Internet Engineering Task Force (IETF) W. Mills

Internet Engineering Task Force (IETF) W. Mills
Request for Comments: 7628 Microsoft
Category: Standards Track T. Showalter
ISSN: 2070-1721
H. Tschofenig
ARM Ltd.
August 2015

A Set of Simple Authentication and Security Layer (SASL) Mechanisms
for OAuth

Abstract

OAuth enables a third-party application to obtain limited access to a
protected resource, either on behalf of a resource owner by
orchestrating an approval interaction or by allowing the third-party
application to obtain access on its own behalf.

This document defines how an application client uses credentials
obtained via OAuth over the Simple Authentication and Security Layer
(SASL) to access a protected resource at a resource server. Thereby,
it enables schemes defined within the OAuth framework for non-HTTP-
based application protocols.

Clients typically store the user's long-term credential. This does,
however, lead to significant security vulnerabilities, for example,
when such a credential leaks. A significant benefit of OAuth for
usage in those clients is that the password is replaced by a shared
secret with higher entropy, i.e., the token. Tokens typically
provide limited access rights and can be managed and revoked
separately from the user's long-term password.

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/rfc7628.

Mills, et al. Standards Track [Page 1]

RFC 7628 SASL OAuth August 2015

Copyright Notice

Copyright (c) 2015 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
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. OAuth SASL Mechanism Specifications . . . . . . . . . . . . . 6
3.1. Initial Client Response . . . . . . . . . . . . . . . . . 7
3.1.1. Reserved Key/Values . . . . . . . . . . . . . . . . . 8
3.2. Server's Response . . . . . . . . . . . . . . . . . . . . 8
3.2.1. OAuth Identifiers in the SASL Context . . . . . . . . 9
3.2.2. Server Response to Failed Authentication . . . . . . 9
3.2.3. Completing an Error Message Sequence . . . . . . . . 10
3.3. OAuth Access Token Types using Keyed Message Digests . . 11
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. Successful Bearer Token Exchange . . . . . . . . . . . . 12
4.2. Successful OAuth 1.0a Token Exchange . . . . . . . . . . 13
4.3. Failed Exchange . . . . . . . . . . . . . . . . . . . . . 14
4.4. SMTP Example of a Failed Negotiation . . . . . . . . . . 15
5. Security Considerations . . . . . . . . . . . . . . . . . . . 16
6. Internationalization Considerations . . . . . . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7.1. SASL Registration . . . . . . . . . . . . . . . . . . . . 18
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. Normative References . . . . . . . . . . . . . . . . . . 19
8.2. Informative References . . . . . . . . . . . . . . . . . 20
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21

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RFC 7628 SASL OAuth August 2015

1. Introduction

OAuth 1.0a [RFC5849] and OAuth 2.0 [RFC6749] are protocol frameworks
that enable a third-party application to obtain limited access to a
protected resource, either by orchestrating an approval interaction
on behalf of a resource owner or by allowing the third-party
application to obtain access on its own behalf.

The core OAuth 2.0 specification [RFC6749] specifies the interaction
between the OAuth client and the authorization server; it does not
define the interaction between the OAuth client and the resource
server for the access to a protected resource using an access token.
Instead, the OAuth client to resource server interaction is described
in separate specifications, such as the bearer token specification
[RFC6750]. OAuth 1.0a includes the protocol specification for the
communication between the OAuth client and the resource server in
[RFC5849].

The main use cases for OAuth 1.0a and OAuth 2.0 have so far focused
on an HTTP-based [RFC7230] environment only. This document
integrates OAuth 1.0a and OAuth 2.0 into non-HTTP-based applications
using the integration into the Simple Authentication and Security
Layer (SASL) [RFC4422]. Hence, this document takes advantage of the
OAuth protocol and its deployment base to provide a way to use SASL
to gain access to resources when using non-HTTP-based protocols, such
as the Internet Message Access Protocol (IMAP) [RFC3501] and the
Simple Mail Transfer Protocol (SMTP) [RFC5321]. This document gives
examples of use in IMAP and SMTP.

To illustrate the impact of integrating this specification into an
OAuth-enabled application environment, Figure 1 shows the abstract
message flow of OAuth 2.0 [RFC6749]. As indicated in the figure,
this document impacts the exchange of messages (E) and (F) since SASL
is used for interaction between the client and the resource server
instead of HTTP.

