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Keywords: [--------], Generic Security Service Application Program Interface, SAML 2.0
Internet Engineering Task Force (IETF) K. Wierenga
Request for Comments: 6595 Cisco Systems, Inc.
Category: Standards Track E. Lear
ISSN: 2070-1721 Cisco Systems GmbH
S. Josefsson
SJD AB
April 2012
A Simple Authentication and Security Layer (SASL) and GSS-API Mechanism
for the Security Assertion Markup Language (SAML)
Abstract
The Security Assertion Markup Language (SAML) has found its usage on
the Internet for Web Single Sign-On. The Simple Authentication and
Security Layer (SASL) and the Generic Security Service Application
Program Interface (GSS-API) are application frameworks to generalize
authentication. This memo specifies a SASL mechanism and a GSS-API
mechanism for SAML 2.0 that allows the integration of existing SAML
Identity Providers with applications using SASL and GSS-API.
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
http://www.rfc-editor.org/info/rfc6595.
Wierenga, et al. Standards Track [Page 1]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Copyright Notice
Copyright (c) 2012 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.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................4
1.2. Applicability ..............................................4
2. Authentication Flow .............................................5
3. SAML SASL Mechanism Specification ...............................7
3.1. Initial Response ...........................................8
3.2. Authentication Request .....................................8
3.3. Outcome and Parameters .....................................9
4. SAML GSS-API Mechanism Specification ...........................10
4.1. GSS-API Principal Name Types for SAML .....................11
5. Examples .......................................................11
5.1. XMPP ......................................................11
5.2. IMAP ......................................................15
6. Security Considerations ........................................17
6.1. Man-in-the-Middle and Tunneling Attacks ...................17
6.2. Binding SAML Subject Identifiers to Authorization
Identities ................................................17
6.3. User Privacy ..............................................18
6.4. Collusion between RPs .....................................18
6.5. Security Considerations Specific to GSS-API ...............18
7. IANA Considerations ............................................18
7.1. IANA Mech-Profile .........................................18
7.2. IANA OID ..................................................19
8. References .....................................................19
8.1. Normative References ......................................19
8.2. Informative References ....................................21
Appendix A. Acknowledgments .......................................22
Wierenga, et al. Standards Track [Page 2]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
1. Introduction
Security Assertion Markup Language (SAML) 2.0 [OASIS-SAMLv2-CORE] is
a set of specifications that provide various means for a user to be
identified to a Relying Party (RP) through the exchange of (typically
signed) assertions issued by an Identity Provider (IdP). It includes
a number of protocols, protocol bindings [OASIS-SAMLv2-BIND], and
interoperability profiles [OASIS-SAMLv2-PROF] designed for different
use cases.
The Simple Authentication and Security Layer (SASL) [RFC4422] is a
generalized mechanism for identifying and authenticating a user and
for optionally negotiating a security layer for subsequent protocol
interactions. SASL is used by application protocols like IMAP
[RFC3501], the Post Office Protocol (POP) [RFC1939], and the
Extensible Message and Presence Protocol (XMPP) [RFC6120]. The
effect is to make modular authentication, so that newer
authentication mechanisms can be added as needed. This memo
specifies just such a mechanism.
The Generic Security Service Application Program Interface (GSS-API)
[RFC2743] provides a framework for applications to support multiple
authentication mechanisms through a unified programming interface.
This document defines a pure SASL mechanism for SAML, but it conforms
to the new bridge between SASL and the GSS-API called GS2 [RFC5801].
This means that this document defines both a SASL mechanism and a
GSS-API mechanism. The GSS-API interface is OPTIONAL for SASL
implementers, and the GSS-API considerations can be avoided in
environments that use SASL directly without GSS-API.
As currently envisioned, this mechanism enables interworking between
SASL and SAML in order to assert the identity of the user and other
attributes to RPs. As such, while servers (as RPs) will advertise
SASL mechanisms (including SAML), clients will select the SAML SASL
mechanism as their SASL mechanism of choice.
