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Posted to dev@knox.apache.org by "Sandor Molnar (Jira)" <ji...@apache.org> on 2026/10/20 08:20:00 UTC
[jira] [Updated] (KNOX-3520) Token exchange: support multi-hop RFC 8693 delegation by binding re-exchanged subject tokens to the actor (delegation policy subjectAudiences)
[ https://issues.apache.org/jira/browse/KNOX-3520?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Sandor Molnar updated KNOX-3520:
--------------------------------
Description:
h2. Summary
The RFC 8693 token exchange topology validates every presented JWT (both {{subject_token}} and {{{}actor_token{}}}) against a single {{knox.token.audiences}} list. Tokens minted by a delegated exchange carry only the requested target audience, so they can never be presented again as a {{{}subject_token{}}}. As a result, multi-hop on-behalf-of chains (A -> B -> C -> D) stop after the first delegated hop, even though {{delegation.max.actor.chain.depth}} suggests deeper chains are supported.
The two available workarounds either do not scale (adding the token-service audience to every downstream token) or remove an important protection (dropping {{{}knox.token.audiences{}}}, which lets any stolen service-account token, minted for any audience, act as a delegation actor).
This JIRA proposes per-role audience rules: actor tokens stay strictly bound to the token-service audience, while a re-exchanged {{subject_token}} is accepted only when it is addressed to the actor presenting it. The actor's own audiences are declared in a new delegation policy field, {{{}subjectAudiences{}}}.
h2. Current configuration
{code:xml}
<topology>
<gateway>
<provider>
<role>federation</role>
<name>JWTProvider</name>
<enabled>true</enabled>
<param>
<name>jwt.expected.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<param>
<name>knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>jwt.instance.key.fallback</name>
<value>true</value>
</param>
<param>
<name>delegation.server.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.requested.subject.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.enforce.requested.audience.required</name>
<value>true</value>
</param>
<param>
<name>delegation.enforce.requested.audience.max.one</name>
<value>true</value>
</param>
<param>
<name>token.exchange.same.subject.requested.audience.enabled</name>
<value>true</value>
</param>
</provider>
<provider>
<role>identity-assertion</role>
<name>Default</name>
<enabled>true</enabled>
</provider>
</gateway>
<service>
<role>KNOXIDF</role>
<param>
<name>knoxidf.knox.token.ttl</name>
<value>3600000</value>
</param>
<param>
<name>delegation.max.actor.chain.depth</name>
<value>6</value>
</param>
<param>
<name>knoxidf.knox.token.limit.per.user</name>
<value>500</value>
</param>
<param>
<name>knoxidf.knox.token.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<param>
<name>knoxidf.knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>knoxidf.knox.token.enable.delegated.auth</name>
<value>true</value>
</param>
<param>
<name>knoxidf.knox.token.audience.validator</name>
<value>passthrough</value>
</param>
</service>
</topology>
{code}
* First-hop subject tokens (user access tokens) are minted by another Knox topology with {{{}aud=knoxidf{}}}.
* Actor tokens are Kubernetes projected service-account (SA) tokens with {{{}audience: knoxidf{}}}, from an issuer registered in the Trusted OIDC Issuer registry.
* The {{passthrough}} audience validator makes the minted token's {{aud}} exactly the requested {{{}resource{}}}/{{{}audience{}}} values.
h2. Problem: multi-hop delegation does not work
Example chain: user {{alice}} -> {{dataservice}} (SA {{{}ds-sa{}}}) -> {{mcp-server}} (SA {{{}mcp-sa{}}}) -> {{search-api}} (SA {{{}search-sa{}}}) -> {{{}vector-db{}}}.
# *Hop 1* ({{{}dataservice{}}} exchanges {{{}alice{}}}'s token, {{{}resource=[https://svc.cluster.local/tools/mcp-server]{}}}): succeeds. The minted token T1 has {{{}sub=alice{}}}, {{{}act.sub=system:serviceaccount:dataservice:ds-sa{}}}, and {{aud}} contains only {{{}[https://svc.cluster.local/tools/mcp-server]{}}}.
