| OAuth Working Group | J. Richer, Ed. |
| Internet-Draft | March 27, 2015 |
| Intended status: Standards Track | |
| Expires: September 28, 2015 |
OAuth 2.0 Token Introspection
draft-ietf-oauth-introspection-07
This specification defines a method for a protected resource to query an OAuth 2.0 authorization server to determine the active state of an OAuth 2.0 token and to determine meta-information about this token. OAuth 2.0 deployments can use this method to convey information about the authorization context of the token from the authorization server to the protected resource.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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In OAuth 2.0, the contents of tokens are opaque to clients. This means that the client does not need to know anything about the content or structure of the token itself, if there is any. However, there is still a large amount of metadata that may be attached to a token, such as its current validity, approved scopes, and information about the context in which the token was issued. These pieces of information are often vital to protected resources making authorization decisions based on the tokens being presented. Since OAuth 2.0 [RFC6749] does not define a protocol for the resource server to learn meta-information about a token that is has received from an authorization server, several different approaches have been developed to bridge this gap. These include using structured token formats such as JWT [JWT] or proprietary inter-service communication mechanisms (such as shared databases and protected enterprise service buses) that convey token information.
This specification defines an interoperable web API that allows authorized protected resources to query the authorization server to determine the set of metadata for a given token that was presented to them by an OAuth 2.0 client. This metadata includes whether or not the token is currently active (or if it has expired or otherwise been revoked), what rights of access the token carries (usually conveyed through OAuth 2.0 scopes), and the authorization context in which the token was granted (including who authorized the token and which client it was issued to). Token introspection allows a protected resource to query this information regardless of whether or not it is carried in the token itself, allowing this method to be used along with or independently of structured token values. Additionally, a protected resource can use the mechanism described in this specification to introspect the token in a particular authorization decision context and ascertain the relevant metadata about the token in order to make this authorization decision appropriately.
This section defines the terminology used by this specification. This section is a normative portion of this specification, imposing requirements upon implementations.
This specification uses the terms "access token", "authorization endpoint", "authorization grant", "authorization server", "client", "client identifier", "protected resource", "refresh token", "resource owner", "resource server", and "token endpoint" defined by OAuth 2.0 [RFC6749], and the terms "claim names" and "claim values" defined by JSON Web Token (JWT) [JWT].
The introspection endpoint is an OAuth 2.0 endpoint that takes a parameter representing an OAuth 2.0 token and returns a JSON [RFC7159] document representing the meta information surrounding the token, including whether this token is currently active. The definition of an active token is up to the authorization server, but this is commonly a token that has been issued by this authorization server, is not expired, has not been revoked, and is within the purview of the protected resource making the introspection call.
The introspection endpoint MUST be protected by a transport-layer security mechanism as described in Section 4.
The protected resource calls the introspection endpoint using an HTTP POST [RFC7231] request with parameters sent as application/x-www-form-urlencoded data as defined in [W3C.REC-html5-20141028]. The authorization server MAY allow an HTTP GET [RFC7231] request with parameters passed in the query string as defined in [W3C.REC-html5-20141028]. The protected resource sends a parameter representing the token along with optional parameters representing additional context that is known by the protected resource to aid the authorization server in its response.
The endpoint MAY allow other parameters to provide further context to the query. For instance, an authorization service may need to know the IP address of the client accessing the protected resource in order to determine the appropriateness of the token being presented.
To prevent unauthorized token scanning attacks, the endpoint MUST also require some form of authorization to access this endpoint, such as client authentication as described in OAuth 2.0 [RFC6749] or a separate OAuth 2.0 access token such as the bearer token described in OAuth 2.0 Bearer Token Usage [RFC6750]. The methods of managing and validating these authentication credentials are out of scope of this specification.
For example, the following example shows a protected resource calling the token introspection endpoint to query about an OAuth 2.0 bearer. The protected resource is using a separate OAuth 2.0 bearer token to authorize this call.
Following is a non-normative example request:
POST /introspect HTTP/1.1 Host: server.example.com Accept: application/json Content-Type: application/x-www-form-urlencoded Authorization: Bearer 23410913-abewfq.123483 token=2YotnFZFEjr1zCsicMWpAA
In this example, the protected resource uses a client identifier and client secret to authenticate itself to the introspection endpoint as well as send a token type hint.