The SAML mechanism described in this memo aims to reuse the Web
Browser Single Sign-On (SSO) profile defined in Section 4.1 of the
SAML 2.0 profiles specification [OASIS-SAMLv2-PROF] to the maximum
extent and therefore does not establish a separate authentication,
integrity, and confidentiality mechanism. The mechanism assumes that
a security layer, such as Transport Layer Security (TLS) [RFC5246],
will continue to be used. This specification is appropriate for use
when a browser instance is available. In the absence of a browser
instance, SAML profiles that don't require a browser, such as the
Enhanced Client or Proxy profile (as defined in Section 4.2 of
[OASIS-SAMLv2-PROF], may be used, but that is outside the scope of
this specification.
Wierenga, et al. Standards Track [Page 3]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Figure 1 describes the interworking between SAML and SASL: this
document requires enhancements to the RP (the SASL server) and to the
client, as the two SASL communication end points, but no changes to
the SAML IdP are necessary. To accomplish this goal, some indirect
messaging is tunneled within SASL, and some use of external methods
is made.
+-----------+
| |
>| Relying |
/ | Party |
// | |
// +-----------+
SAML/ // ^
HTTPS // +--|--+
// | S| |
/ S | A| |
// A | M| |
// S | L| |
// L | | |
// | | |
</ +--|--+
+------------+ v
| | +----------+
| SAML | HTTPS | |
| Identity |<--------------->| Client |
| Provider | | |
+------------+ +----------+
Figure 1: Interworking Architecture
1.1. Terminology
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 RFC 2119 [RFC2119].
The reader is assumed to be familiar with the terms used in the
SAML 2.0 core specification [OASIS-SAMLv2-CORE].
1.2. Applicability
Because this mechanism transports information that should not be
controlled by an attacker, the SAML mechanism MUST only be used over
channels protected by TLS, or over similar integrity-protected and
authenticated channels. In addition, when TLS is used, the client
MUST successfully validate the server's certificate ([RFC5280],
[RFC6125]).
Wierenga, et al. Standards Track [Page 4]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Note: An Intranet does not constitute such an integrity-protected and
authenticated channel!
2. Authentication Flow
While SAML itself is merely a markup language, its common use case
these days is with HTTP [RFC2616] or HTTPS [RFC2818] and HTML
[W3C-REC-HTML401]. What follows is a typical flow:
1. The browser requests a resource of an RP (via an HTTP request).
2. The RP redirects the browser via an HTTP redirect (as described
in Section 10.3 of [RFC2616]) to the IdP or an IdP discovery
service. When it does so, it includes the following parameters:
(1) an authentication request that contains the name of the
resource being requested, (2) a browser cookie, and (3) a return
URL as specified in Section 3.1 of [OASIS-SAMLv2-PROF].
3. The user authenticates to the IdP and perhaps authorizes the
release of user attributes to the RP.
4. In its authentication response, the IdP redirects (via an HTTP
redirect) the browser back to the RP with an authentication
assertion (stating that the IdP vouches that the subject has
successfully authenticated), optionally along with some
additional attributes.
5. The RP now has sufficient identity information to approve access
to the resource or not, and acts accordingly. The authentication
is concluded.
When considering this flow in the context of SASL, we note that while
the RP and the client both must change their code to implement this
SASL mechanism, the IdP can remain untouched. The RP already has
some sort of session (probably a TCP connection) established with the
client. However, it may be necessary to redirect a SASL client to
another application or handler. The steps are as follows:
1. The SASL server (RP) advertises support for the SASL SAML20
mechanism to the client.
2. The client initiates a SASL authentication with SAML20 and sends
a domain name that allows the SASL server to determine the
appropriate IdP.
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3. The SASL server transmits an authentication request encoded using
a Uniform Resource Identifier (URI) as described in RFC 3986
[RFC3986] and an HTTP redirect to the IdP corresponding to the
domain.
4. The SASL client now sends a response consisting of "=".
Authentication continues via the normal SAML flow, and the SASL
server will receive the answer to the challenge out of band from
the SASL conversation.
5. At this point, the SASL client MUST construct a URL containing
the content received in the previous message from the SASL
server. This URL is transmitted to the IdP either by the SASL
client application or an appropriate handler, such as a browser.
6. Next, the user authenticates to the IdP. The manner in which the
end user is authenticated to the IdP, and any policies
surrounding such authentication, are out of scope for SAML and
hence for this document. This step happens out of band from
SASL.