# *Hop 2* ({{{}mcp-server{}}} presents T1 as {{subject_token}} and its own SA token as {{{}actor_token{}}}): rejected with {{{}401{}}}. {{TokenExchangeHandler#handle}} validates the {{subject_token}} through {{{}JWTFederationFilter#parseAndValidateJWT{}}}, which applies {{AbstractJWTFilter#matchesConfiguredAudiences}} against {{{}knox.token.audiences=knoxidf{}}}. T1 does not carry {{{}knoxidf{}}}.
The same check runs on the {{actor_token}} in {{{}TokenExchangeHandler#handleDelegationExchange{}}}, so one list governs two tokens with very different provenance.
h3. Workaround 1: add {{knoxidf}} to every downstream token (does not scale, weakens binding)
The caller requests {{resource=<target>}} plus {{audience=knoxidf}} on every hop whose token may be exchanged again. This requires:
* {{{}delegation.enforce.requested.audience.max.one=false{}}};
* {{knoxidf}} as a key in every actor's {{resourcePolicy}} (otherwise {{evaluate()}} denies with {{{}resource_not_allowed{}}});
* every caller to know in advance whether its callee will re-exchange;
* every downstream service, including third-party tools and MCP servers, to accept a multi-valued {{aud}} that names the token service.
It also removes recipient binding. Every downstream token becomes a valid {{subject_token}} for *any* actor whose policy allows the user and the next resource, not only for the service it was addressed to. A token leaked from logs, a proxy or a misbehaving client can be exchanged by a different, authorized-for-that-user workload.
h3. Workaround 2: remove {{knox.token.audiences}} (opens the gate)
With the parameter unset, {{configuredAudiences == null}} and {{matchesConfiguredAudiences}} accepts *any* audience, for both subject and actor tokens. Hop 2 then succeeds, but:
* *Any SA token from a trusted issuer, minted for any audience, becomes a valid {{{}actor_token{}}}.* This includes the automounted token ({{{}aud=[https://kubernetes.default.svc|https://kubernetes.default.svc/]...{}}}), and tokens a workload sends to Vault or to any other service that accepts SA tokens. Whoever obtains such a token (the receiving service, its logs, an intermediary) can exchange it for tokens for every user the SA's policy allows.
* *The same applies to headless exchanges* ({{{}requested_subject{}}}). There the {{subject_token}} is the actor, and no user token is needed at all. With {{{}allowHeadlessExchange=true{}}}, a single stolen SA token is enough to mint tokens for any user in {{{}canActForUsers{}}}, to any resource in {{{}resourcePolicy{}}}.
* *Same-subject exchanges* accept any SA token and convert it into a Knox-issued token.
* *Re-exchanged subject tokens are still not bound to their recipient* (same issue as workaround 1).
Delegation policies cannot compensate. A policy is keyed by actor identity ({{{}ActorIdentity{}}}: {{iss:namespace:serviceaccount}} for {{{}K8S_SA{}}}), so an SA token minted for Vault and one minted for {{knoxidf}} resolve to the same policy. Neither {{DelegationPolicy}} nor {{TrustedOidcIssuer}} has any notion of the presented token's audience.
Stealing the actor's own {{{}knoxidf{}}}-audience token remains a full compromise of that actor. That case is unavoidable, and the mitigation is short-lived projected tokens. The goal is to keep every other token useless at the token endpoint.