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RFC 7628 SASL OAuth August 2015

----+
+--------+ +---------------+ |
| |--(A)-- Authorization Request --->| Resource | |
| | | Owner | |Plain
| |<-(B)------ Access Grant ---------| | |OAuth
| | +---------------+ |2.0
| | |
| | Client Credentials & +---------------+ |
| |--(C)------ Access Grant -------->| Authorization | |
| Client | | Server | |
| |<-(D)------ Access Token ---------| | |
| | (w/ Optional Refresh Token) +---------------+ |
| | ----+
| | ----+
| | +---------------+ |
| | | | |OAuth
| |--(E)------ Access Token -------->| Resource | |over
| | | Server | |SASL
| |<-(F)---- Protected Resource -----| | |
| | | | |
+--------+ +---------------+ |
----+

Figure 1: OAuth 2.0 Protocol Flow

SASL is a framework for providing authentication and data security
services in connection-oriented protocols via replaceable
authentication mechanisms. It provides a structured interface
between protocols and mechanisms. The resulting framework allows new
protocols to reuse existing authentication mechanisms and allows old
protocols to make use of new authentication mechanisms. The
framework also provides a protocol for securing subsequent exchanges
within a data security layer.

When OAuth is integrated into SASL, the high-level steps are as
follows:

(A) The client requests authorization from the resource owner. The
authorization request can be made directly to the resource owner
(as shown) or indirectly via the authorization server as an
intermediary.

(B) The client receives an authorization grant, which is a
credential representing the resource owner's authorization,
expressed using one of the grant types defined in [RFC6749] or
[RFC5849] or using an extension grant type. The authorization
grant type depends on the method used by the client to request

Mills, et al. Standards Track [Page 4]

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authorization and the types supported by the authorization
server.

(C) The client requests an access token by authenticating with the
authorization server and presenting the authorization grant.

(D) The authorization server authenticates the client and validates
the authorization grant, and if valid, it issues an access
token.

(E) The client requests the protected resource from the resource
server and authenticates it by presenting the access token.

(F) The resource server validates the access token, and if valid, it
indicates a successful authentication.

Again, steps (E) and (F) are not defined in [RFC6749] (but are
described in, for example, [RFC6750] for the OAuth bearer token
instead) and are the main functionality specified within this
document. Consequently, the message exchange shown in Figure 1 is
the result of this specification. The client will generally need to
determine the authentication endpoints (and perhaps the service
endpoints) before the OAuth 2.0 protocol exchange messages in steps
(A)-(D) are executed. The discovery of the resource owner,
authorization server endpoints, and client registration are outside
the scope of this specification. The client must discover the
authorization endpoints using a discovery mechanism such as OpenID
Connect Discovery (OIDCD) [OpenID.Discovery] or WebFinger using host-
meta [RFC7033]. Once credentials are obtained, the client proceeds
to steps (E) and (F) defined in this specification. Authorization
endpoints MAY require client registration, and generic clients SHOULD
support the Dynamic Client Registration protocol [RFC7591].

OAuth 1.0a follows a similar model but uses a different terminology
and does not separate the resource server from the authorization
server.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].

The reader is assumed to be familiar with the terms used in the OAuth
2.0 specification [RFC6749] and SASL [RFC4422].

Mills, et al. Standards Track [Page 5]

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In examples, "C:" and "S:" indicate lines sent by the client and
server, respectively. Line breaks have been inserted for
readability.

Note that the IMAP SASL specification requires base64 encoding, as
specified in Section 4 of [RFC4648].

3. OAuth SASL Mechanism Specifications

SASL is used as an authentication framework in a variety of
application-layer protocols. This document defines the following
SASL mechanisms for usage with OAuth:

OAUTHBEARER: OAuth 2.0 bearer tokens, as described in [RFC6750].
RFC 6750 uses Transport Layer Security (TLS) [RFC5246] to
secure the protocol interaction between the client and the
resource server.

OAUTH10A: OAuth 1.0a Message Authentication Code (MAC) tokens
(using the HMAC-SHA1 keyed message digest), as described in
Section 3.4.2 of [RFC5849].

New extensions may be defined to add additional OAuth Access Token
Types. Such a new SASL OAuth mechanism can be added by registering
the new name(s) with IANA in the SASL Mechanisms registry and citing
this specification for the further definition.

