Internet Engineering Task Force (IETF) E. Lear

Internet Engineering Task Force (IETF) E. Lear
Request for Comments: 6616 Cisco Systems GmbH
Category: Standards Track H. Tschofenig
ISSN: 2070-1721 Nokia Siemens Networks
H. Mauldin
Cisco Systems, Inc.
S. Josefsson
SJD AB
May 2012

A Simple Authentication and Security Layer (SASL) and
Generic Security Service Application Program Interface (GSS-API)
Mechanism for OpenID

Abstract

OpenID has found its usage on the Internet for Web Single Sign-On.
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 and GSS-API mechanism for OpenID that allows the
integration of existing OpenID 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/rfc6616.

Lear, et al. Standards Track [Page 1]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 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. Applicability for Application Protocols other than HTTP . . . 4
2.1. Binding SASL to OpenID in the Relying Party . . . . . . . 7
2.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . . 8
3. OpenID SASL Mechanism Specification . . . . . . . . . . . . . 8
3.1. Initiation . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Authentication Request . . . . . . . . . . . . . . . . . . 9
3.3. Server Response . . . . . . . . . . . . . . . . . . . . . 10
3.4. Error Handling . . . . . . . . . . . . . . . . . . . . . . 11
4. OpenID GSS-API Mechanism Specification . . . . . . . . . . . . 11
4.1. GSS-API Principal Name Types for OpenID . . . . . . . . . 12
5. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6.1. Binding OpenIDs to Authorization Identities . . . . . . . 14
6.2. RP Redirected by Malicious URL to Take an Improper
Action . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3. User Privacy . . . . . . . . . . . . . . . . . . . . . . . 14
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 17

Lear, et al. Standards Track [Page 2]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

1. Introduction

OpenID 2.0 [OpenID] is a web-based three-party protocol that provides
a means for a user to offer identity assertions and other attributes
to a web server (Relying Party) via the help of an identity provider.
The purpose of this system is to provide a way to verify that an end
user controls an identifier.

Simple Authentication and Security Layer (SASL) [RFC4422] is used by
application protocols such as IMAP [RFC3501], Post Office Protocol
(POP) [RFC1939], and Extensible Messaging and Presence Protocol
(XMPP) [RFC6120], with the goal of modularizing authentication and
security layers, so that newer 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 interface. This document
defines a pure SASL mechanism for OpenID, 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. Implementors of the SASL component MAY implement the GSS-
API interface as well.

This mechanism specifies interworking between SASL and OpenID in
order to assert identity and other attributes to Relying Parties. As
such, while SASL servers (as Relying Parties) will advertise SASL
mechanisms, clients will select the OpenID mechanism.

The OpenID mechanism described in this memo aims to reuse the OpenID
mechanism to the maximum extent and therefore does not establish a
separate authentication, integrity, and confidentiality mechanism.
It is anticipated that existing security layers, such as Transport
Layer Security (TLS) [RFC5246], continue to be used. Minimal changes
are required to non-web applications, as most of the transaction
occurs through a normal web browser. Hence, this specification is
only appropriate for use when such a browser is available.

Figure 1 describes the interworking between OpenID and SASL. This
document requires enhancements to the Relying Party and to the Client
(as the two SASL communication end points), but no changes to the
OpenID Provider (OP) are necessary. To accomplish this goal,
indirect messaging required by the OpenID specification is tunneled
through the SASL/GSS-API mechanism.

Lear, et al. Standards Track [Page 3]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

+-----------+
| Relying |
>| Party / |
/ | SASL |
// | Server |
// +-----------+
// ^
OpenID // +--|--+
// | O| | G
/ S | p| | S
// A | e| | S
// S | n| | A
// L | I| | P
// | D| | I
</ +--|--+
+------------+ v
| | +----------+
| OpenID | OpenID | |
| Provider |<--------------->| Client |
| | | |
+------------+ +----------+

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
OpenID 2.0 specification.

1.2. Applicability

Because this mechanism transports information that should not be
controlled by an attacker, the OpenID mechanism MUST only be used
over channels protected by TLS, and the client MUST successfully
validate the server certificate [RFC5280][RFC6125].

