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rfc:rfc7628

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

Mills, et al. Standards Track [Page 2] 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.

Mills, et al. Standards Track [Page 3] RFC 7628 SASL OAuth August 2015

  1. —+

+——–+ +—————+ |

 |        |--(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] RFC 7628 SASL OAuth August 2015

      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] RFC 7628 SASL OAuth August 2015

 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

Mills, et al. Standards Track [Page 6] RFC 7628 SASL OAuth August 2015

 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.

Mills, et al. Standards Track [Page 7] RFC 7628 SASL OAuth August 2015

    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

Mills, et al. Standards Track [Page 8] 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.

Mills, et al. Standards Track [Page 9] 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.

Mills, et al. Standards Track [Page 10] 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,a=user@example.com,^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,a=user@example.com,^Ahost=server.example.com^Aport=143^A
 auth=Bearer vF9dft4qmTc2Nvb3RlckBhbHRhdmlzdGEuY29tCg==^A^A

Mills, et al. Standards Track [Page 12] 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,a=user@example.com,^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

Mills, et al. Standards Track [Page 13] RFC 7628 SASL OAuth August 2015

 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,a=user@example.com,^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,a=user@example.com,^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"
}

Mills, et al. Standards Track [Page 14] 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,user=someuser@example.com,^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: wmills_92105@yahoo.com
 Tim Showalter
 Email: tjs@psaux.com
 Hannes Tschofenig
 ARM Ltd.
 110 Fulbourn Rd
 Cambridge  CB1 9NJ
 United Kingdom
 Email: Hannes.tschofenig@gmx.net
 URI:   http://www.tschofenig.priv.at

Mills, et al. Standards Track [Page 21]

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