SASL mechanisms using this document as their definition do not
provide a data security layer; that is, they cannot provide integrity
or confidentiality protection for application messages after the
initial authentication. If such protection is needed, TLS or some
similar solution should be used. Additionally, for the two
mechanisms specified in this document, TLS MUST be used for
OAUTHBEARER to protect the bearer token; for OAUTH10A, the use of TLS
is RECOMMENDED.

These mechanisms are client initiated and in lockstep, with the
server always replying to a client message. In the case where the
client has and correctly uses a valid token, the flow is:

1. Client sends a valid and correct initial client response.

2. Server responds with a successful authentication.

In the case where authentication fails, the server sends an error
result; the client MUST then send an additional message to the server
in order to allow the server to finish the exchange. Some protocols
and common SASL implementations do not support both sending a SASL

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message and finalizing a SASL negotiation. The additional client
message in the error case deals with this problem. This exchange is:

1. Client sends an invalid initial client response.

2. Server responds with an error message.

3. Client sends a dummy client response.

4. Server fails the authentication.

3.1. Initial Client Response

Client responses are a GS2 [RFC5801] header followed by zero or more
key/value pairs, or it may be empty. The gs2-header rule is defined
here as a placeholder for compatibility with GS2 if a GS2 mechanism
is formally defined, but this document does not define one. The key/
value pairs take the place of the corresponding HTTP headers and
values to convey the information necessary to complete an OAuth-style
HTTP authorization. Unknown key/value pairs MUST be ignored by the
server. The ABNF [RFC5234] syntax is:

kvsep = %x01
key = 1*(ALPHA)
value = *(VCHAR / SP / HTAB / CR / LF )
kvpair = key "=" value kvsep
;;gs2-header = See RFC 5801
client-resp = (gs2-header kvsep *kvpair kvsep) / kvsep

The GS2 header MAY include the username associated with the resource
being accessed, the "authzid". It is worth noting that application
protocols are allowed to require an authzid, as are specific server
implementations.

The client response consisting of only a single kvsep is used only
when authentication fails and is only valid in that context. If sent
as the first message from the client, the server MAY simply fail the
authentication without returning discovery information since there is
no user or server name indication.

The following keys and corresponding values are defined in the client
response:

auth (REQUIRED): The payload that would be in the HTTP
Authorization header if this OAuth exchange was being carried
out over HTTP.

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host: Contains the hostname to which the client connected. In an
HTTP context, this is the value of the HTTP Host header.

port: Contains the destination port that the client connected to,
represented as a decimal positive integer string without
leading zeros.

For OAuth token types such as OAuth 1.0a that use keyed message
digests, the client MUST send host and port number key/values, and
the server MUST fail an authorization request requiring keyed message
digests that are not accompanied by host and port values. In OAuth
1.0a, for example, the so-called "signature base string calculation"
includes the reconstructed HTTP URL.

3.1.1. Reserved Key/Values

In these mechanisms, values for path, query string and post body are
assigned default values. OAuth authorization schemes MAY define
usage of these in the SASL context and extend this specification.
For OAuth Access Token Types that include a keyed message digest of
the request, the default values MUST be used unless explicit values
are provided in the client response. The following key values are
reserved for future use:

mthd (RESERVED): HTTP method; the default value is "POST".

path (RESERVED): HTTP path data; the default value is "/".

post (RESERVED): HTTP post data; the default value is the empty
string ("").

qs (RESERVED): The HTTP query string; the default value is the
empty string ("").

3.2. Server's Response

The server validates the response according to the specification for
the OAuth Access Token Types used. If the OAuth Access Token Type
utilizes a keyed message digest of the request parameters, then the
client must provide a client response that satisfies the data
requirements for the scheme in use.

The server fully validates the client response before generating a
server response; this will necessarily include the validation steps
listed in the specification for the OAuth Access Token Type used.
However, additional validation steps may be needed, depending on the
particular application protocol making use of SASL. In particular,
values included as kvpairs in the client response (such as host and

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RFC 7628 SASL OAuth August 2015

port) that correspond to values known to the application server by
some other mechanism (such as an application protocol data unit or
preconfigured values) MUST be validated to match between the initial
client response and the other source(s) of such information. As a
concrete example, when SASL is used over IMAP to an IMAP server for a
single domain, the hostname can be available via configuration; this
hostname must be validated to match the value sent in the 'host'
kvpair.

