OAuth Protocol Flow - Token Exchange
The token exchange protocol is an extension to OAuth 2.0 that allows one OAuth 2.0 token to be exchanged for another type of token. This exchange typically occurs between different entities in a system, such as a client exchanging an access token for another access token with different scopes, etc.
Endpoint | Parameters | Response |
---|---|---|
/oauth2.0/accessToken |
grant_type=...&resource=https://...&subject_token=...&subject_token_type=...&requested_token_type=... |
New exchanged token and its type. |
Application Configuration
The requested grant type must be urn:ietf:params:oauth:grant-type:token-exchange
. This grant type must also be
specified for
the relevant registered service definition.
-
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{ "@class" : "org.apereo.cas.support.oauth.services.OAuthRegisteredService", "clientId": "clientid", "clientSecret": "clientSecret", "serviceId" : "^(https|imaps)://<redirect-uri>.*", "name" : "OAuthService", "id" : 100, "supportedGrantTypes": [ "java.util.HashSet", [ "urn:ietf:params:oauth:grant-type:token-exchange" ] ], "tokenExchangePolicy": { "@class": "org.apereo.cas.support.oauth.services.DefaultRegisteredServiceOAuthTokenExchangePolicy", "allowedResources": [ "java.util.HashSet", [ "..." ] ], "allowedAudience": [ "java.util.HashSet", [ "..." ] ], "allowedTokenTypes": [ "java.util.HashSet", [ "urn:ietf:params:oauth:token-type:access_token", "urn:ietf:params:oauth:token-type:jwt" ] ] } }
Furthermore, as it’s shown the above example, one may define a dedicated token exchange policy that would control the allowed resources, audience and token types for the token exchange operation. All fields defined for the token exchange policy can accept regular expression patterns.
-
The following configuration is applicable to delegation token exchange flows where the client would provide both a
subject_token
and anactor_token
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{ "@class" : "org.apereo.cas.support.oauth.services.OAuthRegisteredService", "clientId": "clientid", "clientSecret": "clientSecret", "serviceId" : "^(https|imaps)://<redirect-uri>.*", "name" : "OAuthService", "id" : 100, "supportedGrantTypes": [ "java.util.HashSet", [ "urn:ietf:params:oauth:grant-type:token-exchange" ] ], "tokenExchangePolicy": { "@class": "org.apereo.cas.support.oauth.services.DefaultRegisteredServiceOAuthTokenExchangePolicy", "allowedActorTokenTypes": [ "java.util.HashSet", [ "urn:ietf:params:oauth:token-type:access_token", "urn:ietf:params:oauth:token-type:jwt" ]], "requiredActorTokenAttributes" : { "@class" : "java.util.HashMap", "memberOf" : [ "java.util.HashSet", [ ".*can-act-as.*" ] ] } } }
The subject (
sub
claim) of the subject_token is used to make the request. The actor (act
claim) of the produced token in the end is the same as the subject of theactor_token
used to make the request. This for example indicates delegation and identifies anadmin
as the current actor to whom authority has been delegated to act on behalf of auser
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{ "aud": "...", "iss":"...", "exp": 1234, "scope": "...", "sub": "user", "act": { "sub":"admin" } }
Delegation semantics are typically expressed in a token by including information about both the primary subject of the token as well as the actor to whom that subject has delegated some of its rights. Typically, in the request, the
subject_token
represents the identity of the party on behalf of whom the token is being requested while theactor_token
represents the identity of the party to whom the access rights of the issued token are being delegated.
Supported Exchanges
The following combinations of token types are supported for impersonation operations:
Subject Token Type | Issued Token Type | Description |
---|---|---|
urn:ietf:params:oauth:token-type:access_token |
urn:ietf:params:oauth:token-type:access_token |
Exchange an access token for another. |
urn:ietf:params:oauth:token-type:access_token |
urn:ietf:params:oauth:token-type:jwt |
Exchange an access token for another as JWT. |
urn:ietf:params:oauth:token-type:access_token |
urn:ietf:params:oauth:token-type:id_token |
Exchange an access token for an OpenID Connect ID token. |
The following combinations of token types are supported for delegation operations:
Subject Token Type | Issued Token Type | Description |
---|---|---|
urn:ietf:params:oauth:token-type:access_token |
urn:ietf:params:oauth:token-type:access_token |
Exchange an access token for another. |
urn:ietf:params:oauth:token-type:access_token |
urn:ietf:params:oauth:token-type:jwt |
Exchange an access token for another as JWT. |
Delegation vs. Impersonation
When principal A
impersonates principal B
, A
is given all the rights that B
has within some defined rights context and
is indistinguishable from B
in that context. Thus, when principal A
impersonates principal B
, then insofar as any
entity receiving such a token is concerned, they are actually dealing with B
. It is true that some members of the
identity system might have awareness that impersonation is going on, but it is not a requirement. For all intents
and purposes, when A
is impersonating B, A
is B
within the context of the rights authorized by the token.
With delegation semantics, principal A
still has its own identity separate from B
, and it is explicitly understood that while B
may have delegated some of its rights to A
, any actions taken are being taken by A
representing B
.
In a sense, A
is an agent for B
.