| OAuth Working Group | B. Campbell |
| Internet-Draft | Ping Identity |
| Intended status: Standards Track | J. Bradley |
| Expires: January 23, 2020 | Yubico |
| H. Tschofenig | |
| Arm Limited | |
| July 22, 2019 |
Resource Indicators for OAuth 2.0
draft-ietf-oauth-resource-indicators-04
An extension to the OAuth 2.0 Authorization Framework defining request parameters that enable a client to explicitly signal to an authorization server about the identity of the protected resource(s) to which it is requesting access.
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Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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Several years of deployment and implementation experience with the OAuth 2.0 Authorization Framework has uncovered a need, in some circumstances, for the client to explicitly signal to the authorization server where it intends to use the access token it is requesting.
Knowing the protected resource (a.k.a. resource server, application, API, etc.) that will process the access token enables the authorization server to construct the token as necessary for that entity. Properly encrypting the token (or content within the token) to a particular resource, for example, requires knowing which resource will receive and decrypt the token. Furthermore, various resources oftentimes have different requirements with respect to the data contained in, or referenced by, the token and knowing the resource where the client intends to use the token allows the authorization server to mint the token accordingly.
Specific knowledge of the intended recipient(s) of the access token also helps facilitate improved security characteristics of the token itself. Bearer tokens, currently the most commonly utilized type of OAuth access token, allow any party in possession of a token to get access to the associated resources. To prevent misuse, several important security assumptions must hold, one of which is that an access token must only be valid for use at a specific protected resource and for a specific scope of access. Section 5.2 of [RFC6750], for example, prescribes including the token's intended recipients within the token to prevent token redirect. When the authorization server is informed of the resource that will process the access token, it can restrict the intended audience of that token to the given resource such that the token cannot be used successfully at other resources.
OAuth scope, from Section 3.3 of [RFC6749], is sometimes overloaded to convey the location or identity of the protected resource, however, doing so isn't always feasible or desirable. Scope is typically about what access is being requested rather than where that access will be redeemed (e.g. email, admin:org, user_photos, channels:read, and channels:write are a small sample of scope values in use in the wild that convey only the type of access and not the location or identity).
In some circumstances and for some deployments, a means for the client to signal to the authorization server where it intends to use the access token it's requesting is important and useful. A number of implementations and deployments of OAuth 2.0 have already employed proprietary parameters toward that end. Going forward, this specification aspires to provide a standardized and interoperable alternative to the proprietary approaches.
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 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This specification uses the terms "access token", "refresh token", "authorization server", "resource server", "authorization endpoint", "authorization request", "authorization response", "token endpoint", "grant type", "access token request", "access token response", and "client" defined by The OAuth 2.0 Authorization Framework.
In requests to the authorization server, a client MAY indicate the protected resource (a.k.a. resource server, application, API, etc.) to which it is requesting access by including the following parameter in the request.
The parameter value identifies a resource to which the client is requesting access. The parameter can carry the location of a protected resource, typically as an https URL, or a more abstract identifier. This enables the authorization server to apply policy as appropriate for the resource, such as determining the type and content of tokens to be issued, if and how tokens are encrypted, and applying appropriate audience restrictions.
The client SHOULD provide the most specific URI that it can for the complete API or set of resources it intends to access. In practice a client will know a base URI for the application or resource that it interacts with, which is appropriate to use as the value of the resource parameter. The client SHOULD use the base URI of the API as the resource parameter value unless specific knowledge of the resource dictates otherwise. For example, the value https://api.example.com/ would be used for a resource that is the exclusive application on that host, however, if the resource is one of many applications on that host, something like https://api.example.com/app/ would be used as a more specific value. Another example, for an API like SCIM that has multiple endpoints such as https://apps.example.com/scim/Users, https://apps.example.com/scim/Groups, and https://apps.example.com/scim/Schemas The client would use https://apps.example.com/scim/ as the resource so that the issued access token is valid for all the endpoints of the SCIM API.
The following error code is provided for an authorization server to indicate problems with the requested resource(s) in response to an authorization request or access token request. It can also be used to inform the client that it has requested an invalid combination of resource and scope.
The authorization server SHOULD audience restrict issued access tokens to the resource(s) indicated by the resource parameter. Audience restrictions can be communicated in JSON Web Tokens with the aud claim and the top-level member of the same name provides the audience restriction information in a Token Introspection response. The authorization server may use the exact resource value as the audience or it may map from that value to a more general URI or abstract identifier for the given resource.
When the resource parameter is used in an authorization request to the authorization endpoint, it indicates the identity of the protected resource(s) to which access is being requested. When an access token will be returned directly from the authorization endpoint via the implicit flow (Section 4.2 of OAuth 2.0), the requested resource is applicable to that access token. In the code flow (Section 4.1 of OAuth 2.0) where an intermediate representation of the authorization grant (the authorization code) is returned from the authorization endpoint, the requested resource is applicable to the full authorization grant.