The server responds to a successfully verified client message by
completing the SASL negotiation. The authenticated identity reported
by the SASL mechanism is the identity securely established for the
client with the OAuth credential. The application, not the SASL
mechanism, based on local access policy determines whether the
identity reported by the mechanism is allowed access to the requested
resource. Note that the semantics of the authzid are specified by
the SASL framework [RFC4422].

3.2.1. OAuth Identifiers in the SASL Context

In the OAuth framework, the client may be authenticated by the
authorization server, and the resource owner is authenticated to the
authorization server. OAuth access tokens may contain information
about the authentication of the resource owner and about the client
and may therefore make this information accessible to the resource
server.

If both identifiers are needed by an application the developer will
need to provide a way to communicate that from the SASL mechanism
back to the application.

3.2.2. Server Response to Failed Authentication

For a failed authentication, the server returns an error result in
JSON [RFC7159] format and fails the authentication. The error result
consists of the following values:

status (REQUIRED): The authorization error code. Valid error
codes are defined in the IANA "OAuth Extensions Error Registry"
as specified in the OAuth 2.0 core specification.

scope (OPTIONAL): An OAuth scope that is valid to access the
service. This may be omitted, which implies that unscoped
tokens are required. If a scope is specified, then a single
scope is preferred. At the time this document was written,
there are several implementations that do not properly support
space-separated lists of scopes, so the use of a space-
separated list of scopes is NOT RECOMMENDED.

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RFC 7628 SASL OAuth August 2015

openid-configuration (OPTIONAL): The URL for a document following
the OpenID Provider Configuration Information schema as
described in OIDCD [OpenID.Discovery], Section 3 that is
appropriate for the user. As specified in OIDCD, this will
have the "https" URL scheme. This document MUST have all
OAuth-related data elements populated. The server MAY return
different URLs for users in different domains, and the client
SHOULD NOT cache a single returned value and assume it applies
for all users/domains that the server supports. The returned
discovery document SHOULD have all data elements required by
the OpenID Connect Discovery specification populated. In
addition, the discovery document SHOULD contain the
'registration_endpoint' element to identify the endpoint to be
used with the Dynamic Client Registration protocol [RFC7591] to
obtain the minimum number of parameters necessary for the OAuth
protocol exchange to function. Another comparable discovery or
client registration mechanism MAY be used if available.

The use of the 'offline_access' scope, as defined in
[OpenID.Core], is RECOMMENDED to give clients the capability to
explicitly request a refresh token.

If the resource server provides a scope, then the client MUST always
request scoped tokens from the token endpoint. If the resource
server does not return a scope, the client SHOULD presume an unscoped
token is required to access the resource.

Since clients may interact with a number of application servers, such
as email servers and Extensible Messaging and Presence Protocol
(XMPP) [RFC6120] servers, they need to have a way to determine
whether dynamic client registration has been performed already and
whether an already available refresh token can be reused to obtain an
access token for the desired resource server. This specification
RECOMMENDS that a client uses the information in the 'iss' element
defined in OpenID Connect Core [OpenID.Core] to make this
determination.

3.2.3. Completing an Error Message Sequence

Section 3.6 of SASL [RFC4422] explicitly prohibits additional
information in an unsuccessful authentication outcome. Therefore,
the error message is sent in a normal message. The client MUST then
send either an additional client response consisting of a single %x01
(control A) character to the server in order to allow the server to
finish the exchange or a SASL abort message as generally defined in
Section 3.5 of SASL [RFC4422]. A specific example of an abort
message is the "BAD" response to an AUTHENTICATE in IMAP [RFC3501],
Section 6.2.2.

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RFC 7628 SASL OAuth August 2015

3.3. OAuth Access Token Types using Keyed Message Digests

OAuth Access Token Types may use keyed message digests, and the
client and the resource server may need to perform a cryptographic
computation for integrity protection and data origin authentication.

OAuth is designed for access to resources identified by URIs. SASL
is designed for user authentication and has no facility for more
fine-grained access control. In this specification, we require or
define default values for the data elements from an HTTP request that
allows the signature base string to be constructed properly. The
default HTTP path is "/", and the default post body is empty. These
atoms are defined as extension points so that no changes are needed
if there is a revision of SASL that supports more specific resource
authorization, e.g., IMAP access to a specific folder or FTP access
limited to a specific directory.