2. Applicability for Application Protocols other than HTTP

OpenID was originally envisioned for HTTP- [RFC2616] and HTML-based
[W3C.REC-html401-19991224] communications, and with the associated
semantic; the idea being that the user would be redirected by the
Relying Party (RP) to an identity provider (IdP) who authenticates
the user and then sends identity information and other attributes
(either directly or indirectly) to the Relying Party. The identity

Lear, et al. Standards Track [Page 4]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

provider in the OpenID specifications is referred to as an OpenID
Provider (OP). The actual protocol flow can be found in Section 3 of
the OpenID 2.0 specification [OpenID]. The reader is strongly
encouraged to be familiar with that specification before continuing.

When considering that flow in the context of SASL, we note that while
the RP and the client both need to change their code to implement
this SASL mechanism, it is a design constraint that the OP behavior
remain untouched, in order for implementations to interoperate with
existing IdPs. Hence, an analog flow that interfaces the three
parties needs to be created. In the analog, we note that unlike a
web server, the SASL server already has some sort of session
(probably a TCP connection) established with the client. However, it
may be necessary for a SASL client to invoke to another application.
This will be discussed below. By doing so, we externalize much of
the authentication from SASL.

The steps are listed below:

1. The SASL server advertises support for the SASL OpenID mechanism
to the client.

2. The client initiates a SASL authentication and transmits the
User-Supplied Identifier as its first response. The SASL
mechanism is client-first, and, as explained in [RFC4422], the
server will send an empty challenge if needed.

3. After normalizing the User-Supplied Identifier as discussed in
[OpenID], the Relying Party performs discovery on it and
establishes the OP Endpoint URL that the end user uses for
authentication.

4. The Relying Party and the OP optionally establish an association
-- a shared secret established using Diffie-Hellman Key
Exchange. The OP uses an association to validate those messages
through the use of a Hashed Message Authentication Code (HMAC);
this removes the need for subsequent direct requests to verify
the signature after each authentication request/response.

5. The Relying Party transmits an authentication request to the OP
to obtain an assertion in the form of an indirect request.
These messages are passed through the client rather than
directly between the RP and the OP. OpenID defines two methods
for indirect communication -- namely, HTTP redirects and HTML
form submission. Neither mechanism is directly applicable for
usage with SASL. To ensure that an OP that is OpenID 2.0
capable can be used, a new method is defined in this document
that requires the OpenID message content to be encoded using a

Lear, et al. Standards Track [Page 5]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

Universal Resource Identifier (URI) [RFC3986]. Note that any
Internationalized Resource Identifiers (IRIs) must be normalized
to URIs by the SASL client, as specified in [RFC3987], prior to
transmitting them to the SASL server.

6. The SASL client now sends a response consisting of "=" to the
server, to indicate that authentication continues via the normal
OpenID flow.

7. At this point, the client application MUST construct a URL
containing the content received in the previous message from the
RP. This URL is transmitted to the OP by either the SASL client
application or an appropriate handler, such as a browser.

8. Next, the end user optionally authenticates to the OP and then,
depending on the OP, may approve or disapprove authentication to
the Relying Party. For reasons of its own, the OP has the
option of not authenticating a request. The manner in which the
end user is authenticated to their respective OP and any
policies surrounding such authentication are out of scope of
OpenID and, hence, also out of scope for this specification.
This step happens out of band from SASL.

9. The OP will convey information about the success or failure of
the authentication phase back to the RP, again using an indirect
response via the client browser or handler. The client
transmits to the RP (over HTTP/TLS) the redirect of the OP
result. This step happens out of band from SASL.

10. The RP MAY send an OpenID check_authentication request directly
to the OP, if no association has been established, and the OP
should respond. Again, this step happens out of band from SASL.

11. The SASL server sends an appropriate SASL response to the
client, with optional Open Simple Registry (SREG) attributes.

Lear, et al. Standards Track [Page 6]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

SASL Serv. RP/Client OP
|>-----(1)----->| | Advertisement
| | |
|<-----(2)-----<| | Initiation
| | |
|> - - (3) - - - - - - - - - ->| Discovery
| |
|>- - -(4)- - - - - - - - - - >| Association
|<- - -(4)- - - - - - - - - - <|
| | |
|>-----(5)----->| | Indirect Auth Request
| | |
|<-----(6)-----<| | Client "=" Response
| | |
| |>- - (7)- - ->| Client GET to the OP (ext.)
| | |
| |<- - (8)- - ->| Client / OP Auth. (ext.)
| | |
|<- - -(9)- - - + - - - - - - <| HTTPS Indirect id_res
| | |
|<- - -(10)- - - - - - - - - ->| Optional
| | | check_authentication
| | |
|>-----(11)---->| | SASL completion with status

----- = SASL
- - - = HTTPS

Note the directionality in SASL is such that the client MUST send the
"=" response. Specifically, the SASL client processes the redirect
and then awaits a final SASL decision, while the rest of the OpenID
authentication process continues.