h2. Proposal
h3. Audience rules per token role
||#||Token||Rule||Status||Why||
|1|{{{}actor_token{}}}, and the {{subject_token}} of a headless exchange (it is the actor)|{{aud}} must contain a {{knox.token.audiences}} value|Existing|Blocks replay of SA tokens minted for other audiences (kube-apiserver, Vault, ...). Only tokens deliberately minted for the token service can act.|
|2|{{subject_token}} of a delegated exchange, first hop (e.g. a user access token)|{{aud}} must contain a {{knox.token.audiences}} value|Existing|Entry into the delegation system is limited to tokens deliberately minted for the token service.|
|3|{{subject_token}} of a delegated exchange, later hops (a token minted by this topology for the actor's service)|Accepted without a {{knox.token.audiences}} value only if: an {{actor_token}} is present, the token's {{iss}} is in {{{}token.exchange.subject.token.policy.audiences.issuers{}}}, and its {{aud}} intersects the actor policy's {{subjectAudiences}}|*New*|The only relaxation; it enables multi-hop. A service can re-exchange only a token addressed to itself, and only while proving its identity with its own rule-1 actor token. Other actors, even ones authorized for the same user, cannot use it.|
|4|{{subject_token}} of a same-subject exchange|{{aud}} must contain a {{knox.token.audiences}} value (no exception)|Existing|No policy is consulted in same-subject exchanges, so there is no recipient binding to fall back on.|
|5|{{subject_token}} of a headless exchange|Must not carry an {{act}} claim ({{{}delegation.headless.reject.subject.with.act{}}})|*New*|Defense in depth: a delegated token, however it got a {{knoxidf}} audience, must never become a headless actor, which would mean acting as anyone in the policy without a user token.|
Applied per exchange type:
* *Delegated* ({{{}actor_token{}}} present): rule 1 on the actor, rule 2 or rule 3 on the subject.
* *Headless* ({{{}requested_subject{}}}, no actor): rules 1 and 5 on the {{{}subject_token{}}}. Rule 3 never applies.
* {*}Same-subject{*}: rule 4.
h3. New delegation policy field: {{subjectAudiences}}
An optional set of audiences that identify the actor {*}as a recipient{*}, i.e. the audiences under which other actors mint tokens for it. Defaults to empty, which means rule 3 never matches for that actor (fail-safe, backward compatible).
{code:yaml}
{
"actorAuthority": "K8S_SA",
"actorId": "https://kubernetes.default.svc.cluster.local:tools:mcp-sa",
"name": "mcp-sa-delegation",
"status": "active",
"allowHeadlessExchange": false,
"canActForUsers": ["alice"],
"canActForGroups": [],
"subjectAudiences": ["https://svc.cluster.local/tools/mcp-server"],
"resourcePolicy": {
"https://svc.cluster.local/search/search-api": []
}
}
{code}
h3. Why {{resourcePolicy}} cannot be reused for this check
{{resourcePolicy}} and {{subjectAudiences}} answer opposite questions:
||Field||Direction||Question||Example for {{mcp-sa}}||
|{{resourcePolicy}}|Outbound|Which services may this actor mint tokens {_}for{_}?|{{search-api}} (the services it calls)|
|{{subjectAudiences}}|Inbound|Which tokens addressed _to_ this actor may it present as {{{}subject_token{}}}?|{{mcp-server}} (the service it is)|
# *The legitimate flow fails.* T1's {{aud}} is {{{}mcp-server{}}}, but {{{}mcp-sa{}}}'s {{resourcePolicy}} lists {{{}search-api{}}}. A service rarely lists itself as a target.
# *It binds the token to the wrong parties.* The actors that list {{mcp-server}} in {{resourcePolicy}} are its {_}callers{_}. An "aud in resourcePolicy" check would let any caller of {{mcp-server}} that obtains T1 exchange it. That is a confused-deputy gap: it proves the presenter may _send to_ the service, not that it _is_ the service.
# *Making #1 pass widens outbound permissions.* Adding {{mcp-server}} to {{{}mcp-sa{}}}'s own {{resourcePolicy}} also authorizes {{mcp-sa}} to mint tokens addressed to itself for any user in its policy, including headless exchanges when {{{}allowHeadlessExchange=true{}}}.