Using the example in the OAuth 1.0a specification as a starting
point, below is the authorization request in OAuth 1.0a style (with
%x01 shown as ^A and line breaks added for readability), assuming it
is on an IMAP server running on port 143:

n,[email protected],^A
host=example.com^A
port=143^A
auth=OAuth realm="Example",
oauth_consumer_key="9djdj82h48djs9d2",
oauth_token="kkk9d7dh3k39sjv7",
oauth_signature_method="HMAC-SHA1",
oauth_timestamp="137131201",
oauth_nonce="7d8f3e4a",
oauth_signature="Tm90IGEgcmVhbCBzaWduYXR1cmU"^A^A

The signature base string would be constructed per the OAuth 1.0a
specification [RFC5849] with the following things noted:

o The method value is defaulted to POST.

o The scheme defaults to be "http", and any port number other than
80 is included.

o The path defaults to "/".

o The query string defaults to "".

Mills, et al. Standards Track [Page 11]

RFC 7628 SASL OAuth August 2015

In this example, the signature base string with line breaks added for
readability would be:

POST&http%3A%2F%2Fexample.com:143%2F&oauth_consumer_key%3D9djdj82h4
8djs9d2%26oauth_nonce%3D7d8f3e4a%26oauth_signature_method%3DHMAC-SH
A1%26oauth_timestamp%3D137131201%26oauth_token%3Dkkk9d7dh3k39sjv7

4. Examples

These examples illustrate exchanges between IMAP and SMTP clients and
servers. All IMAP examples use SASL-IR [RFC4959] and send payload in
the initial client response. The bearer token examples assume
encrypted transport; if the underlying connection is not already TLS,
then STARTTLS MUST be used as TLS is required in the bearer token
specification.

Note to implementers: The SASL OAuth method names are case
insensitive. One example uses "Bearer" but that could as easily be
"bearer", "BEARER", or "BeArEr".

4.1. Successful Bearer Token Exchange

This example shows a successful OAuth 2.0 bearer token exchange in
IMAP. Note that line breaks are inserted for readability.

[Initial connection and TLS establishment...]
S: * OK IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER SASL-IR
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTHBEARER bixhPXVzZXJAZXhhbXBsZS5jb20sAWhv
c3Q9c2VydmVyLmV4YW1wbGUuY29tAXBvcnQ9MTQzAWF1dGg9QmVhcmVyI
HZGOWRmdDRxbVRjMk52YjNSbGNrQmhiSFJoZG1semRHRXVZMjl0Q2c9PQ
EB
S: t1 OK SASL authentication succeeded

As required by IMAP [RFC3501], the payloads are base64 encoded. The
decoded initial client response (with %x01 represented as ^A and long
lines wrapped for readability) is:

n,[email protected],^Ahost=server.example.com^Aport=143^A
auth=Bearer vF9dft4qmTc2Nvb3RlckBhbHRhdmlzdGEuY29tCg==^A^A

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RFC 7628 SASL OAuth August 2015

The same credential used in an SMTP exchange is shown below. Again,
this example assumes that TLS is already established per the bearer
token specification requirements.

[connection begins]
S: 220 mx.example.com ESMTP 12sm2095603fks.9
C: EHLO sender.example.com
S: 250-mx.example.com at your service,[172.31.135.47]
S: 250-SIZE 35651584
S: 250-8BITMIME
S: 250-AUTH LOGIN PLAIN OAUTHBEARER
S: 250-ENHANCEDSTATUSCODES
S: 250-STARTTLS
S: 250 PIPELINING
[Negotiate TLS...]
C: t1 AUTH OAUTHBEARER bixhPXVzZXJAZXhhbXBsZS5jb20sAWhvc3Q9c2Vy
dmVyLmV4YW1wbGUuY29tAXBvcnQ9NTg3AWF1dGg9QmVhcmVyIHZGOWRmd
DRxbVRjMk52YjNSbGNrQmhiSFJoZG1semRHRXVZMjl0Q2c9PQEB
S: 235 Authentication successful.
[connection continues...]

The decoded initial client response is:

n,[email protected],^Ahost=server.example.com^Aport=587^A
auth=Bearer vF9dft4qmTc2Nvb3RlckBhbHRhdmlzdGEuY29tCg==^A^A

4.2. Successful OAuth 1.0a Token Exchange

This IMAP example shows a successful OAuth 1.0a token exchange. Note
that line breaks are inserted for readability. This example assumes
that TLS is already established. Signature computation is discussed
in Section 3.3.

