Internet DRAFT - draft-vanrein-httpauth-sasl
draft-vanrein-httpauth-sasl
Network Working Group R. Van Rein
Internet-Draft ARPA2.net
Intended status: Standards Track 8 November 2022
Expires: 12 May 2023
HTTP Authentication with SASL
draft-vanrein-httpauth-sasl-08
Abstract
Most application-level protocols standardise their authentication
exchanges under the SASL framework. HTTP has taken another course,
and often ends up replicating the work to allow individual
mechanisms. This specification adopts full SASL authentication into
HTTP.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 12 May 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Embedding SASL in HTTP . . . . . . . . . . . . . . . . . . . 3
2.1. HTTP Request and Response Messages . . . . . . . . . . . 4
2.2. Authentication Field Definitions . . . . . . . . . . . . 5
2.3. Caching Authentication Results . . . . . . . . . . . . . 6
3. Server-Side User Name . . . . . . . . . . . . . . . . . . . . 6
4. Authentication Session Example . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. HTTP Server Environment Variables . . . . . . . . . 13
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
HTTP has historically followed its own path for client
authentication, while many other end-user protocols standardised on
SASL; examples of SASL protocols include SMTP, IMAP, POP, XMPP, LDAP,
AMQP and MQTT. This specification introduces SASL to HTTP, so it may
share in past and future work done for SASL in general.
Among the work that could be shared is backend authentication
integration, which is possible due to protocol-independent SASL
exchanges for any given method, making it easy to take them out of
one protocol and inserting them into another. Although HTTP has
adopted several SASL-compatible authentication methods, it uses
various notations and so it still needs method-specific support at
the HTTP level to translate them to a SASL backend.
In front-ends, a similar situation has arisen. The varying syntaxes
for authentication methods have made it difficult to rely on support
in most or all HTTP clients. When such clients could externalise
their SASL handling to generic software such as a SASL library, then
any extension to a library automatically spills over into the HTTP
sphere. It is common for developers of web clients to also produce
email clients, so a shared code base (and credential store) is not
difficult to imagine.
Sharing of authentication mechanisms is beneficial in both
directions. HTTP benefits by being able to use anything from strong
password mechanisms [RFC5802] without explicit support [RFC7804] in
applications, up to GS2 mechanisms [RFC5801] with channel binding
[RFC5056] [RFC5554] to TLS [RFC5929] based on pinning either the
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certificate for the TLS server or even a unique part of the
individual TLS connection; for instance Kerberos5 [RFC4120] currently
uses Negotiate authentication [RFC4559] which is not as secure as
GS2-KRB5-PLUS over SASL.
SASL also benefits; had it been the norm for HTTP, then the work to
pass SAML over it [RFC6595] would probably have been done
immediately. In fact, HTTP can still benefit from receiving
standardised SAML20 inquiries over SASL, because it resolves the need
for configuration of initiation paths and practices. Also, it
removes authentication data from URIs, where they are not ideally
placed.
In terms of security for HTTP applications, it appears beneficial to
have very good authentication capabilities in the layers below the
application; this is specifically true for applications developed in
HTML and JavaScript, which tend to load code from various places,
including code that is not always in the end user's interest; since
it already is a concern what identity information passes through
these applications, it is not advisable to use credentials in those
places. The HTTP layer is in a better position to take control over
these assets, at the protocol levels of HTTP and TLS, and conceal
credentials and possibly also identity from applications running on
top. Inasfar as tokens are needed, they can be derived from session
keys using generally accepted key derivation schemes, but the session
keys can be isolated from dynamic layers above HTTP.
2. Embedding SASL in HTTP
This specification integrates the SASL framework [RFC4422] into
mainstream HTTP [RFC7231], [RFC7232]. The SASL Authentication scheme
follows the general structure for HTTP Authentication [RFC7235]. It
uses the WWW-Authenticate and Proxy-Authenticate headers in responses
from web servers and web proxies, respectively, and correspondingly
the Authorization and Proxy-Authorization request header to answer to
requests.
The SASL service name for the following embedding of SASL is HTTP;
contrary to most other service names, it is spelled in uppercase, in
line with what has become general practice in Kerberos and GSSAPI.