Following is a non-normative example request:
POST /introspect HTTP/1.1 Host: server.example.com Accept: application/json Content-Type: application/x-www-form-urlencoded Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW token=mF_9.B5f-4.1JqM&token_type_hint=access_token
The server responds with a JSON object [RFC7159] in application/json format with the following top-level members.
Specific implementations MAY extend this structure with their own service-specific response names as top-level members of this JSON object. Response names intended to be used across domains MUST be registered in the OAuth Token Introspection Response registry defined in Section 3.1.
The authorization server MAY respond differently to different protected resources making the same request. For instance, an authorization server MAY limit which scopes from a given token are returned for each protected resource in order to prevent protected resources from learning more about the larger network than is necessary for their function.
The response MAY be cached by the protected resource to improve performance and reduce load on the introspection endpoint, but at the cost of liveness of the information used by the protected resource. See Section 4 for more information regarding the trade off when the response is cached.
For example, the following response contains a set of information about an active token:
Following is a non-normative example response:
HTTP/1.1 200 OK
Content-Type: application/json
{
"active": true,
"client_id": "l238j323ds-23ij4",
"username": "jdoe",
"scope": "read write dolphin",
"sub": "Z5O3upPC88QrAjx00dis",
"aud": "https://protected.example.net/resource",
"iss": "https://server.example.com/",
"exp": 1419356238,
"iat": 1419350238,
"extension_field": "twenty-seven"
}
If the introspection call is properly authorized but the token is not active, does not exist on this server, or the protected resource is not allowed to introspect this particular token, the authorization server MUST return an introspection response with the active field set to false. Note that in order to avoid disclosing too much of the authorization server's state to a third party, the authorization server SHOULD NOT include any additional information about an inactive token, including why the token is inactive. For example, the response for a token that has been revoked or is otherwise invalid would look like the following:
Following is a non-normative example response:
HTTP/1.1 200 OK
Content-Type: application/json
{
"active": false
}
If the protected resource uses OAuth 2.0 client credentials to authenticate to the introspection endpoint and its credentials are invalid, the authorization server responds with an HTTP 401 (Unauthorized) as described in section 5.2 of OAuth 2.0 [RFC6749].
If the protected resource uses an OAuth 2.0 bearer token to authorize its call to the introspection endpoint and the token used for authorization does not contain sufficient privileges or is otherwise invalid for this request, the authorization server responds with an HTTP 401 code as described in section 3 of OAuth 2.0 Bearer Token Usage [RFC6750].
Note that a properly formed and authorized query for an inactive or otherwise invalid token (or a token the protected resource is not allowed to know about) is not considered an error response by this specification. In these cases, the authorization server MUST instead respond with an introspection response with the active field set to false as described in Section 2.2.
This specification establishes the OAuth Token Introspection Response registry.
OAuth registration client metadata names and descriptions are registered with a Specification Required ([RFC5226]) after a two-week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of names prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published.
Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register OAuth Token Introspection Response name: example").
Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.
IANA must only accept registry updates from the Designated Expert(s) and should direct all requests for registration to the review mailing list.
The initial contents of the OAuth Token Introspection Response registry are:
Since there are many different and valid ways to implement an OAuth 2.0 system, there are consequently many ways for an authorization server to determine whether or not a token is currently active or not. However, since resource servers using token introspection rely on the authorization server to determine the state of a token, the authorization server MUST perform all applicable checks against a token's state. For instance:
If an authorization server fails to perform any applicable check, the resource server could make an errant security decision based on that response. Note that not all of these checks will be applicable to all OAuth 2.0 deployments and it is up to the authorization server to determine which of these checks (and any other checks) apply.
If left unprotected and un-throttled, the introspection endpoint could present a means for an attacker to poll a series of possible token values, fishing for a valid token. To prevent this, the authorization server MUST require authentication of protected resources that need to access the introspection endpoint and SHOULD require protected resources to be specifically authorized to call the introspection endpoint. The specifics of this authentication credentials are out of scope of this specification, but commonly these credentials could take the form of any valid client authentication mechanism used with the token endpoint, an OAuth 2.0 access token, or other HTTP authorization or authentication mechanism. A single piece of software acting as both a client and a protected resource MAY re-use the same credentials between the token endpoint and the introspection endpoint, though doing so potentially conflates the activities of the client and protected resource portions of the software and the authorization server MAY require separate credentials for each mode.