For authorization requests sent as a JWTs, such as when using JWT Secured Authorization Request, a single resource parameter value is represented as a JSON string while multiple values are represented as an array of strings.
If the client omits the resource parameter when requesting authorization, the authorization server MAY process the request with no specific resource or by using a pre-defined default resource value. Alternatively, the authorization server MAY require clients to specify the resource(s) they intend to access and MAY fail requests that omit the parameter with an invalid_target error. The authorization server might use this data to inform the user about the resources the client is going to access on her behalf, to meet policy decision (e.g. refuse the request due to unknown resources), and determine the set of resources that can be used in subsequent access token requests.
If the authorization server fails to parse the provided value(s) or does not consider the resource(s) acceptable, it should reject the request with an error response using the error code invalid_target as the value of the error parameter and can provide additional information regarding the reasons for the error using the error_description and/or error_uri parameters.
GET /as/authorization.oauth2?response_type=token
&client_id=example-client
&state=XzZaJlcwYew1u0QBrRv_Gw
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
&resource=https%3A%2F%2Fapi.example.com%2Fapp%2F HTTP/1.1
Host: authorization-server.example.com
Figure 1: Implicit Flow Authorization Request
An example of an authorization request where the client tells the authorization server that it wants an access token for use at https://api.example.com/app/ is shown in Figure 1 below (extra line breaks and indentation are for display purposes only).
GET /as/authorization.oauth2?response_type=code
&client_id=s6BhdRkqt3
&state=tNwzQ87pC6llebpmac_IDeeq-mCR2wLDYljHUZUAWuI
&redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
&scope=calendar%20contacts
&resource=https%3A%2F%2Fcal.example.com%2F
&resource=https%3A%2F%2Fcontacts.example.com%2F HTTP/1.1
Host: authorization-server.example.com
Figure 2: Code Flow Authorization Request
Below in Figure 2 is an example of an authorization request using the code response type where the client is requesting access to the resource owner's contacts and calendar data at https://cal.example.com/ and https://contacts.example.com/ (extra line breaks and indentation are for display purposes only).
When the resource parameter is used on an access token request made to the token endpoint, for all grant types, it indicates the target service or protected resource where the client intends to use the requested access token.
The resource value(s) that are acceptable to an authorization server in fulfilling an access token request are at its sole discretion based on local policy or configuration. In the case of a refresh_token or authorization_code grant type request, such policy may limit the acceptable resources to those that were originally granted by the resource owner or a subset thereof. In the authorization_code case where the requested resources are a subset of the set of resources originally granted, the authorization server will issue an access token based on that subset of requested resources while any refresh token that is returned is bound to the full original grant.
When requesting a token, the client can indicate the desired target service(s) where it intends to use that token by way of the resource parameter and can indicate the desired scope of the requested token using the scope parameter. The semantics of such a request are that the client is asking for a token with the requested scope that is usable at all the requested target services. Effectively, the requested access rights of the token are the cartesian product of all the scopes at all the target services. To the extent possible, when issuing access tokens, the authorization server should downscope the scope value associated with an access token to the value the respective resource is able to process and needs to know. This further improves privacy as scope values give an indication of what services the resource owner uses and downscoping a token to only that which is needed for a particular service can limit the extent to which such information is revealed across different services. As specified in Section 5.1 of [RFC6749], the authorization server must indicate the access token's effective scope to the client in the scope response parameter value when it differs from the scope requested by the client.
POST /as/token.oauth2 HTTP/1.1 Host: authorization-server.example.com Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ Content-Type: application/x-www-form-urlencoded grant_type=authorization_code &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb &code=10esc29BWC2qZB0acc9v8zAv9ltc2pko105tQauZ &resource=https%3A%2F%2Fcal.example.com%2F
Figure 3: Access Token Request
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-cache, no-store
{
"access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi
JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI
iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgwMCwic2NvcGUiOiJjYWxlbmRhciIs
ImF1ZCI6Imh0dHBzOi8vY2FsLmV4YW1wbGUuY29tLyJ9.nNWJ2dXSxaDRdMUK
lzs-cYIj8MDoM6Gy7pf_sKrLGsAFf1C2bDhB60DQfW1DZL5npdko1_Mmk5sUf
zkiQNVpYw",
"token_type":"Bearer",
"expires_in":3600,
"refresh_token":"4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2",
"scope":"calendar"
}
Figure 4: Access Token Response
Following from the code flow authorization request shown in Figure 2, the below examples show an authorization_code grant type access token request (Figure 3) and response (Figure 4) where the client tells the authorization server that it wants the access token for use at https://cal.example.com/ (extra line breaks and indentation are for display purposes only).