Since SASL prescribes channel binding to occur relative to TLS
instead of to the application protocol, we can add that when the
HTTPS transport is used. Whether channel binding is used SHOULD
remain a configuration choice in HTTP software, as it might interfere
with intentional HTTPS proxying. Unintended proxying on the other
hand, might lead to tapping of credentials under certain SASL
mechanisms, and it may be considered helpful to prevent such
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situations by requiring channel binding for those situations. HTTP
in general allows a user session to hop between connections, and
browsers are likely to do this; to support this, the support of tls-
server-end-point channel binding [Section 4 of [RFC5929]] is
RECOMMENDED. Specific HTTP clients may exercise more control over
connections to achieve stronger security; for those use cases, tls-
unique channel binding [Section 3 of [RFC5929]] is RECOMMENDED.
Generic web servers SHOULD support both forms of channel binding.
2.1. HTTP Request and Response Messages
This section defines a few names for HTTP request and response
messages, to be used in the remainder of this specification.
Initial Responses are HTTP responses that normally set a status code
401 or 407, and that are sent when the HTTP server decides to
initiate an authentication exchange. In addition, the server MAY
send Initial Responses in other responses, to indicate to the client
that it MAY try again to achieve better results [Section 4.1 of
[RFC7235]].
Initial Requests are those HTTP requests that a client sends to
initiate a fresh SASL authentication. The identity SHOULD be
selected by the user independently from the URI; prior settings MAY
however be remembered by a client for the combination of resource
authority (scheme, host and possibly a separately communicated
resource user name) with the server-sent realm string. The server
can support a mixture of client identities for various roles or
access levels through variation of realm strings. There is no
current practice of server-side resource names in HTTP, but the
generic URI schema presents this logic and it is easy to imagine an
HTTP User header that a client could support.
Intermediate Responses are HTTP responses to SASL authentication,
with a status code set to 401 or 407. Intermediate Requests are
those HTTP requests that a client sends to continue a SASL
authentication after an Intermediate Response.
Positive Responses set a 200 status code to depict success.
Information in this response is provided in an Authentication-Info or
Proxy-Authentication-Info header [RFC7615] instead of the headers
used in Initial Responses and Intermediate Responses [RFC7235].
Proper interpretation of a Positive Response requires client state
indicating that SASL authentication was used, or else the optional
fields are not completely reliable information sources; cryptographic
markers in the c2c field MAY be used to overcome this in a manner
that defies abuse by rogue servers.
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Negative Responses also set a 401 or 407 status code and will often
return the client to an earlier state that it recognises as one it
has tried before. These responses should therefore offer
authentication to start again. In contrast to the Initial Response,
there is now a c2c field that helps the client evaluate the request.
The following fields, defined in upcoming sections, MUST and MAY be
present in HTTP authentication exchanges for SASL:
Request or Response | MUST have fields | MAY have fields
----------------------+------------------+-----------------
Initial Response | s2s,mech | realm
Initial Request | mech | c2s,realm,s2s
Intermediate Response | s2s | s2c
Intermediate Request | s2s | c2s
Positive Response | | s2s
Negative Response | s2s,mech | realm
2.2. Authentication Field Definitions
Data for SASL is transported in the following fields:
c2s holds SASL token data from client to server. This field is
transmitted with base64 encoding. The field is absent when the
SASL client sends no token.
s2c holds SASL token data from server to client. This field is
transmitted with base64 encoding. The field is absent when the
SASL server sends no token.
s2s holds opaque server data which the client MUST reflect in
Intermediate Requests and, when responding to an Initial
Response, in the Initial Request. This is a necessity for a
stateless HTTP Authentication framework [Section 5.1.2 of
[RFC7235]]. It MAY be used in a Positive Response to pass a
cacheable Section 2.3 authentication token in a future Initial
Request.
The following fields support SASL within the HTTP Authentication
Framework:
realm optionally names a scope of authorisation under the
combination of scheme, server host name and possibly a HTTP
user to implement the semantics of the generic URI username for
resource selection. The realm does not necessarily match a
domain name, which is used elsewhere as a realm notation.
mech In an Initial Response, the field is filled with a space-
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separated list of SASL mechanism names; In an Initial Request,
the client chooses one SASL mechanism name.
2.3. Caching Authentication Results
When an HTTP server sends a Positive Response, it MAY include an
"s2s" field. If it does this, then it should be prepared to accept
the field value for authentication in an Initial Request. However,
credentials can expire or fall in disgrace for other reasons, so the
server MAY still choose to reject the provided field.
When an HTTP client receives a Positive Response with an "s2s" field,
it MAY memorise the fields for future reuse in an Initial Request,
either with or without preceding Initial Response from the server.
The HTTP client MUST use the realm as part of the decision which
cached result to use, but it MAY extrapolate the results from one
resource retrieval in an attempt to authenticate another.