S: * OK IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER AUTH=OAUTH10A SASL-IR
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTH10A bixhPXVzZXJAZXhhbXBsZS5jb20sAWhvc3Q9ZXhhb
XBsZS5jb20BcG9ydD0xNDMBYXV0aD1PQXV0aCByZWFsbT0iRXhhbXBsZSIsb2F1
dGhfY29uc3VtZXJfa2V5PSI5ZGpkajgyaDQ4ZGpzOWQyIixvYXV0aF90b2tlbj0
ia2trOWQ3ZGgzazM5c2p2NyIsb2F1dGhfc2lnbmF0dXJlX21ldGhvZD0iSE1BQy
1TSEExIixvYXV0aF90aW1lc3RhbXA9IjEzNzEzMTIwMSIsb2F1dGhfbm9uY2U9I
jdkOGYzZTRhIixvYXV0aF9zaWduYXR1cmU9IlRtOTBJR0VnY21WaGJDQnphV2R1
WVhSMWNtVSUzRCIBAQ==
S: t1 OK SASL authentication succeeded

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As required by IMAP [RFC3501], the payloads are base64 encoded. The
decoded initial client response (with %x01 represented as ^A and
lines wrapped for readability) is:

n,[email protected],^A
host=example.com^A
port=143^A
auth=OAuth realm="Example",
oauth_consumer_key="9djdj82h48djs9d2",
oauth_token="kkk9d7dh3k39sjv7",
oauth_signature_method="HMAC-SHA1",
oauth_timestamp="137131201",
oauth_nonce="7d8f3e4a",
oauth_signature="SSdtIGEgbGl0dGxlIHRlYSBwb3Qu"^A^A

4.3. Failed Exchange

This IMAP example shows a failed exchange because of the empty
Authorization header, which is how a client can query for the needed
scope. Note that line breaks are inserted for readability.

S: * OK IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER SASL-IR
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTHBEARER bixhPXVzZXJAZXhhbXBsZS5jb20sAW
hvc3Q9c2VydmVyLmV4YW1wbGUuY29tAXBvcnQ9MTQzAWF1dGg9AQE=
S: + eyJzdGF0dXMiOiJpbnZhbGlkX3Rva2VuIiwic2NvcGUiOiJleGFtcGxl
X3Njb3BlIiwib3BlbmlkLWNvbmZpZ3VyYXRpb24iOiJodHRwczovL2V4
YW1wbGUuY29tLy53ZWxsLWtub3duL29wZW5pZC1jb25maWd1cmF0aW9u
In0=
C: AQ==
S: t1 NO SASL authentication failed

The decoded initial client response is:

n,[email protected],^Ahost=server.example.com^A
port=143^Aauth=^A^A

The decoded server error response is:

{
"status":"invalid_token",
"scope":"example_scope",
"openid-configuration":"https://example.com/.well-known/openid-config"
}

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RFC 7628 SASL OAuth August 2015

The client responds with the required dummy response; "AQ==" is the
base64 encoding of the ASCII value 0x01. The same exchange using the
IMAP-specific method of canceling an AUTHENTICATE command sends "*"
and is shown below.

S: * OK IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER SASL-IR IMAP4rev1
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTHBEARER bixhPXVzZXJAZXhhbXBsZS5jb20sAW
hvc3Q9c2VydmVyLmV4YW1wbGUuY29tAXBvcnQ9MTQzAWF1dGg9AQE=
S: + eyJzdGF0dXMiOiJpbnZhbGlkX3Rva2VuIiwic2NvcGUiOiJleGFtcGxl
X3Njb3BlIiwib3BlbmlkLWNvbmZpZ3VyYXRpb24iOiJodHRwczovL2V4
YW1wbGUuY29tLy53ZWxsLWtub3duL29wZW5pZC1jb25maWd1cmF0aW9u
In0=
C: *
S: t1 NO SASL authentication failed

4.4. SMTP Example of a Failed Negotiation

This example shows an authorization failure in an SMTP exchange. TLS
negotiation is not shown, but as noted above, it is required for the
use of bearer tokens.