# *Different administrative constraints.* {{resourcePolicy}} keys must be able to name services in other namespaces or tenants, because services call across them. {{subjectAudiences}} can be restricted by admission control or the admin API to the actor's own namespace, which prevents team A from claiming team B's audience. One shared field would allow no such restriction.
# {{resourcePolicy}} values are scopes for the outbound target; they have no meaning for an inbound check.
Nothing in the current policy model links the actor identity to the audience it serves. {{subjectAudiences}} is that link.
h3. Proposed topology
Unchanged except for the two new JWTProvider parameters (both default to unset or {{{}false{}}}, which keeps today's behavior):
{code:xml}
<topology>
<gateway>
<provider>
<role>federation</role>
<name>JWTProvider</name>
<enabled>true</enabled>
<param>
<name>jwt.expected.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<!-- Rules 1, 2, 4: unchanged, still applies to every actor token and to non re-exchanged subject tokens. -->
<param>
<name>knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>jwt.instance.key.fallback</name>
<value>true</value>
</param>
<!-- NEW, rule 3: on a delegated exchange (actor_token present), a subject_token issued by one of these
issuers that lacks knox.token.audiences is accepted if its aud intersects the actor policy's
subjectAudiences. Unset: rule 3 is disabled (today's behavior). Never applies to actor tokens,
headless or same-subject exchanges. -->
<param>
<name>token.exchange.subject.token.policy.audiences.issuers</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<!-- NEW, rule 5: reject a headless subject_token that carries an act claim. -->
<param>
<name>delegation.headless.reject.subject.with.act</name>
<value>true</value>
</param>
<param>
<name>delegation.server.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.requested.subject.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.enforce.requested.audience.required</name>
<value>true</value>
</param>
<!-- Can stay true: minted tokens no longer need the token-service audience to be re-exchanged. -->
<param>
<name>delegation.enforce.requested.audience.max.one</name>
<value>true</value>
</param>
<param>
<name>token.exchange.same.subject.requested.audience.enabled</name>
<value>true</value>
</param>
</provider>
<provider>
<role>identity-assertion</role>
<name>Default</name>
<enabled>true</enabled>
</provider>
</gateway>
<service>
<role>KNOXIDF</role>
<param>
<name>knoxidf.knox.token.ttl</name>
<value>3600000</value>
</param>
<param>
<name>delegation.max.actor.chain.depth</name>
<value>6</value>
</param>
<param>
<name>knoxidf.knox.token.limit.per.user</name>
<value>500</value>
</param>
<param>
<name>knoxidf.knox.token.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<param>
<name>knoxidf.knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>knoxidf.knox.token.enable.delegated.auth</name>
<value>true</value>
</param>
<param>
<name>knoxidf.knox.token.audience.validator</name>
<value>passthrough</value>
</param>
</service>
</topology>
{code}
h3. Implementation notes
* {{{}TokenExchangeHandler#handle{}}}: the {{subject_token}} audience can no longer be decided when it is parsed, because the actor and its policy are only known later. Validate signature, issuer and expiry with the {{JWTFederationFilter#parseAndValidateJWT}} overload that takes a {{{}RequestAudienceValidator{}}}, and record whether the {{knox.token.audiences}} check passed.
** Same-subject or headless and the check failed: reject as today ({{{}401{}}}).
** Delegated and the check failed: only continue if the token's {{iss}} is in {{{}token.exchange.subject.token.policy.audiences.issuers{}}}. Otherwise reject as today.
* {{actor_token}} validation is unchanged (rule 1).
* {{PolicyCheckRequest}} carries the subject token's audiences and whether rule 3 is needed. {{JdbcDelegationPolicyService#evaluate}} denies with a new reason {{subject_audience_not_allowed}} when they do not intersect {{{}subjectAudiences{}}}. The client still gets the generic {{{}400 invalid_request{}}}; the reason is audited.
* {{{}DelegationPolicy{}}}: new {{subjectAudiences}} field (DB schema migration, admin API request/response, validation of non-blank values).