When cached fields result in a Negative Response then the HTTP client
SHOULD remove the failing cache entry, and it SHOULD try again by
going through a full SASL authentication cycle. The stateless nature
of HTTP authentication is helpful in the sense that a new Initial
Request can be sent to an older Initial Response.
3. Server-Side User Name
HTTP does not define a mechanism to specifically select the user as
an authoritative resource name space on the server. Local syntax
conventions exist, but lack universally reliable semantics. Basic
authentication has been used to this effect, but this conflates the
client identity with the server-side name space, which is not
necessarily the same.
To allow HTTP servers to zoom in on user-specific information, the
User header is hereby introduced. Its syntax matches the userinfo
part of a URI, up to but excluding any colons in it:
User = *( unreserved / pct-encoded / sub-delims )
The value of the header MUST be percent-decoded before the server can
use it to identify a local user.
The User header MAY be sent by clients, and HTTP servers MAY ignore
it for any reason, including local user identities that do not comply
to a more restrictive local user name syntax.
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When an HTTP server makes use of the User header, it MUST include a
Vary header in its response, with either a single "*" in it or the
name "User". This informs caches that the response must be
considered specific to the User header value in the matching request.
The User header may be used with or without any form of
authentication. When used with authentication, the value of the
percent-decoded header is considered part of the authority component
of the resource, and therefore of the naming scope for the realm.
Clients can use this refined notion of realm to select an
authentication identity; when the value is known early enough, this
may even help to select an X.509 client certificate. Note that the
User header might be used together with the aforementioned practice
of Basic authentication, but it can also replace it with an even
simpler mechanism to free up the authentication exchange for HTTP
SASL.
The distinction of a client-side user from a server-side user can
benefit the use of credential schemes that are not tied to the HTTP
server. A specific example of this is the current work on realm
crossover with GS2-SXOVER-PLUS. The use of such a mechanism may
offload security concerns from the application layer.
4. Authentication Session Example
This section is non-normative.
When an HTTP server receives a request for a protected page, it will
send an Initial Response to ask for authentication with a special
status code 401; for proxy access that would be 407, and header names
change accordingly. Stripped down to the bare essentials, the server
sends (this section adds whitespace for clarity)
HTTP/1.1 401 Unauthorized
WWW-Authenticate: SASL
realm="members only",
mech="SCRAM-SHA-256 SCRAM-SHA-256-PLUS
SCRAM-SHA-1 SCRAM-SHA-1-PLUS
GS2-KRB5-PLUS GS2-KRB5",
s2s="[xxxxx]"
The server offers SCRAM-* and GS2-KRB5 mechanisms. The variants with
-PLUS provide additional channel binding, to ensure that
authentication is specific to the current HTTPS connection, thus
avoiding replay of the session across connections. Clients aware of
HTTP connections may use connection-specific channel binding (tls-
unique) while those that abstract from the connections must resort to
weaker name-based channel binding (tls-server-end-point).
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The server might have additionally offered the ANONYMOUS mechanism to
allow the client to select "guest mode" access; the interaction would
continue as authenticated, but presumably with limited access to HTTP
resources and continued WWW-Authenticate headers to continue to offer
authentication to improve resource information content. The server
might have offered EXTERNAL to allow the client to incorporate a TLS
credential for authentication and possibly change to an authorization
identity. The server might have offered GS2-SXOVER-PLUS if it is
willing to connect to the client's home realm over Diameter, and
thereby support realm crossover of SASL credentials.
The client initiates the SCRAM-SHA-256-PLUS mechanism, and to that
end sends an Initial Request (this section shows square brackets
around text that is transmitted with base64-encoding)
Authorization: SASL
realm="members only",
mech="SCRAM-SHA-256-PLUS",
c2s="[n,,n=user,r=rOprNGfwEbeRWgbNEkqO]",
s2s="[xxxxx]"
This mechanism is initiated by the client, hence the inclusion of the
c2s token in the Initial Request. The contents of this field are
specific to the selected mechanism, so SCRAM-SHA-256-PLUS in this
case.
The SCRAM mechanism implementation is now initiated with the c2s
token, and the server produces a followup challenge in a s2c token.