[connection begins]
S: 220 mx.example.com ESMTP 12sm2095603fks.9
C: EHLO sender.example.com
S: 250-mx.example.com at your service,[172.31.135.47]
S: 250-SIZE 35651584
S: 250-8BITMIME
S: 250-AUTH LOGIN PLAIN OAUTHBEARER
S: 250-ENHANCEDSTATUSCODES
S: 250 PIPELINING
[Negotiate TLS...]
C: AUTH OAUTHBEARER bix1c2VyPXNvbWV1c2VyQGV4YW1wbGUuY29tLAFhdXRoPUJlYXJl
ciB2RjlkZnQ0cW1UYzJOdmIzUmxja0JoZEhSaGRtbHpkR0V1WTI5dENnPT0BAQ==
S: 334 eyJzdGF0dXMiOiJpbnZhbGlkX3Rva2VuIiwic2NoZW1lcyI6ImJlYXJlciBtYWMiL
CJzY29wZSI6Imh0dHBzOi8vbWFpbC5leGFtcGxlLmNvbS8ifQ==
C: AQ==
S: 535-5.7.1 Username and Password not accepted. Learn more at
S: 535 5.7.1 http://support.example.com/mail/oauth
[connection continues...]

The initial client response is:

n,[email protected],^A
auth=Bearer vF9dft4qmTc2Nvb3RlckBhdHRhdmlzdGEuY29tCg==^A^A

Mills, et al. Standards Track [Page 15]

RFC 7628 SASL OAuth August 2015

The server returned an error message in the 334 SASL message; the
client responds with the required dummy response, and the server
finalizes the negotiation.

{
"status":"invalid_token",
"schemes":"bearer mac",
"scope":"https://mail.example.com/"
}

5. Security Considerations

OAuth 1.0a and OAuth 2.0 allow for a variety of deployment scenarios,
and the security properties of these profiles vary. As shown in
Figure 1, this specification is aimed to be integrated into a larger
OAuth deployment. Application developers therefore need to
understand their security requirements based on a threat assessment
before selecting a specific SASL OAuth mechanism. For OAuth 2.0, a
detailed security document [RFC6819] provides guidance to select
those OAuth 2.0 components that help to mitigate threats for a given
deployment. For OAuth 1.0a, Section 4 of [RFC5849] provides guidance
specific to OAuth 1.0a.

This document specifies two SASL Mechanisms for OAuth and each comes
with different security properties.

OAUTHBEARER: This mechanism borrows from OAuth 2.0 bearer tokens
[RFC6750]. It relies on the application using TLS to protect the
OAuth 2.0 bearer token exchange; without TLS usage at the
application layer, this method is completely insecure.
Consequently, TLS MUST be provided by the application when
choosing this authentication mechanism.

OAUTH10A: This mechanism reuses OAuth 1.0a MAC tokens (using the
HMAC-SHA1 keyed message digest), as described in Section 3.4.2 of
[RFC5849]. To compute the keyed message digest in the same way as
in RFC 5839, this specification conveys additional parameters
between the client and the server. This SASL mechanism only
supports client authentication. If server-side authentication is
desirable, then it must be provided by the application underneath
the SASL layer. The use of TLS is strongly RECOMMENDED.

Mills, et al. Standards Track [Page 16]

RFC 7628 SASL OAuth August 2015

Additionally, the following aspects are worth pointing out:

An access token is not equivalent to the user's long term password.

Care has to be taken when these OAuth credentials are used for
actions like changing passwords (as it is possible with some
protocols, e.g., XMPP [RFC6120]). The resource server should
ensure that actions taken in the authenticated channel are
appropriate to the strength of the presented credential.

Lifetime of the application sessions.

It is possible that SASL will be used to authenticate a
connection, and the life of that connection may outlast the life
of the access token used to establish it. This is a common
problem in application protocols where connections are long lived
and not a problem with this mechanism, per se. Resource servers
may unilaterally disconnect clients in accordance with the
application protocol.

Access tokens have a lifetime.

Reducing the lifetime of an access token provides security
benefits, and OAuth 2.0 introduces refresh tokens to obtain new
access tokens on the fly without any need for human interaction.
Additionally, a previously obtained access token might be revoked
or rendered invalid at any time. The client MAY request a new
access token for each connection to a resource server, but it
SHOULD cache and reuse valid credentials.