2.1. Binding SASL to OpenID in the Relying Party

OpenID is meant to be used in serial within the web, where browser
cookies are easily accessible. As such, there are no transaction IDs
within the protocol. To ensure that a specific request is bound, and
in particular to ease inter-process communication, the Relying Party
MUST encode a nonce or transaction ID in the URIs it transmits
through the client for success or failure, as either a base URI or
fragment component to the "return_to" URI. This value is to be used
to uniquely identify each authentication transaction. The nonce
value MUST be at least 2^32 bits and large enough to handle well in
excess of the number of concurrent transactions a SASL server shall
see.

Lear, et al. Standards Track [Page 7]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

2.2. Discussion

As mentioned above, OpenID is primarily designed to interact with
web-based applications. Portions of the authentication stream are
only defined in the crudest sense. That is, when one is prompted to
approve or disapprove an authentication, anything that one might find
on a browser is allowed, including JavaScript, complex style-sheets,
etc. Because of this lack of structure, implementations will need to
invoke a rich browser in order to ensure that the authentication can
be completed.

Once there is an outcome, the SASL server needs to know about it.
The astute reader will hopefully by now have noticed an "=" client
SASL response. This is not to say that nothing is happening, but
rather that authentication flow has shifted from SASL and the client
application to OpenID within the browser, and it will return to the
client application when the server has an outcome to hand to the
client. The alternative to this flow would be some sort of signal
from the HTML browser to the SASL client of the results that would in
turn be passed to the SASL server. The inter-process communication
issue this raises is substantial. Better, we conclude, to
externalize the authentication to the browser and have an "=" client
response.

3. OpenID SASL Mechanism Specification

This section specifies the details of the OpenID SASL mechanism.
Recall Section 5 of [RFC4422] for what needs to be described here.

The name of this mechanism is "OPENID20". The mechanism is capable
of transferring an authorization identity (via "gs2-header"). The
mechanism does not offer a security layer.

The mechanism is client-first. The first mechanism message is from
the client to the server, and it is the "initial-response" described
below. 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, and it
is the "authentication_request" described below.

The third mechanism message is from client to the server, and it is
the fixed message consisting of "=".

Lear, et al. Standards Track [Page 8]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

The fourth mechanism message is from the server to the client,
described below as "outcome_data" (with SREG attributes), sent as
additional data when indicating a successful outcome.

3.1. Initiation

A client initiates an OpenID authentication with SASL by sending the
GS2 header followed by the URI, as specified in the OpenID
specification.

The ABNF [RFC5234] syntax is as follows:

initial-response = gs2-header Auth-Identifier
Auth-Identifier = Identifier ; authentication identifier
Identifier = URI ; Identifier is specified in
; Sec. 7.2 of the OpenID 2.0 spec.

The syntax and semantics of the "gs2-header" are specified in
[RFC5801], and we use it here with the following limitations: The
"gs2-nonstd-flag" MUST NOT be present. The "gs2-cb-flag" MUST be "n"
because channel binding is not supported by this mechanism.

URI is specified in [RFC3986]. Extensible Resource Identifiers
(XRIs) [XRI2.0] MUST NOT be used.

3.2. Authentication Request

The SASL server sends the URL resulting from the OpenID
authentication request, containing an "openid.mode" of either
"checkid_immediate" or "checkid_setup", as specified in Section 9.1
of the OpenID 2.0 specification [OpenID].

authentication-request = URI

As part of this request, the SASL server MUST append a unique
transaction ID to the "return_to" portion of the request. The form
of this transaction is left to the RP to decide, but it SHOULD be
large enough to be resistant to being guessed or attacked.

The client now sends that request via an HTTP GET to the OP, as if
redirected to do so from an HTTP server.

The client MUST handle both user authentication to the OP and
confirmation or rejection of the authentication by the RP via this
SASL mechanism.

Lear, et al. Standards Track [Page 9]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

After all authentication has been completed by the OP, and after the
response has been sent to the client, the client will relay the
response to the Relying Party via HTTP/TLS, as specified previously
in the transaction ("return_to").