To be able to validate future client messages against server-side
state, it includes such state in an s2s token. This token is
presumably protected from abuse with a signature and/or encryption,
and it would likely identify the selected mechanism to validate
during later rounds. The server packs all this in an Intermediate
Response
HTTP/1.1 401 Unauthorized
WWW-Authentication: SASL
s2c="[r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxF
Ilj)hNlF$k0,s=W22ZaJ0SNY7soEsUEjb6gQ==,
i=4096]",
s2s="[yyyyy]"
Given that all server state is contained in this message, the client
is free at any time to give up authentication and perhaps try another
method. Normally however, it would proceed with the ongoing
transaction.
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The SCRAM mechanism continues with another round. The client engages
in the prescribed cryptographic computations and packs an
Intermediate Request along with updated state in the new c2s token
Authorization: SASL
c2s="[c=biws,r=rOprNGfwEbeRWgbNEkqO%hvYDpWUa2RaTCAfuxFIlj)hN
lF$k0,p=dHzbZapWIk4jUhN+Ute9ytag9zjfMHgsqmmiz7AndVQ=]",
s2s="[yyyyy]"
When the client has performed authentication properly, as determined
by a server-side check of the c2s response token with the prior state
in the s2s token, it can send a Positive Response along with the
requested resource
HTTP/1.1 200 OK
WWW-Authentication: SASL
s2c="[v=6rriTRBi23WpRR/wtup+mMhUZUn/dB5nLTJRsjl95G4=]",
s2s="[zzzzz]"
The s2s token in a Positive Response is an optional extension. It is
presented by the server to allow the client to speed up
authentication in future requests. The client may send it whenever
the server asks for the same realm string under the same scheme and
authority; the client may make proactive assumpions about the realm
string for new requests. Authentication must never be reused in
another context than bound by channel binding. When used, the client
immediate sends an Intermediate Response holding
Authorization: SASL
realm="members only",
s2s="[zzzzz]"
The server always has an option to refuse repeated authentication and
forcing the client into a new authentication round. One reason for
this could be that a session timed out. Another might be that the
client is trying to use a credential outside a scope set by channel
binding.
5. Security Considerations
It is not generally safe for SASL mechanisms to exchange c2s and s2c
messages over unprotected transports. Furthermore, the SASL exchange
may be at risk of tampering when the sequence of HTTP messages is not
secured to form one stream. This means that a secure transport layer
must be used, like TLS. The termination of such a secure layer MUST
also terminate any ongoing SASL handshakes.
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The s2s field MUST be protected against tampering by rogue peers, and
such protection also protects against tampering by rogue
intermediates when using an unprotected transport. In addition, but
dependent on the mechanism used, the s2s field may also need
encryption to conceal their data from peers and intermediates.
SASL EXTERNAL can be a very efficient mechanism to combine with a
secure transport layer if that includes authentication. This may be
the case for TLS, especially when client-side authentication is
deployed. Mechanisms other than EXTERNAL should take into account
that a relation may exist between identities negotiated in the
protective layer and the SASL exchange over HTTP. For example, a
login account may be exchanged for an alias or group identity.
Channel binding is available in some SASL mechanisms. When used with
HTTP SASL over TLS, it binds to the TLS channel. When doing so, it
is vital that either there be no renegotiation of the TLS handshake,
or both secure renegotiation [RFC5746] and the extended master secret
[RFC7627] are used.
The User header field as defined herein is orthogonal to issues of
authentication and authorisation, and adds no security concerns.
6. IANA Considerations
This specification extends the "Hypertext Transfer Protocol (HTTP)
Authentication Scheme Registry" with an "Authentication Scheme Name"
SASL, referencing this specification.
This specification defines an additional entry in the registry
"Generic Security Service Application Program Interface
(GSSAPI)/Kerberos/Simple Authentication and Security Layer (SASL)
Service Names" namely:
Service Name: HTTP
Usage: Web authentication using the SASL framework
Reference: TBD:this specification
The capitalisation of the service name has historic origins and is
now the preferred spelling for reasons of compatibility.
Please add the following entry to the Message Headers registry:
Header Field Name Template Protocol Status Reference
------------------ --------- --------- ------- ----------
User http TBD TBD:THIS_SPEC
7. References
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7.1. Normative References
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<https://www.rfc-editor.org/info/rfc4120>.
[RFC4559] Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
Kerberos and NTLM HTTP Authentication in Microsoft
Windows", RFC 4559, DOI 10.17487/RFC4559, June 2006,
<https://www.rfc-editor.org/info/rfc4559>.
[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<https://www.rfc-editor.org/info/rfc4422>.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, DOI 10.17487/RFC5056, November 2007,
<https://www.rfc-editor.org/info/rfc5056>.