* Headless: when {{{}delegation.headless.reject.subject.with.act=true{}}}, a {{subject_token}} with an {{act}} claim is rejected (new audit reason {{{}headless_subject_has_act{}}}).
* Fail-safe defaults: no new parameter set and empty {{subjectAudiences}} keeps today's behavior.
h3. Related (optional, could be a separate JIRA)
With the {{passthrough}} validator, a same-subject exchange that requests no audience mints a token with *no* {{aud}} claim ({{{}TokenResource#getJWT{}}} only sets audiences when the list is non-empty). Such a token cannot be exchanged again and is rejected by most receivers. Suggestion: {{passthrough}} should fall back to the configured {{knox.token.audiences}} when the request names none.
h2. 3-hop example once implemented
Chain: {{alice}} -> {{dataservice}} ({{{}ds-sa{}}}) -> {{mcp-server}} ({{{}mcp-sa{}}}) -> {{search-api}} ({{{}search-sa{}}}) -> {{{}vector-db{}}}.
Delegation policies (only the relevant fields):
{code:yaml}
// ds-sa: the first hop needs no subjectAudiences (the subject token carries knoxidf, rule 2)
{ "actorId": "https://kubernetes.default.svc.cluster.local:dataservice:ds-sa",
"canActForUsers": ["alice"],
"subjectAudiences": [],
"resourcePolicy": { "https://svc.cluster.local/tools/mcp-server": [] } }
// mcp-sa
{ "actorId": "https://kubernetes.default.svc.cluster.local:tools:mcp-sa",
"canActForUsers": ["alice"],
"subjectAudiences": ["https://svc.cluster.local/tools/mcp-server"],
"resourcePolicy": { "https://svc.cluster.local/search/search-api": [] } }
// search-sa
{ "actorId": "https://kubernetes.default.svc.cluster.local:search:search-sa",
"canActForUsers": ["alice"],
"subjectAudiences": ["https://svc.cluster.local/search/search-api"],
"resourcePolicy": { "https://svc.cluster.local/data/vector-db": [] } }
{code}
h3. Hop 1: {{dataservice}} -> token for {{mcp-server}}
{code:bash}
POST /gateway/knox-knoxidf-tokenexchange/knoxidf/api/v1/token
grant_type=urn:ietf:params:oauth:grant-type:token-exchange
subject_token=<T0> subject_token_type=urn:ietf:params:oauth:token-type:jwt
actor_token=<ds-sa> actor_token_type=urn:ietf:params:oauth:token-type:jwt
resource=https://svc.cluster.local/tools/mcp-server
// subject_token T0 (user access token) -> rule 2: aud contains knoxidf
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-tokenexchange",
"sub": "alice", "aud": ["knoxidf"] }
// actor_token (ds-sa projected SA token) -> rule 1: aud contains knoxidf
{ "iss": "https://kubernetes.default.svc.cluster.local",
"sub": "system:serviceaccount:dataservice:ds-sa", "aud": ["knoxidf"] }
// policy ds-sa: alice allowed, mcp-server in resourcePolicy -> minted T1
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
"sub": "alice",
"aud": ["https://svc.cluster.local/tools/mcp-server"],
"act": { "sub": "system:serviceaccount:dataservice:ds-sa" } }
{code}
{{dataservice}} calls {{mcp-server}} with T1. {{mcp-server}} performs an ordinary audience check: its own audience is in {{{}aud{}}}. It does not need {{knoxidf}} in it.
