| KITTEN | W. Mills |
| Internet-Draft | Microsoft |
| Intended status: Standards Track | T. Showalter |
| Expires: May 16, 2015 | |
| H. Tschofenig | |
| ARM Ltd. | |
| November 12, 2014 |
A set of SASL Mechanisms for OAuth
draft-ietf-kitten-sasl-oauth-17.txt
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 serve. 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.
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Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved.
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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 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.
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] and the MAC Token specification [I-D.ietf-oauth-v2-http-mac]. OAuth 1.0a included the protocol specification for the communication between the OAuth client and the resource server in [RFC5849].
The main use cases for OAuth 2.0 and OAuth 1.0a have so far focused on an HTTP-based [RFC2616] environment only. This document integrates OAuth 1.0a and OAuth 2.0 into non-HTTP-based applications using the integration into SASL. Hence, this document takes advantage of the OAuth protocol and its deployment base to provide a way to use the Simple Authentication and Security Layer (SASL) [RFC4422] 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], which is what this memo uses in the examples.
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.
----+
+--------+ +---------------+ |
| |--(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
The Simple Authentication and Security Layer (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 protocols 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:
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 [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 [I-D.ietf-oauth-dyn-reg].
OAuth 1.0 follows a similar model but uses a different terminology and does not separate the resource server from the authorization server.
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 [RFC2119].
The reader is assumed to be familiar with the terms used in the OAuth 2.0 specification [RFC6749] and SASL [RFC4422].
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, see Section 4 of [RFC4648], not this memo.
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:
New extensions may be defined to add additional OAuth Access Token Types. Such a new SASL OAuth mechanism can be added by simply registering the new name(s) and citing this specification for the further definition.
These mechanisms are client initiated and lock-step, the server always replying to a client message. In the case where the client has and correctly uses a valid token the flow is:
In the case where authorization fails the server sends an error result, then 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 message and finalizing a SASL negotiation, the additional client message in the error case deals with this problem. This exchange is:
Client responses are a GS2 [RFC5801] header followed by zero or more key/value pairs, or may be empty. The gs2-header is defined here for compatibility with GS2 if a GS2 mechanism is formally defined, but this document does not define one. These 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 0*kvpair kvsep) / kvsep
The GS2 header MAY include the user name 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 following keys and corresponding values are defined in the client response:
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.
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 use request keyed message digest the default values MUST be used unless explicit values are provided in the client response. The following key values are reserved for future use:
The server validates the response according 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 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 authz-id is specified by the SASL framework [RFC4422].
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.
For a failed authentication the server returns a JSON [RFC4627] formatted error result, and fails the authentication. The error result consists of the following values:
If the resource server provides a scope then the client MUST always request scoped tokens from the token endpoint. If the resource server provides no scope to the client then the client SHOULD presume an empty scope (unscoped token) is required to access the resource.
Since clients may interact with a number of application servers, such as email servers and XMPP 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 re-used to obtain an access token for the desired resource server. This specification RECOMMENDs that a client uses the information in the 'issue' element to make this determination.
Section 3.6 of [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 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.
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 which allow 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 which 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, on an IMAP server running on port 143 and given the OAuth 1.0a style authorization request (with %x01 shown as ^A and line breaks added for readability) below:
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 1.0 specification [RFC5849] with the following things noted:
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
These examples illustrate exchanges between IMAP and SMTP clients and servers.
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".
This example shows a successful OAuth 2.0 bearer token exchange in IMAP. Note that line breaks are inserted for readability and the underlying TLS establishment is not shown either.
S: * OK IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER SASL-IR
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTHBEARER bixhPXVzZXJAZXhhbXBsZS5jb20sAWhvc3Q9c2
VydmVyLmV4YW1wbGUuY29tAXBvcnQ9MTQzAWF1dGg9QmVhcmVyIHZGOWRmdDRxb
VRjMk52YjNSbGNrQmhiSFJoZG1semRHRXVZMjl0Q2c9PQEB
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
The same credential used in an SMTP exchange is shown below. Note that line breaks are inserted for readability, and that the SMTP protocol terminates lines with CR and LF characters (ASCII values 0x0D and 0x0A), these are not displayed explicitly in the example.