[RFC5554] Williams, N., "Clarifications and Extensions to the
Generic Security Service Application Program Interface
(GSS-API) for the Use of Channel Bindings", RFC 5554,
DOI 10.17487/RFC5554, May 2009,
<https://www.rfc-editor.org/info/rfc5554>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<https://www.rfc-editor.org/info/rfc5746>.
[RFC5801] Josefsson, S. and N. Williams, "Using Generic Security
Service Application Program Interface (GSS-API) Mechanisms
in Simple Authentication and Security Layer (SASL): The
GS2 Mechanism Family", RFC 5801, DOI 10.17487/RFC5801,
July 2010, <https://www.rfc-editor.org/info/rfc5801>.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
<https://www.rfc-editor.org/info/rfc5929>.
[RFC6595] Wierenga, K., Lear, E., and S. Josefsson, "A Simple
Authentication and Security Layer (SASL) and GSS-API
Mechanism for the Security Assertion Markup Language
(SAML)", RFC 6595, DOI 10.17487/RFC6595, April 2012,
<https://www.rfc-editor.org/info/rfc6595>.
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[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
DOI 10.17487/RFC7232, June 2014,
<https://www.rfc-editor.org/info/rfc7232>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<https://www.rfc-editor.org/info/rfc7235>.
[RFC7615] Reschke, J., "HTTP Authentication-Info and Proxy-
Authentication-Info Response Header Fields", RFC 7615,
DOI 10.17487/RFC7615, September 2015,
<https://www.rfc-editor.org/info/rfc7615>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<https://www.rfc-editor.org/info/rfc7627>.
7.2. Informative References
[RFC5802] Newman, C., Menon-Sen, A., Melnikov, A., and N. Williams,
"Salted Challenge Response Authentication Mechanism
(SCRAM) SASL and GSS-API Mechanisms", RFC 5802,
DOI 10.17487/RFC5802, July 2010,
<https://www.rfc-editor.org/info/rfc5802>.
[RFC7804] Melnikov, A., "Salted Challenge Response HTTP
Authentication Mechanism", RFC 7804, DOI 10.17487/RFC7804,
March 2016, <https://www.rfc-editor.org/info/rfc7804>.
[I-D.vanrein-dnstxt-krb1]
van Rein, R., "Declaring Kerberos Realm Names in DNS
(_kerberos TXT)", Work in Progress, Internet-Draft, draft-
vanrein-dnstxt-krb1-10, 21 October 2022,
<https://www.ietf.org/archive/id/draft-vanrein-dnstxt-
krb1-10.txt>.
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Appendix A. HTTP Server Environment Variables
We define a number of variables that SHOULD be passed from an HTTP
SASL stack (and from User header processing) to applications run on
top of it. The intention of defining these is to obtain maximum
interoperability between these layers of software.
The following variables MUST NOT be available until SASL
authentication is successful; it would be available when the server
could send a 200 OK response:
SASL_SECURE is only "yes" (without the quotes) when a client is
authenticated to the current resource. It never has another
value; it is simply undefined when not secured by SASL.
SASL_REALM is the realm for which the secure exchange succeeded. A
realm is not always used, because sites only need it when there
are more than one in the same name space. When undefined in
the SASL flow, this variable will not be set.
REMOTE_USER is the client identity as confirmed through SASL
authentication. Its content is formatted like an email
address, and includes a domain name. That domain need not be
related to the web server; it is possible for a web server to
welcome foreign clients.
SASL_MECH indicates the mechanism used, and is one of the
standardised SASL mechanism names. It may be used to detect
the level of security.
SASL_S2S holds the accepted s2s field, and could be used as a random
session identifier. It would normally be encrypted
information.
SASL_S2S_ is a prefix for extra information that the server may
extract from the s2s field in the HTTP SASL protocol flow.
This depends on the authentication stack used in the web
server.
The following variable SHOULD be available while processing a request
with a User header with locally acceptable syntax:
LOCAL_USER gives the HTTP User header value after syntax checking
and percent-decoding. If used at all, it MUST be treated as a
resource name space selector. This header does not describe
the authenticated client identity, which is usually passed in a
variable REMOTE_USER.
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Appendix B. Acknowledgements
Thanks to Henri Manson for making the first implementation of this
specification and for feedback on the header formats. The
specification also benefited from input by Daniel Stenberg.
This work was supported with an open source development fund from
NLNet.
Author's Address
Rick van Rein
ARPA2.net
Haarlebrink 5
Enschede
Email: rick@openfortress.nl
Van Rein Expires 12 May 2023 [Page 14]