3.3. Server Response

The Relying Party now validates the response it received from the
client via HTTP/TLS, as specified in the OpenID specification, using
the "return_to" URI given previously in the transaction.

The response by the Relying Party constitutes a SASL mechanism
outcome, and it SHALL be used to set state in the server accordingly.
Also, it SHALL be used by the server to report that state to the SASL
client as described in Section 3.6 of [RFC4422]. In the additional
data, the server MAY include OpenID Simple Registry (SREG) attributes
that are listed in Section 4 of [SREG1.0]. SREG attributes are
encoded as follows:

1. Strip "openid.sreg." from each attribute name.

2. Treat the concatenation of results as URI parameters that are
separated by an ampersand (&) and encode as one would a URI,
absent the scheme, authority, and the question mark.

For example: [email protected]&fullname=Eliot%20Lear

More formally:

outcome-data = [ sreg-avp *( "," sreg-avp ) ]
sreg-avp = sreg-attr "=" sreg-val
sreg-attr = sreg-word
sreg-val = sreg-word
sreg-word = 1*( unreserved / pct-encoded )
; pct-encoded from Section 2.1 of RFC 3986
; unreserved from Section 2.3 of RFC 3986

A client who does not support SREG MUST ignore SREG attributes sent
by the server. Similarly, a client MUST ignore unknown attributes.

In the case of failures, the response MUST follow this syntax:

outcome-data = "openid.error" "=" sreg-val *( "," sregp-avp )

Lear, et al. Standards Track [Page 10]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

3.4. Error Handling

Section 3.6 of [RFC4422] explicitly prohibits additional information
in an unsuccessful authentication outcome. Therefore, the
openid.error and openid.error_code are to be sent as an additional
challenge in the event of an unsuccessful outcome. In this case, as
the protocol is in lockstep, the client will follow with an
additional exchange containing "=", after which the server will
respond with an application-level outcome.

4. OpenID GSS-API Mechanism Specification

This section MUST be observed to properly implement the GSS-API
mechanism that is described below.

The OpenID SASL mechanism is actually also a GSS-API mechanism. The
OpenID user takes the role of the GSS-API Initiator and the OpenID
Relying Party takes the role of the GSS-API Acceptor. The OpenID
Provider does not have a role in GSS-API and is considered an
internal matter for the OpenID mechanism. The messages are the same,
but a) the GS2 header on the client's first message and channel
binding data are excluded when OpenID is used as a GSS-API mechanism,
and b) the initial context token header (described in Section 3.1 of
RFC 2743) is prefixed to the client's first authentication message
(context token).

The GSS-API OID for the OpenID 2.0 mechanism is 1.3.6.1.5.5.16 (see
Section 7 for more information). The DER encoding of the OID is 0x2b
0x06 0x01 0x05 0x05 0x10.

OpenID security contexts MUST have the mutual_state flag
(GSS_C_MUTUAL_FLAG) set to TRUE. OpenID does not support credential
delegation; therefore, OpenID 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 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 identity with the target name, as discussed in
[RFC6125].

The OpenID mechanism does not support per-message tokens or
GSS_Pseudo_random.

Lear, et al. Standards Track [Page 11]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

The [RFC5587] mechanism attributes for this mechanism are
GSS_C_MA_MECH_CONCRETE, GSS_C_MA_ITOK_FRAMED, and GSS_C_MA_AUTH_INIT.

4.1. GSS-API Principal Name Types for OpenID

OpenID supports standard generic name syntaxes for acceptors such as
GSS_C_NT_HOSTBASED_SERVICE (see Section 4.1 of [RFC2743]).

OpenID supports only a single name type for initiators:
GSS_C_NT_USER_NAME. GSS_C_NT_USER_NAME is the default name type for
OpenID.

OpenID name normalization is covered by the OpenID specification; see
Section 7.2 of [OpenID].

The query, display, and exported name syntaxes for OpenID principal
names are all the same. There are no OpenID-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 Section 4 of
[RFC2743]). The exported name token does, of course, conform to
Section 3.2 of [RFC2743], but the "NAME" part of the token should be
treated as a potential input string to the OpenID name normalization
rules. For example, the OpenID Identifier "https://openid.example/"
will have a GSS_C_NT_USER_NAME value of "https://openid.example/".

GSS-API name attributes may be defined in the future to hold the
normalized OpenID Identifier.