h3. Hop 2: {{mcp-server}} -> token for {{search-api}}
{code:yaml}
subject_token=<T1> actor_token=<mcp-sa> resource=https://svc.cluster.local/search/search-api
// subject_token T1: aud lacks knoxidf -> rule 3:
// actor_token present, iss = knox-knoxidf-tokenexchange (listed), aud intersects mcp-sa subjectAudiences
// actor_token (mcp-sa projected SA token) -> rule 1
{ "iss": "https://kubernetes.default.svc.cluster.local",
"sub": "system:serviceaccount:tools:mcp-sa", "aud": ["knoxidf"] }
// policy mcp-sa: alice allowed, search-api in resourcePolicy, chain depth 2 <= 6 -> minted T2
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
"sub": "alice",
"aud": ["https://svc.cluster.local/search/search-api"],
"act": { "sub": "system:serviceaccount:tools:mcp-sa",
"act": { "sub": "system:serviceaccount:dataservice:ds-sa" } } }
{code}
h3. Hop 3: {{search-api}} -> token for {{vector-db}}
{code:yaml}
subject_token=<T2> actor_token=<search-sa> resource=https://svc.cluster.local/data/vector-db
// subject_token T2 -> rule 3: aud intersects search-sa subjectAudiences
// actor_token (search-sa projected SA token, aud knoxidf) -> rule 1
// policy search-sa: alice allowed, vector-db in resourcePolicy, chain depth 3 <= 6 -> minted T3
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
"sub": "alice",
"aud": ["https://svc.cluster.local/data/vector-db"],
"act": { "sub": "system:serviceaccount:search:search-sa",
"act": { "sub": "system:serviceaccount:tools:mcp-sa",
"act": { "sub": "system:serviceaccount:dataservice:ds-sa" } } } }
{code}
h3. Attack cases
||Attacker has||Result||Rule||
|{{ds-sa}} automounted SA token ({{{}aud=[https://kubernetes.default.svc|https://kubernetes.default.svc/]...{}}}) or one minted for Vault|Rejected as {{actor_token}} and as headless {{subject_token}}|1|
|T1, presented with another workload's {{knoxidf}} actor token (even one allowed to act for {{{}alice{}}})|Rejected: {{mcp-server}} is not in that actor's {{subjectAudiences}}|3|
|T1 as a headless {{subject_token}}|Rejected: no {{knoxidf}} audience, and it carries {{act}}|1, 5|
|T1 in a same-subject exchange|Rejected: no {{knoxidf}} audience|4|
|An SA token with {{aud=[https://svc.cluster.local/tools/mcp-server]}} sent directly to {{{}mcp-server{}}}, used as {{subject_token}}|Rejected: its issuer is not in {{token.exchange.subject.token.policy.audiences.issuers}}|3|
|{{{}mcp-sa{}}}'s own {{knoxidf}} projected SA token|Attacker acts as {{{}mcp-sa{}}}, within its policy. Unavoidable; mitigate with short {{expirationSeconds}}|-|
h3. Prior art
Other authorization servers that support chained token exchange restrict re-exchanged subject tokens in the same two ways this proposal does:
# the subject token must be *issued by the authorization server itself* (or a trusted sibling);
# the subject token must be *addressed to the party presenting it*.
||Server||Subject token issuer||Subject token audience||Requester authenticated by||Chaining / {{act}}||
|Keycloak, standard token exchange V2|Same realm only. External tokens are not accepted; a JWT Authorization Grant is recommended instead|"The {{subject_token}} sent to the token exchange endpoint must have the requester client set as an audience in the {{aud}} claim."|Client authentication ({{client_id}} + secret or certificate)|Chaining works; no {{actor_token}}/{{act}} in V2 (delegation and impersonation were only preview features in legacy V1)|
|Microsoft Entra On-Behalf-Of|Entra itself|"This token must have an audience ({{aud}}) claim of the app making this OBO request ... Applications can't redeem a token for a different app."|Client credentials (secret or certificate)|Chaining works; no {{act}} claim|
|Okta On-Behalf-Of token exchange|Same authorization server, or a trusted authorization server in the same Okta org|Authorization server audience|Service app client credentials|Nested {{act.act}}, at most 5 actors|
How this maps to Knox:
* *Issuer restriction:* {{token.exchange.subject.token.policy.audiences.issuers}} limits the relaxed path (rule 3) to tokens minted by Knox itself, as the servers above do. External and first-hop tokens still enter only through rule 2 ({{aud}} contains {{knox.token.audiences}}), which matches Keycloak's separate grant for external tokens and Okta's trusted servers.
