Status Code Registry" located
at <http://www.iana.org/assignments/http-status-codes> has been
updated with the registrations below:
| Value | Description | Reference |
| 401 | Unauthorized | Section 3.1 |
| 407 | Proxy Authentication Required | Section 3.2 |
5.3. Header Field Registration
HTTP header fields are registered within the "Message Headers"
registry maintained at
This document defines the following HTTP header fields, so the
"Permanent Message Header Field Names" registry has been updated
accordingly (see [BCP90]).
| Header Field Name | Protocol | Status | Reference |
| Authorization | http | standard | Section 4.2 |
| Proxy-Authenticate | http | standard | Section 4.3 |
| Proxy-Authorization | http | standard | Section 4.4 |
| WWW-Authenticate | http | standard | Section 4.1 |
The change controller is: "IETF (email@example.com) - Internet
Engineering Task Force".
6. Security Considerations
This section is meant to inform developers, information providers,
and users of known security concerns specific to HTTP authentication.
More general security considerations are addressed in HTTP messaging
[RFC7230] and semantics [RFC7231].
Everything about the topic of HTTP authentication is a security
consideration, so the list of considerations below is not exhaustive.
Furthermore, it is limited to security considerations regarding the
authentication framework, in general, rather than discussing all of
the potential considerations for specific authentication schemes
(which ought to be documented in the specifications that define those
schemes). Various organizations maintain topical information and
links to current research on Web application security (e.g.,
[OWASP]), including common pitfalls for implementing and using the
authentication schemes found in practice.
6.1. Confidentiality of Credentials
The HTTP authentication framework does not define a single mechanism
for maintaining the confidentiality of credentials; instead, each
authentication scheme defines how the credentials are encoded prior
to transmission. While this provides flexibility for the development
of future authentication schemes, it is inadequate for the protection
of existing schemes that provide no confidentiality on their own, or
that do not sufficiently protect against replay attacks.
Furthermore, if the server expects credentials that are specific to
each individual user, the exchange of those credentials will have the
effect of identifying that user even if the content within
credentials remains confidential.
HTTP depends on the security properties of the underlying transport-
or session-level connection to provide confidential transmission of
header fields. In other words, if a server limits access to
authenticated users using this framework, the server needs to ensure
that the connection is properly secured in accordance with the nature
of the authentication scheme used. For example, services that depend
on individual user authentication often require a connection to be
secured with TLS ("Transport Layer Security", [RFC5246]) prior to
exchanging any credentials.
6.2. Authentication Credentials and Idle Clients
Existing HTTP clients and user agents typically retain authentication
information indefinitely. HTTP does not provide a mechanism for the
origin server to direct clients to discard these cached credentials,
since the protocol has no awareness of how credentials are obtained
or managed by the user agent. The mechanisms for expiring or
revoking credentials can be specified as part of an authentication
Circumstances under which credential caching can interfere with the
application's security model include but are not limited to:
o Clients that have been idle for an extended period, following
which the server might wish to cause the client to re-prompt the
user for credentials.
o Applications that include a session termination indication (such
as a "logout" or "commit" button on a page) after which the server
side of the application "knows" that there is no further reason
for the client to retain the credentials.
User agents that cache credentials are encouraged to provide a
readily accessible mechanism for discarding cached credentials under
6.3. Protection Spaces
Authentication schemes that solely rely on the "realm" mechanism for
establishing a protection space will expose credentials to all
resources on an origin server. Clients that have successfully made
authenticated requests with a resource can use the same
authentication credentials for other resources on the same origin
server. This makes it possible for a different resource to harvest
authentication credentials for other resources.
This is of particular concern when an origin server hosts resources
for multiple parties under the same canonical root URI (Section 2.2).
Possible mitigation strategies include restricting direct access to
authentication credentials (i.e., not making the content of the
Authorization request header field available), and separating
protection spaces by using a different host name (or port number) for
This specification takes over the definition of the HTTP
Authentication Framework, previously defined in RFC 2617. We thank
John Franks, Phillip M. Hallam-Baker, Jeffery L. Hostetler, Scott D.
Lawrence, Paul J. Leach, Ari Luotonen, and Lawrence C. Stewart for
their work on that specification. See Section 6 of [RFC2617] for
See Section 10 of [RFC7230] for the Acknowledgments related to this
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, June 2014.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 7234, June 2014.
8.2. Informative References
[BCP90] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
[OWASP] van der Stock, A., Ed., "A Guide to Building Secure Web
Applications and Web Services", The Open Web Application
Security Project (OWASP) 2.0.1, July 2005,
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Appendix A. Changes from RFCs 2616 and 2617
The framework for HTTP Authentication is now defined by this
document, rather than RFC 2617.
The "realm" parameter is no longer always required on challenges;
consequently, the ABNF allows challenges without any auth parameters.
The "token68" alternative to auth-param lists has been added for
consistency with legacy authentication schemes such as "Basic".
This specification introduces the Authentication Scheme Registry,
along with considerations for new authentication schemes.
Appendix B. Imported ABNF
The following core rules are included by reference, as defined in
Appendix B.1 of [RFC5234]: ALPHA (letters), CR (carriage return),
CRLF (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double
quote), HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any
8-bit sequence of data), SP (space), and VCHAR (any visible US-ASCII
The rules below are defined in [RFC7230]:
BWS = <BWS, see [RFC7230], Section 3.2.3>
OWS = <OWS, see [RFC7230], Section 3.2.3>
quoted-string = <quoted-string, see [RFC7230], Section 3.2.6>
token = <token, see [RFC7230], Section 3.2.6>
401 Unauthorized (status code) 6
407 Proxy Authentication Required (status code) 6
Authorization header field 8
Canonical Root URI 5
Protection Space 5
Proxy-Authenticate header field 8
Proxy-Authorization header field 9
WWW-Authenticate header field 7