Since the introspection endpoint takes in OAuth 2.0 tokens as parameters and responds with information used to make authorization decisions, the server MUST support TLS 1.2 RFC 5246 [RFC5246] and MAY support additional transport-layer mechanisms meeting its security requirements. When using TLS, the client or protected resource MUST perform a TLS/SSL server certificate check, per RFC 6125 [RFC6125]. Implementation security considerations can be found in Recommendations for Secure Use of TLS and DTLS [TLS.BCP].
In order to prevent the values of access tokens being from leaking into server-side logs via query parameters, an authorization server offering token introspection MAY disallow HTTP GET and instead require an HTTP POST method to be used at the introspection endpoint.
In order to avoid disclosing internal server state, an introspection response for an inactive token SHOULD NOT contain any additional claims beyond the required active claim (with its value set to false).
Since a protected resource MAY cache the response of the introspection endpoint, designers of an OAuth 2.0 system using this protocol MUST consider the performance and security trade-offs inherent in caching security information such as this. A less aggressive cache with a short timeout will provide the protected resource with more up to date information (due to it needing to query the introspection endpoint more often) at the cost of increased network traffic and load on the introspection endpoint. A more aggressive cache with a longer duration will minimize network traffic and load on the introspection endpoint, but at the cost of liveness of information about the token. For example, the token may be revoked while the protected resource is relying on the value of the cached response to make authorization decisions. This creates a window during which a revoked token could be used at the protected resource. Consequently, an acceptable cache validity duration needs to be carefully considered given the concerns and sensitivities of the protected resource being accessed and the likelihood of a token being revoked or invalidated in the interim period. Highly sensitive environments can opt to disable caching entirely on the protected resource in order to maximize liveness of information.
An authorization server offering token introspection MUST be able to understand the token values being presented to it during this call. The exact means by which this happens is an implementation detail and outside the scope of this specification. For unstructured tokens, this could take the form of a simple server-side database query against a data store containing the context information for the token. For structured tokens, this could take the form of the server parsing the token, validating its signature or other protection mechanisms, and returning the information contained in the token back to the protected resource (allowing the protected resource to be unaware of the token's contents, much like the client).
Note that for tokens carrying encrypted information that is needed during the introspection process, the authorization server MUST be able to decrypt and validate the token in order to access this information. Also note that in cases where the authorization server stores no information about the token and has no means of accessing information about the token by parsing the token itself, it can not likely offer an introspection service.
The introspection response may contain privacy-sensitive information such as user identifiers for resource owners. When this is the case, measures MUST be taken to prevent disclosure of this information to unintended parties. One way to limit disclosure is to require authorization to call the introspection endpoint and to limit calls to only registered and trusted protected resource servers. Another method is to transmit user identifiers as opaque service-specific strings, potentially returning different identifiers to each protected resource. Omitting privacy-sensitive information from an introspection response is the simplest way of minimizing privacy issues.
Thanks to the OAuth Working Group and the UMA Working Group for feedback.
| [JWT] | Jones, M., Bradley, J. and N. Sakimura, "JSON Web Token (JWT)", Internet-Draft draft-ietf-oauth-json-web-token, July 2014. |
| [TLS.BCP] | Sheffer, Y., Holz, R. and P. Saint-Andre, "Recommendations for Secure Use of TLS and DTLS", November 2014. |
With bearer tokens such as those defined by OAuth 2.0 Bearer Token Usage [RFC6750], the protected resource will have in its possession the entire secret portion of the token for submission to the introspection service. However, for proof-of-possession style tokens, the protected resource will have only a token identifier used during the request, along with the cryptographic signature on the request. The protected resource would be able to submit the token identifier to the authorization server's token endpoint in order to obtain the necessary key information needed to validate the signature on the request. The details of this usage are outside the scope of this specification and will be defined in an extension to this specification.
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