7. The IdP will convey information about the success or failure of
the authentication back to the SASL server (RP) in the form of an
authentication statement or failure, using an indirect response
via the client browser or the handler (and with an external
browser, client control should be passed back to the SASL
client). This step happens out of band from SASL.
8. The SASL server sends an appropriate SASL response to the client.
Please note: What is described here is the case in which the client
has not previously authenticated. It is possible that the client
already holds a valid SAML authentication token so that the user does
not need to be involved in the process anymore, but that would still
be external to SASL. This is classic Web Single Sign-On, in which
the Web Browser client presents the authentication token (cookie) to
the RP without renewed user authentication at the IdP.
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RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
With all of this in mind, the flow appears as follows in Figure 2:
SASL Serv. Client IdP
|>-----(1)----->| | Advertisement
| | |
|<-----(2)-----<| | Initiation
| | |
|>-----(3)----->| | Authentication Request
| | |
|<-----(4)-----<| | Response of "="
| | |
| |<- -(5,6) - ->| Client<>IdP
| | | Authentication
| | |
|<- - - - - - - - - - -(7)- - -| Authentication Statement
| | |
|>-----(8)----->| | SASL Completion with
| | | Status
| | |
----- = SASL
- - - = HTTP or HTTPS (external to SASL)
Figure 2: Authentication Flow
3. SAML SASL Mechanism Specification
This section specifies the details of the SAML SASL mechanism. See
Section 5 of [RFC4422] for additional details.
The name of this mechanism is "SAML20". The mechanism is capable of
transferring an authorization identity (via the "gs2-header"). The
mechanism does not offer a security layer.
The mechanism is client-first. The first mechanism message from the
client to the server is the "initial-response". As described in
[RFC4422], if the application protocol does not support sending a
client response together with the authentication request, the server
will send an empty server challenge to let the client begin. The
second mechanism message is from the server to the client, containing
the SAML "authentication-request". The third mechanism message is
from the client to the server and is the fixed message consisting of
"=". The fourth mechanism message is from the server to the client,
indicating the SASL mechanism outcome.
Wierenga, et al. Standards Track [Page 7]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
3.1. Initial Response
A client initiates a SAML20 authentication with SASL by sending the
GS2 header followed by the Identity Provider identifier (message 2 in
Figure 2) and is defined using ABNF [RFC5234] as follows:
initial-response = gs2-header IdP-Identifier
IdP-Identifier = domain ; domain name with corresponding IdP
The gs2-header is used as follows:
- The "gs2-nonstd-flag" MUST NOT be present.
- The "gs2-cb-flag" MUST be set to "n" because channel-binding
[RFC5056] data cannot be integrity protected by the SAML
negotiation. (Note: In theory, channel-binding data could be
inserted in the SAML flow by the client and verified by the
server, but that is currently not supported in SAML.)
- The "gs2-authzid" carries the optional authorization identity as
specified in [RFC5801] (not to be confused with the
IdP-Identifier).
A domain name is either a "traditional domain name" as described in
[RFC1035] or an "internationalized domain name" as described in
[RFC5890]. Clients and servers MUST treat the IdP-Identifier as a
domain name slot [RFC5890]. They also SHOULD support
internationalized domain names (IDNs) in the IdP-Identifier field; if
they do so, all of the domain name's labels MUST be A-labels or
NR-LDH labels [RFC5890]. If necessary, internationalized labels MUST
be converted from U-labels to A-labels by using the Punycode encoding
[RFC3492] for A-labels prior to sending them to the SASL server, as
described in the protocol specification for Internationalized Domain
Names in Applications [RFC5891].
3.2. Authentication Request
The SASL server transmits to the SASL client a URI that redirects the
SAML client to the IdP (corresponding to the domain that the user
provided), with a SAML authentication request as one of the
parameters (message 3 in Figure 2) using the following ABNF:
authentication-request = URI
The URI is specified in [RFC3986] and is encoded according to
Section 3.4 ("HTTP Redirect Binding") of the SAML 2.0 bindings
specification [OASIS-SAMLv2-BIND]. The SAML authentication request
is encoded according to Section 3.4 ("Authentication Request
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RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Protocol") of [OASIS-SAMLv2-CORE]. Should the client support
Internationalized Resource Identifiers (IRIs) [RFC3987], it MUST
first map the IRI to a URI before transmitting it to the server, as
defined in Section 3.1 of [RFC3987].