* *Recipient binding:* In Keycloak and Entra the requester is an OAuth client, and its {{client_id}} is the audience, so "{{aud}} must be the requester" needs no extra configuration. In Knox the requester is identified by an {{actor_token}} (e.g. {{system:serviceaccount:tools:mcp-sa}}), and that identity is not the audience it serves (e.g. {{https://svc.cluster.local/tools/mcp-server}}). {{subjectAudiences}} is the explicit actor-identity-to-audience mapping that a client registration provides implicitly in Keycloak and Entra.
* *Actor credential (rule 1):* The servers above authenticate the requester with a client secret or certificate. A {{private_key_jwt}} client assertion, which must be addressed to the token endpoint, is the closest equivalent of requiring {{aud=knoxidf}} on the {{actor_token}}.
* *Delegation chain:* Like Okta, Knox records the chain as nested {{act}} and caps its length ({{delegation.max.actor.chain.depth}}).
RFC 8693 leaves which subject and actor tokens are acceptable to authorization-server policy (Section 2.1), so this proposal follows common practice; the spec doesn't mandate it.
References:
* [Keycloak: Configuring and using token exchange|https://www.keycloak.org/securing-apps/token-exchange]
* [Microsoft Entra: OAuth 2.0 On-Behalf-Of flow|https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-on-behalf-of-flow]
* [Okta: Set up On-Behalf-Of token exchange|https://developer.okta.com/docs/guides/set-up-token-exchange/main/]
* [RFC 8693: OAuth 2.0 Token Exchange|https://www.rfc-editor.org/rfc/rfc8693]
was:
h2. Summary
The RFC 8693 token exchange topology validates every presented JWT (both {{subject_token}} and {{{}actor_token{}}}) against a single {{knox.token.audiences}} list. Tokens minted by a delegated exchange carry only the requested target audience, so they can never be presented again as a {{{}subject_token{}}}. As a result, multi-hop on-behalf-of chains (A -> B -> C -> D) stop after the first delegated hop, even though {{delegation.max.actor.chain.depth}} suggests deeper chains are supported.
The two available workarounds either do not scale (adding the token-service audience to every downstream token) or remove an important protection (dropping {{{}knox.token.audiences{}}}, which lets any stolen service-account token, minted for any audience, act as a delegation actor).
This JIRA proposes per-role audience rules: actor tokens stay strictly bound to the token-service audience, while a re-exchanged {{subject_token}} is accepted only when it is addressed to the actor presenting it. The actor's own audiences are declared in a new delegation policy field, {{{}subjectAudiences{}}}.
h2. Current configuration
{code:xml}
<topology>
<gateway>
<provider>
<role>federation</role>
<name>JWTProvider</name>
<enabled>true</enabled>
<param>
<name>jwt.expected.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<param>
<name>knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>jwt.instance.key.fallback</name>
<value>true</value>
</param>
<param>
<name>delegation.server.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.requested.subject.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.enforce.requested.audience.required</name>
<value>true</value>
</param>
<param>
<name>delegation.enforce.requested.audience.max.one</name>
<value>true</value>
</param>
<param>
<name>token.exchange.same.subject.requested.audience.enabled</name>
<value>true</value>
</param>
</provider>
<provider>
<role>identity-assertion</role>
<name>Default</name>
<enabled>true</enabled>
</provider>
</gateway>
<service>
<role>KNOXIDF</role>
<param>
<name>knoxidf.knox.token.ttl</name>
<value>3600000</value>
</param>
<param>
<name>delegation.max.actor.chain.depth</name>
<value>6</value>
</param>
<param>
<name>knoxidf.knox.token.limit.per.user</name>
<value>500</value>
</param>
<param>
<name>knoxidf.knox.token.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<param>
<name>knoxidf.knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>knoxidf.knox.token.enable.delegated.auth</name>
<value>true</value>
</param>
<param>
<name>knoxidf.knox.token.audience.validator</name>
<value>passthrough</value>
</param>
</service>
</topology>
{code}
* First-hop subject tokens (user access tokens) are minted by another Knox topology with {{{}aud=knoxidf{}}}.