[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
C: t1 AUTHENTICATE OAUTHBEARER bixhPXVzZXJAZXhhbXBsZS5jb20sAWhvc3Q9c
2VydmVyLmV4YW1wbGUuY29tAXBvcnQ9MTQzAWF1dGg9QmVhcmVyIHZGOWRmdDR
xbVRjMk52YjNSbGNrQmhiSFJoZG1semRHRXVZMjl0Q2c9PQEB
S: 235 Authentication successful.
[connection continues...]
This IMAP example shows a successful OAuth 1.0a token exchange. Note that line breaks are inserted for readability and the underlying TLS establishment is not shown. Signature computation is discussed in Section 3.3.
S: * OK IMAP4rev1 Server Ready
C: t0 CAPABILITY
S: * CAPABILITY IMAP4rev1 AUTH=OAUTHBEARER OAUTH10A SASL-IR
S: t0 OK Completed
C: t1 AUTHENTICATE OAUTH10A bixhPXVzZXJAZXhhbXBsZS5jb20sAWhvc3Q9ZXhhb
XBsZS5jb20BcG9ydD0xNDMBYXV0aD1PQXV0aCByZWFsbT0iRXhhbXBsZSIsb2F1
dGhfY29uc3VtZXJfa2V5PSI5ZGpkajgyaDQ4ZGpzOWQyIixvYXV0aF90b2tlbj0
ia2trOWQ3ZGgzazM5c2p2NyIsb2F1dGhfc2lnbmF0dXJlX21ldGhvZD0iSE1BQy
1TSEExIixvYXV0aF90aW1lc3RhbXA9IjEzNzEzMTIwMSIsb2F1dGhfbm9uY2U9I
jdkOGYzZTRhIixvYXV0aF9zaWduYXR1cmU9IlRtOTBJR0VnY21WaGJDQnphV2R1
WVhSMWNtVSUzRCIBAQ==
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 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
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 IMAP4rev1 Server
Ready
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-configuration"
}
The client responds with the required dummy response, "AQ==" is the base64 encoding of the ASCII value 0x01.
This example shows an authorization failure in an SMTP exchange. Note that line breaks are inserted for readability, and that the SMTP protocol terminates lines with CR and LF characters (ASCII values 0x0D and 0x0A), these are not displayed explicitly in the example.
[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
C: AUTH OAUTHBEARER bix1c2VyPXNvbWV1c2VyQGV4YW1wbGUuY29tLAFhdXRoPUJlYXJl
ciB2RjlkZnQ0cW1UYzJOdmIzUmxja0JoZEhSaGRtbHpkR0V1WTI5dENnPT0BAQ==
S: 334 eyJzdGF0dXMiOiI0MDEiLCJzY2hlbWVzIjoiYmVhcmVyIG1hYyIsInNjb3BlIjoia
HR0cHM6Ly9tYWlsLmdvb2dsZS5jb20vIn0K
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 server returned an error message in the 334 SASL message, the client responds with the required dummy response, and the server finalizes the negotiation.
OAuth 1.0a and OAuth 2 allows 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 the needs of 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 RFC 5849 [RFC5849] provides guidance specific to OAuth 1.0.
This document specifies two SASL Mechanisms for OAuth and each comes with different security properties.
Additionally, the following aspects are worth pointing out:
The identifer 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.
At the time of writing the standardization of the various claims in the access token (in JSON format) is still ongoing, see [I-D.ietf-oauth-json-web-token]. Once completed it will provide a standardized format for exchanging identity information between the authorization server and the resource server.
The IANA is requested to register the following SASL profile:
The IANA is requested to register the following SASL profile:
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. The supervising area director was Stephen Farrell.
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