Note: The SASL server may have a static mapping of domain to
corresponding IdP or, alternatively, a DNS-lookup mechanism could be
envisioned, but that is out of scope for this document.
Note: While the SASL client MAY sanity-check the URI it received,
ultimately it is the SAML IdP that will be validated by the SAML
client; this topic is out of scope for this document.
The client then sends the authentication request via an HTTP GET
(sent over a server-authenticated TLS channel) to the IdP, as if
redirected to do so from an HTTP server and in accordance with the
Web Browser SSO profile, as described in Section 4.1 of
[OASIS-SAMLv2-PROF] (messages 5 and 6 in Figure 2).
The client handles both user authentication to the IdP and
confirmation or rejection of the authentication of the RP (out of
scope for this document).
After all authentication has been completed by the IdP, the IdP will
send a redirect message to the client in the form of a URI
corresponding to the RP as specified in the authentication request
("AssertionConsumerServiceURL") and with the SAML response as one of
the parameters (message 7 in Figure 2).
Please note: This means that the SASL server needs to implement a
SAML RP. Also, the SASL server needs to correlate the session it has
with the SASL client with the appropriate SAML authentication result.
It can do so by comparing the ID of the SAML authentication request
it has issued with the one it receives in the SAML authentication
statement.
3.3. Outcome and Parameters
The SASL server (in its capacity as a SAML RP) now validates the SAML
authentication response it received from the SAML client via HTTP or
HTTPS.
The outcome of that validation by the SASL server constitutes a SASL
mechanism outcome and therefore (as stated in [RFC4422]) SHALL be
used to set state in the server accordingly, and it SHALL be used by
the server to report that state to the SASL client, as described in
[RFC4422], Section 3.6 (message 8 in Figure 2).
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RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
4. SAML GSS-API Mechanism Specification
This section and its sub-sections are not required for SASL
implementors, but this section MUST be observed to implement the
GSS-API mechanism discussed below.
This section specifies a GSS-API mechanism that, when used via the
GS2 bridge to SASL, behaves like the SASL mechanism defined in this
document. Thus, it can loosely be said that the SAML SASL mechanism
is also a GSS-API mechanism. The SAML user takes the role of the
GSS-API Initiator, and the SAML RP takes the role of the GSS-API
Acceptor. The SAML IdP does not have a role in GSS-API and is
considered an internal matter for the SAML mechanism. The messages
are the same, but
a) the GS2 header on the client's first message and channel-binding
data are excluded when SAML is used as a GSS-API mechanism, and
b) the initial context token header (Section 3.1 of [RFC2743]) is
prefixed to the client's first authentication message (context
token).
The GSS-API mechanism OID for SAML is 1.3.6.1.5.5.17 (see Section 7.2
for more information). The DER encoding of the OID is
0x2b 0x06 0x01 0x05 0x05 0x11.
SAML20 security contexts MUST have the mutual_state flag
(GSS_C_MUTUAL_FLAG) set to TRUE. SAML does not support credential
delegation; therefore, SAML security contexts MUST have the
deleg_state flag (GSS_C_DELEG_FLAG) set to FALSE.
The mutual authentication property of this mechanism relies on
successfully comparing the TLS server's identity with the negotiated
target name. Since the TLS channel is managed by the application
outside of the GSS-API mechanism, the mechanism itself is unable to
confirm the name, while the application is able to perform this
comparison for the mechanism. For this reason, applications MUST
match the TLS server's identity with the target name, as discussed in
[RFC6125]. More precisely, to pass identity validation, the client
uses the securely negotiated targ_name as the reference identifier
and matches it to the DNS-ID of the server's certificate, and it MUST
reject the connection if there is a mismatch. For compatibility with
deployed certificate hierarchies, the client MAY also perform a
comparison with the Common Name ID (CN-ID) when there is no DNS-ID
present. Wildcard matching is permitted. The targ_name reference
identifier is a "traditional domain names"; thus, the comparison is
made using case-insensitive ASCII comparison.
Wierenga, et al. Standards Track [Page 10]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
The SAML mechanism does not support per-message tokens or the
GSS_Pseudo_random() function [RFC4401].