5. Example

Suppose a user has an OpenID of https://openid.example and wishes to
authenticate his IMAP connection to mail.example (where .example is
the top-level domain specified in [RFC2606]). The user would input
his OpenID into his mail user agent when he configures the account.
In this case, no association is attempted between the OpenID RP and
the OP. The client will make use of the "return_to" attribute to
capture results of the authentication to be redirected to the server.
Note the use of [RFC4959] for the initial response. The
authentication on the wire would then look something like the
following:

Lear, et al. Standards Track [Page 12]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

(S = IMAP server; C = IMAP client)

C: < connects to IMAP port>
S: * OK
C: C1 CAPABILITY
S: * CAPABILITY IMAP4rev1 SASL-IR SORT [...] AUTH=OPENID20
S: C1 OK Capability Completed
C: C2 AUTHENTICATE OPENID biwsaHR0cHM6Ly9vcGVuaWQuZXhhbXBsZS8=
[ This is the base64 encoding of "n,,https://openid.example/".
Server performs discovery on http://openid.example/ ]
S: + aHR0cHM6Ly9vcGVuaWQuZXhhbXBsZS9vcGVuaWQvP29wZW5pZC5ucz1
odHRwOi8vc3BlY3Mub3BlbmlkLm5ldC9hdXRoLzIuMCZvcGVuaWQucm
V0dXJuX3RvPWh0dHBzOi8vbWFpbC5leGFtcGxlL2NvbnN1bWVyLzFlZ
jg4OGMmb3BlbmlkLmNsYWltZWRfaWQ9aHR0cHM6Ly9vcGVuaWQuZXhh
bXBsZS8mb3BlbmlkLmlkZW50aXR5PWh0dHBzOi8vb3BlbmlkLmV4YW1
wbGUvJm9wZW5pZC5yZWFsbT1pbWFwOi8vbWFpbC5leGFtcGxlJm9wZW
5pZC5tb2RlPWNoZWNraWRfc2V0dXA=

[ This is the base64 encoding of "https://openid.example/openid/
?openid.ns=http://specs.openid.net/auth/2.0
&openid.return_to=https://mail.example/consumer/1ef888c
&openid.claimed_id=https://openid.example/
&openid.identity=https://openid.example/
&openid.realm=imap://mail.example
&openid.mode=checkid_setup"
with line breaks and spaces added here for readability.
]
C: PQ==
[ The client now sends the URL it received to a browser for
processing. The user logs into https://openid.example and
agrees to authenticate imap://mail.example. A redirect is
passed back to the client browser that then connects to
https://imap.example/consumer via SSL with the results.
From an IMAP perspective, however, the client sends the "="
response, and awaits mail.example.
Server mail.example would now contact openid.example with an
openid.check_authentication message. After that...
]
S: + ZW1haWw9bGVhckBtYWlsLmV4YW1wbGUsZnVsbG5hbWU9RWxp
b3QlMjBMZWFy
[ Here, the IMAP server has returned an SREG attribute of
[email protected],fullname=Eliot%20Lear.
Line break in response added in this example for readability. ]
C:
[ In IMAP, client must send a blank response after receiving
the SREG data. ]
S: C2 OK

Lear, et al. Standards Track [Page 13]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

In this example, the SASL server / RP has made use of a transaction
ID 1ef888c.

6. Security Considerations

This section will address only security considerations associated
with the use of OpenID with SASL and GSS-API. For considerations
relating to OpenID in general, the reader is referred to the OpenID
specification [OpenID] and to other literature [OpReview].
Similarly, for general SASL [RFC4422] and GSS-API [RFC5801] security
considerations, the reader is referred to those specifications.

6.1. Binding OpenIDs to Authorization Identities

As specified in [RFC4422], the server is responsible for binding
credentials to a specific authorization identity. It is therefore
necessary that a registration process takes place in advance that
binds specific OpenIDs to specific authorization identities, or that
only specific trusted OpenID Providers be allowed, where a mapping is
predefined. For example, it could be prearranged between an IdP and
RP that "https://example.com/user" maps to "user" for purposes of
authorization.

6.2. RP Redirected by Malicious URL to Take an Improper Action

In the initial SASL client response, a user or host can transmit a
malicious response to the RP for purposes of taking advantage of
weaknesses in the RP's OpenID implementation. It is possible to add
port numbers to the URL so that the outcome is that the RP does a
port scan of the site. The URL could contain an unauthorized host or
even the local host. The URL could contain a protocol other than
http or https, such as file or ftp.