6. Internationalization Considerations

The identifier asserted by the OAuth authorization server about the
resource owner inside the access token may be displayed to a human.
For example, when SASL is used in the context of IMAP, the client may
assert the resource owner's email address to the IMAP server for
usage in an email-based application. The identifier may therefore
contain internationalized characters, and an application needs to
ensure that the mapping between the identifier provided by OAuth is
suitable for use with the application-layer protocol SASL is
incorporated into. An example of a SASL-compatible container is the
JSON Web Token (JWT) [RFC7519], which provides a standardized format
for exchanging authorization and identity information that supports
internationalized characters.

Mills, et al. Standards Track [Page 17]

RFC 7628 SASL OAuth August 2015

7. IANA Considerations

7.1. SASL Registration

The IANA has registered the following entry in the SASL Mechanisms
registry:

SASL mechanism name: OAUTHBEARER

Security Considerations: See this document

Published Specification: See this document

For further information: Contact the authors of this document.

Intended usage: COMMON

Owner/Change controller: the IESG

Note: None

The IANA has registered the following entry in the SASL Mechanisms
registry:

SASL mechanism name: OAUTH10A

Security Considerations: See this document

Published Specification: See this document

For further information: Contact the authors of this document.

Intended usage: COMMON

Owner/Change controller: the IESG

Note: None

Mills, et al. Standards Track [Page 18]

RFC 7628 SASL OAuth August 2015

8. References

8.1. Normative References

[OpenID.Core]
Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
C. Mortimore, "OpenID Connect Core 1.0", November 2014,
<http://openid.net/specs/openid-connect-core-1_0.html>.

[OpenID.Discovery]
Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
Connect Discovery 1.0", November 2014,
<http://openid.net/specs/
openid-connect-discovery-1_0.html>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC4422] Melnikov, A., Ed. and K. Zeilenga, Ed., "Simple
Authentication and Security Layer (SASL)", RFC 4422,
DOI 10.17487/RFC4422, June 2006,
<http://www.rfc-editor.org/info/rfc4422>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.

[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, <http://www.rfc-editor.org/info/rfc5801>.

[RFC5849] Hammer-Lahav, E., Ed., "The OAuth 1.0 Protocol", RFC 5849,
DOI 10.17487/RFC5849, April 2010,
<http://www.rfc-editor.org/info/rfc5849>.

Mills, et al. Standards Track [Page 19]

RFC 7628 SASL OAuth August 2015

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.

[RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
Framework: Bearer Token Usage", RFC 6750,
DOI 10.17487/RFC6750, October 2012,
<http://www.rfc-editor.org/info/rfc6750>.

[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.

[RFC7591] Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
RFC 7591, DOI 10.17487/RFC7591, July 2015,
<http://www.rfc-editor.org/info/rfc7591>.

8.2. Informative References

[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003,
<http://www.rfc-editor.org/info/rfc3501>.

[RFC4959] Siemborski, R. and A. Gulbrandsen, "IMAP Extension for
Simple Authentication and Security Layer (SASL) Initial
Client Response", RFC 4959, DOI 10.17487/RFC4959,
September 2007, <http://www.rfc-editor.org/info/rfc4959>.

[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.

[RFC6120] Saint-Andre, P., "Extensible Messaging and Presence
Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
March 2011, <http://www.rfc-editor.org/info/rfc6120>.

[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013,
<http://www.rfc-editor.org/info/rfc6819>.

[RFC7033] Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
"WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
2013, <http://www.rfc-editor.org/info/rfc7033>.

Mills, et al. Standards Track [Page 20]

RFC 7628 SASL OAuth August 2015

[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.

[RFC7519] Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
(JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
<http://www.rfc-editor.org/info/rfc7519>.

Acknowledgements

The authors would like to thank the members of the KITTEN working
group and in addition and specifically: Simon Josefson, Torsten
Lodderstadt, Ryan Troll, Alexey Melnikov, Jeffrey Hutzelman, Nico
Williams, Matt Miller, and Benjamin Kaduk.

This document was produced under the chairmanship of Alexey Melnikov,
Tom Yu, Shawn Emery, Josh Howlett, Sam Hartman, Matthew Miller, and
Benjamin Kaduk. The supervising Area Director was Stephen Farrell.

Authors' Addresses

William Mills
Microsoft

Email: [email protected]

Tim Showalter

Email: [email protected]

Hannes Tschofenig
ARM Ltd.
110 Fulbourn Rd
Cambridge CB1 9NJ
United Kingdom

Email: [email protected]
URI: http://www.tschofenig.priv.at

Mills, et al. Standards Track [Page 21]