4.1. GSS-API Principal Name Types for SAML
SAML supports standard generic name syntaxes for acceptors such as
GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743], Section 4.1). SAML
supports only a single name type for initiators: GSS_C_NT_USER_NAME.
GSS_C_NT_USER_NAME is the default name type for SAML. The query,
display, and exported name syntaxes for SAML principal names are all
the same. There are no SAML-specific name syntaxes -- applications
should use generic GSS-API name types, such as GSS_C_NT_USER_NAME and
GSS_C_NT_HOSTBASED_SERVICE (see [RFC2743] Section 4). The exported
name token, of course, conforms to [RFC2743], Section 3.2.
5. Examples
5.1. XMPP
Suppose the user has an identity at the SAML IdP saml.example.org and
a Jabber Identifier (JID) "somenode@example.com" and wishes to
authenticate his XMPP [RFC6120] connection to xmpp.example.com. The
authentication on the wire would then look something like the
following:
Step 1: Client initiates stream to server:
<stream:stream xmlns='jabber:client'
xmlns:stream='http://etherx.jabber.org/streams'
to='example.com' version='1.0'>
Step 2: Server responds with a stream tag sent to client:
<stream:stream
xmlns='jabber:client' xmlns:stream='http://etherx.jabber.org/streams'
id='some_id' from='example.com' version='1.0'>
Step 3: Server informs client of available authentication mechanisms:
<stream:features>
<mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<mechanism>DIGEST-MD5</mechanism>
<mechanism>PLAIN</mechanism>
<mechanism>SAML20</mechanism>
</mechanisms>
</stream:features>
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RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Step 4: Client selects an authentication mechanism and provides the
initial client response -- containing the gs2-header and domain --
that has been encoded in base64 according to Section 4 of [RFC4648]:
<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='SAML20'>
biwsZXhhbXBsZS5vcmc=</auth>
The decoded string is
n,,example.org
Step 5: Server sends a base64-encoded challenge to client in the form
of an HTTP redirect to the SAML IdP corresponding to example.org
(https://saml.example.org) with the SAML authentication request as
specified in the redirection URL:
aHR0cHM6Ly9zYW1sLmV4YW1wbGUub3JnL1NBTUwvQnJvd3Nlcj9TQU1MUmVx
dWVzdD1QSE5oYld4d09rRjFkR2h1VW1WeGRXVnpkQ0I0Yld4dWN6cHpZVzFz
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RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
MjUwWlhoMFEyeGhjM05TWldZK0RRb2dQQzl6WVcxc2NEcFNaWEYxWlhOMFpX
UkJkWFJvYmtOdmJuUmxlSFErSUEwS1BDOXpZVzFzY0RwQmRYUm9ibEpsY1hW
bGMzUSs=
The decoded challenge is as follows:
https://saml.example.org/SAML/Browser?SAMLRequest=PHNhbWxwOk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Where the decoded SAMLRequest looks like the following:
<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
ID="_bec424fa5103428909a30ff1e31168327f79474984" Version="2.0"
IssueInstant="2007-12-10T11:39:34Z" ForceAuthn="false"
IsPassive="false"
ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
AssertionConsumerServiceURL=
"https://xmpp.example.com/SAML/AssertionConsumerService">
<saml:Issuer xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
https://xmpp.example.com
</saml:Issuer>
<samlp:NameIDPolicy xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
Format="urn:oasis:names:tc:SAML:2.0:nameid-format:persistent"
SPNameQualifier="xmpp.example.com" AllowCreate="true" />
<samlp:RequestedAuthnContext
Wierenga, et al. Standards Track [Page 13]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
Comparison="exact">
<saml:AuthnContextClassRef
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
</saml:AuthnContextClassRef>
</samlp:RequestedAuthnContext>
</samlp:AuthnRequest>
Note: The server can use the request ID
("_bec424fa5103428909a30ff1e31168327f79474984") to correlate the SASL
session with the SAML authentication.
Step 5 (alternative): Server returns error to client if no SAML
authentication request can be constructed:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<temporary-auth-failure/>
</failure>
</stream:stream>
Step 6: Client sends the "=" response (base64-encoded) to the
challenge:
<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
PQ==
</response>
The following steps between brackets are out of scope for this
document but are included to better illustrate the entire flow:
[The client now sends the URL to a browser instance for processing.