One mitigation would be for RPs to have a list of authorized URI
bases. OPs SHOULD only redirect to RPs with the same domain
component of the base URI. RPs MUST NOT automatically retry on
failed attempts. A log of those sites that fail SHOULD be kept, and
limitations on queries from clients SHOULD be imposed, just as with
any other authentication attempt. Applications SHOULD NOT invoke
browsers to communicate with OPs that they are not themselves
configured with.

6.3. User Privacy

The OP is aware of each RP that a user logs into. There is nothing
in the protocol to hide this information from the OP. It is not a
requirement to track the visits, but there is nothing that prohibits
the collection of information. SASL servers should be aware that

Lear, et al. Standards Track [Page 14]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

OpenID Providers will be able to track -- to some extent -- user
access to their services and any additional information that OP
provides.

7. IANA Considerations

IANA has updated the "SASL Mechanisms" registry using the following
template, as described in [RFC4422].

SASL mechanism name: OPENID20

Security Considerations: See this document

Published specification: See this document

Person & email address to contact for further information: Authors of
this document

Intended usage: COMMON

Owner/Change controller: IESG

Note: None

IANA has also assigned an OID for this GSS mechanism in the "SMI
Security for Mechanism Codes" registry, with the prefix of
iso.org.dod.internet.security.mechanisms (1.3.6.1.5.5) and
referencing this specification in the registry.

8. Acknowledgments

The authors would like to thank Alexey Melnikov, Joe Hildebrand, Mark
Crispin, Chris Newman, Leif Johansson, Sam Hartman, Nico Williams,
Klaas Wierenga, Stephen Farrell, and Stephen Kent for their review
and contributions.

9. References

9.1. Normative References

[OpenID] OpenID Foundation, "OpenID Authentication 2.0 - Final",
December 2007, <http://specs.openid.net/auth/2.0>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2606] Eastlake, D. and A. Panitz, "Reserved Top Level DNS
Names", BCP 32, RFC 2606, June 1999.

Lear, et al. Standards Track [Page 15]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

[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.

[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. and K. Zeilenga, "Simple Authentication and
Security Layer (SASL)", RFC 4422, June 2006.

[RFC5234] Crocker, D. 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.

[RFC5587] Williams, N., "Extended Generic Security Service Mechanism
Inquiry APIs", RFC 5587, July 2009.

[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.

[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.

[SREG1.0] OpenID Foundation, "OpenID Simple Registration Extension
version 1.0", June 2006, <http://openid.net/sreg/1.0>.

Lear, et al. Standards Track [Page 16]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

9.2. Informative References

[OpReview] "Google Sites OpenID Reference Page",
<http://sites.google.com/site/openidreview/resources>.

[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.

[RFC4959] Siemborski, R. and A. Gulbrandsen, "IMAP Extension for
Simple Authentication and Security Layer (SASL) Initial
Client Response", RFC 4959, September 2007.

[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, March 2011.

[W3C.REC-html401-19991224]
Hors, A., Raggett, D., and I. Jacobs, "HTML 4.01
Specification", World Wide Web Consortium
Recommendation REC-html401-19991224, December 1999,
<http://www.w3.org/TR/1999/REC-html401-19991224>.

[XRI2.0] Reed, D., Ed. and D. McAlpin, Ed., "Extensible Resource
Identifier (XRI) Syntax V2.0", OASIS Standard xri-syntax-
V2.0-cs, September 2005, <http://www.oasis-open.org/
committees/download.php/15376/xri-syntax-V2.0-cs.html>.

Lear, et al. Standards Track [Page 17]

RFC 6616 SASL & GSS-API Mechanism for OpenID May 2012

Authors' Addresses

Eliot Lear
Cisco Systems GmbH
Richtistrasse 7
CH-8304 Wallisellen
Switzerland

Phone: +41 44 878 9200
EMail: [email protected]

Hannes Tschofenig
Nokia Siemens Networks
Linnoitustie 6
Espoo 02600
Finland

Phone: +358 (50) 4871445
EMail: [email protected]
URI: http://www.tschofenig.priv.at

Henry Mauldin
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
USA

Phone: +1 (800) 553-6387
EMail: [email protected]

Simon Josefsson
SJD AB
Johan Olof Wallins vag 13
171 64 Solna
Sweden

EMail: [email protected]
URI: http://josefsson.org/

Lear, et al. Standards Track [Page 18]