POST /as/token.oauth2 HTTP/1.1 Host: authorization-server.example.com Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ Content-Type: application/x-www-form-urlencoded grant_type=refresh_token &refresh_token=4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2 &resource=https%3A%2F%2Fcontacts.example.com%2Fapp%2F
Figure 5: Access Token Request
HTTP/1.1 200 OK
Content-Type: application/json
Cache-Control: no-cache, no-store
{
"access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi
JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI
iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgyNiwic2NvcGUiOiJjb250YWN0cyIs
ImF1ZCI6Imh0dHBzOi8vY29udGFjdHMuZXhhbXBsZS5jb20vIn0.5f4yhqazc
OSlJw4y94KPeWNEFQqj2cfeO8x4hr3YbHtIl3nQXnBMw5wREY5O1YbZED-GfH
UowfmtNaA5EikYAw",
"token_type":"Bearer",
"expires_in":3600,
"scope":"contacts"
}
Figure 6: Access Token Response
A subsequent access token request, using the refresh token, where the client tells the authorization server that it wants an access token for use at https://contacts.example.com/ is shown in Figure 5 below with the response shown in Figure 6 (extra line breaks and indentation are for display purposes only).
An access token that is audience restricted to a protected resource that obtains that token legitimately cannot be used to access resources on behalf of the resource owner at other protected resources. The resource parameter enables a client to indicate the protected resources where the requested access token will be used, which in turn enables the authorization server to apply the appropriate audience restrictions to the token.
Some servers may host user content or be multi-tenant. In order to avoid attacks that might confuse a client into sending an access token to a resource that is user controlled or is owned by a different tenant, it is important to use a specific resource URI including a path component. This will cause any access token issued for accessing the user controlled resource to have an invalid audience if replayed against the legitimate resource API.
Although multiple occurrences of the resource parameter may be included in a request, using only a single resource parameter is encouraged. A bearer token that has multiple intended recipients (audiences) indicating that the token is valid at more than one protected resource can be used by any one of those protected resources to access any of the other protected resources. Thus, a high degree of trust between the involved parties is needed when using access tokens with multiple audiences. Furthermore an authorization server may be unwilling or unable to fulfill a token request with multiple resources.
Whenever feasible, the resource parameter should correspond to the network addressable location of the protected resource. This makes it possible for the client to validate that the resource being requested controls the corresponding network location, reducing the risk of malicious endpoints obtaining tokens meant for other resources. If the resource parameter contains an abstract identifier, it is the client's responsibility to validate out of band that any network endpoint to which tokens are sent are the intended audience for that identifier.
This specification updates the following value in the IANA "OAuth Parameters" registry [IANA.OAuth.Parameters] established by [RFC6749].
This specification updates the following error in the IANA "OAuth Extensions Error Registry" [IANA.OAuth.Parameters] established by [RFC6749].
| [IANA.OAuth.Parameters] | IANA, "OAuth Parameters" |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC3986] | Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005. |
| [RFC6749] | Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
| [I-D.ietf-oauth-jwsreq] | Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization Framework: JWT Secured Authorization Request (JAR)", Internet-Draft draft-ietf-oauth-jwsreq-19, June 2019. |
| [RFC6750] | Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, October 2012. |
| [RFC7519] | Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015. |
| [RFC7644] | Hunt, P., Grizzle, K., Ansari, M., Wahlstroem, E. and C. Mortimore, "System for Cross-domain Identity Management: Protocol", RFC 7644, DOI 10.17487/RFC7644, September 2015. |
| [RFC7662] | Richer, J., "OAuth 2.0 Token Introspection", RFC 7662, DOI 10.17487/RFC7662, October 2015. |
This specification was developed within the OAuth Working Group under the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with Eric Rescorla, Benjamin Kaduk and Roman Danyliw serving as Security Area Directors. Additionally, the following individuals contributed ideas, feedback, and wording that helped shape this specification:
Vittorio Bertocci, Sergey Beryozkin, Roman Danyliw, William Denniss, Vladimir Dzhuvinov, George Fletcher, Dick Hardt, Phil Hunt, Michael Jones, Torsten Lodderstedt, Anthony Nadalin, Justin Richer, Nat Sakimura, Rifaat Shekh-Yusef, Filip Skokan, and Hans Zandbelt.
[[ to be removed by the RFC Editor before publication as an RFC ]]
draft-ietf-oauth-resource-indicators-04
draft-ietf-oauth-resource-indicators-03
draft-ietf-oauth-resource-indicators-02
draft-ietf-oauth-resource-indicators-01
draft-ietf-oauth-resource-indicators-00
draft-campbell-oauth-resource-indicators-02
draft-campbell-oauth-resource-indicators-01
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