The browser engages in a normal SAML authentication flow (external to
SASL), like redirection to the IdP (https://saml.example.org); the
user logs into https://saml.example.org and agrees to authenticate to
xmpp.example.com. A redirect is passed back to the client browser.
The client browser in turn sends the AuthN response, which contains
the subject-identifier as an attribute, to the server. If the AuthN
response doesn't contain the JID, the server maps the subject-
identifier received from the IdP to a JID.]
Step 7: Server informs client of successful authentication:
<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>
Wierenga, et al. Standards Track [Page 14]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Step 7 (alternative): Server informs client of failed authentication:
<failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
<not-authorized/>
</failure>
</stream:stream>
Please note: Line breaks were added to the base64 data for clarity.
5.2. IMAP
The following sequence describes an IMAP exchange. Lines beginning
with 'S:' indicate data sent by the server, and lines starting with
'C:' indicate data sent by the client. Long lines are wrapped for
readability.
S: * OK IMAP4rev1
C: . CAPABILITY
S: * CAPABILITY IMAP4rev1 STARTTLS
S: . OK CAPABILITY Completed
C: . STARTTLS
S: . OK Begin TLS negotiation now
C: . CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=SAML20
S: . OK CAPABILITY Completed
C: . AUTHENTICATE SAML20
S: +
C: biwsZXhhbXBsZS5vcmc=
S: + aHR0cHM6Ly9zYW1sLmV4YW1wbGUub3JnL1NBTUwvQnJvd3Nlcj9TQU1M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, et al. Standards Track [Page 15]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012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C: PQ==
S: . OK Success (TLS protection)
The decoded challenge is as follows:
https://saml.example.org/SAML/Browser?SAMLRequest=PHNhbWxwOkF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RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Where the decoded SAMLRequest looks like the following:
<samlp:AuthnRequest xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
ID="_bec424fa5103428909a30ff1e31168327f79474984" Version="2.0"
IssueInstant="2007-12-10T11:39:34Z" ForceAuthn="false"
IsPassive="false"
ProtocolBinding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST"
AssertionConsumerServiceURL=
"https://mail.example.com/SAML/AssertionConsumerService">
<saml:Issuer xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
https://xmpp.example.com
</saml:Issuer>
<samlp:NameIDPolicy xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
Format="urn:oasis:names:tc:SAML:2.0:nameid-format:persistent"
SPNameQualifier="xmpp.example.com" AllowCreate="true" />
<samlp:RequestedAuthnContext
xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
Comparison="exact">
<saml:AuthnContextClassRef
xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion">
urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport
</saml:AuthnContextClassRef>
</samlp:RequestedAuthnContext>
</samlp:AuthnRequest>
6. Security Considerations
This section addresses only security considerations associated with
the use of SAML with SASL applications. For considerations relating
to SAML in general, and for general SASL security considerations, the
reader is referred to the SAML specifications and to other
literature.
6.1. Man-in-the-Middle and Tunneling Attacks
This mechanism is vulnerable to man-in-the-middle and tunneling
attacks unless a client always verifies the server's identity before
proceeding with authentication (see [RFC6125]). Typically, TLS is
used to provide a secure channel with server authentication.
6.2. Binding SAML Subject Identifiers to Authorization Identities
As specified in [RFC4422], the server is responsible for binding
credentials to a specific authorization identity. It is therefore
necessary that only specific trusted IdPs be allowed. This is a
typical part of SAML trust establishment between RPs and the IdP.
Wierenga, et al. Standards Track [Page 17]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
6.3. User Privacy
The IdP is aware of each RP that a user logs into. There is nothing
in the protocol to hide this information from the IdP. It is not a
requirement to track the visits, but there is nothing that prohibits
the collection of information. SASL server implementers should be
aware that SAML IdPs will be able to track -- to some extent -- user
access to their services.
6.4. Collusion between RPs
It is possible for RPs to link data that they have collected on the
users. By using the same identifier to log into every RP, collusion
between RPs is possible. In SAML, targeted identity was introduced.
Targeted identity allows the IdP to transform the identifier the user
typed in to an RP-specific opaque identifier. This way, the RP would
never see the actual user identifier but instead would see a randomly
generated identifier.
6.5. Security Considerations Specific to GSS-API
Security issues inherent in GSS-API [RFC2743] and GS2 [RFC5801] apply
to the SAML GSS-API mechanism defined in this document. Further, and
as discussed in Section 4, proper TLS server identity verification is
critical to the security of the mechanism.
7. IANA Considerations
7.1. IANA Mech-Profile
The IANA has registered the following SASL profile:
SASL mechanism profile: SAML20
Security Considerations: See this document
Published Specification: See this document
For further information: Contact the authors of this document.
Owner/Change controller: the IETF
Intended usage: COMMON
Note: None
Wierenga, et al. Standards Track [Page 18]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
7.2. IANA OID
The IANA has also assigned a new entry for this GSS mechanism in the
SMI Security for Mechanism Codes sub-registry, whose prefix is
iso.org.dod.internet.security.mechanisms (1.3.6.1.5.5), and
referenced this specification in the registry.
8. References
8.1. Normative References
[OASIS-SAMLv2-BIND]
Cantor, S., Ed., Hirsch, F., Ed., Kemp, J., Ed., Philpott,
R., Ed., and E. Maler, Ed., "Bindings for the OASIS
Security Assertion Markup Language (SAML) V2.0", OASIS
Standard saml-bindings-2.0-os, March 2005, <http://
docs.oasis-open.org/security/saml/v2.0/
saml-bindings-2.0-os.pdf>.
[OASIS-SAMLv2-CORE]
Cantor, S., Ed., Kemp, J., Ed., Philpott, R., Ed., and E.
Maler, Ed., "Assertions and Protocols for the OASIS
Security Assertion Markup Language (SAML) V2.0", OASIS
Standard saml-core-2.0-os, March 2005, <http://
docs.oasis-open.org/security/saml/v2.0/
saml-core-2.0-os.pdf>.
[OASIS-SAMLv2-PROF]
Hughes, J., Ed., Cantor, S., Ed., Hodges, J., Ed., Hirsch,
F., Ed., Mishra, P., Ed., Philpott, R., Ed., and E. Maler,
Ed., "Profiles for the OASIS Security Assertion Markup
Language (SAML) V2.0", OASIS Standard OASIS.saml-profiles-
2.0-os, March 2005, <http://docs.oasis-open.org/security/
saml/v2.0/saml-profiles-2.0-os.pdf>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
Wierenga, et al. Standards Track [Page 19]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC4422] Melnikov, A., Ed., and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
June 2006.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[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, May 2008.
[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, July 2010.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
Wierenga, et al. Standards Track [Page 20]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[W3C-REC-HTML401]
Le Hors, A., Ed., Raggett, D., Ed., and I. Jacobs, Ed.,
"HTML 4.01 Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.
8.2. Informative References
[RFC1939] Myers, J. and M. Rose, "Post Office Protocol - Version 3",
STD 53, RFC 1939, May 1996.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL -
VERSION 4rev1", RFC 3501, March 2003.
[RFC4401] Williams, N., "A Pseudo-Random Function (PRF) API
Extension for the Generic Security Service Application
Program Interface (GSS-API)", RFC 4401, February 2006.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.
Wierenga, et al. Standards Track [Page 21]
RFC 6595 A SASL and GSS-API Mechanism for SAML April 2012
Appendix A. Acknowledgments
The authors would like to thank Scott Cantor, Joe Hildebrand, Josh
Howlett, Leif Johansson, Thomas Lenggenhager, Diego Lopez, Hank
Mauldin, RL "Bob" Morgan, Stefan Plug, and Hannes Tschofenig for
their review and contributions.
Authors' Addresses
Klaas Wierenga
Cisco Systems, Inc.
Haarlerbergweg 13-19
1101 CH Amsterdam
The Netherlands
Phone: +31 20 357 1752
EMail: klaas@cisco.com
Eliot Lear
Cisco Systems GmbH
Richtistrasse 7
CH-8304 Wallisellen
Switzerland
Phone: +41 44 878 9200
EMail: lear@cisco.com
Simon Josefsson
SJD AB
Johan Olof Wallins vag 13
Solna 171 64
Sweden
EMail: simon@josefsson.org
URI: http://josefsson.org/
Wierenga, et al. Standards Track [Page 22]