* Actor tokens are Kubernetes projected service-account (SA) tokens with {{{}audience: knoxidf{}}}, from an issuer registered in the Trusted OIDC Issuer registry.
* The {{passthrough}} audience validator makes the minted token's {{aud}} exactly the requested {{{}resource{}}}/{{{}audience{}}} values.
h2. Problem: multi-hop delegation does not work
Example chain: user {{alice}} -> {{dataservice}} (SA {{{}ds-sa{}}}) -> {{mcp-server}} (SA {{{}mcp-sa{}}}) -> {{search-api}} (SA {{{}search-sa{}}}) -> {{{}vector-db{}}}.
# *Hop 1* ({{{}dataservice{}}} exchanges {{{}alice{}}}'s token, {{{}resource=[https://svc.cluster.local/tools/mcp-server]{}}}): succeeds. The minted token T1 has {{{}sub=alice{}}}, {{{}act.sub=system:serviceaccount:dataservice:ds-sa{}}}, and {{aud}} contains only {{{}[https://svc.cluster.local/tools/mcp-server]{}}}.
# *Hop 2* ({{{}mcp-server{}}} presents T1 as {{subject_token}} and its own SA token as {{{}actor_token{}}}): rejected with {{{}401{}}}. {{TokenExchangeHandler#handle}} validates the {{subject_token}} through {{{}JWTFederationFilter#parseAndValidateJWT{}}}, which applies {{AbstractJWTFilter#matchesConfiguredAudiences}} against {{{}knox.token.audiences=knoxidf{}}}. T1 does not carry {{{}knoxidf{}}}.
The same check runs on the {{actor_token}} in {{{}TokenExchangeHandler#handleDelegationExchange{}}}, so one list governs two tokens with very different provenance.
h3. Workaround 1: add {{knoxidf}} to every downstream token (does not scale, weakens binding)
The caller requests {{resource=<target>}} plus {{audience=knoxidf}} on every hop whose token may be exchanged again. This requires:
* {{{}delegation.enforce.requested.audience.max.one=false{}}};
* {{knoxidf}} as a key in every actor's {{resourcePolicy}} (otherwise {{evaluate()}} denies with {{{}resource_not_allowed{}}});
* every caller to know in advance whether its callee will re-exchange;
* every downstream service, including third-party tools and MCP servers, to accept a multi-valued {{aud}} that names the token service.
It also removes recipient binding. Every downstream token becomes a valid {{subject_token}} for *any* actor whose policy allows the user and the next resource, not only for the service it was addressed to. A token leaked from logs, a proxy or a misbehaving client can be exchanged by a different, authorized-for-that-user workload.
h3. Workaround 2: remove {{knox.token.audiences}} (opens the gate)
With the parameter unset, {{configuredAudiences == null}} and {{matchesConfiguredAudiences}} accepts *any* audience, for both subject and actor tokens. Hop 2 then succeeds, but:
* *Any SA token from a trusted issuer, minted for any audience, becomes a valid {{{}actor_token{}}}.* This includes the automounted token ({{{}aud=[https://kubernetes.default.svc|https://kubernetes.default.svc/]...{}}}), and tokens a workload sends to Vault or to any other service that accepts SA tokens. Whoever obtains such a token (the receiving service, its logs, an intermediary) can exchange it for tokens for every user the SA's policy allows.
* *The same applies to headless exchanges* ({{{}requested_subject{}}}). There the {{subject_token}} is the actor, and no user token is needed at all. With {{{}allowHeadlessExchange=true{}}}, a single stolen SA token is enough to mint tokens for any user in {{{}canActForUsers{}}}, to any resource in {{{}resourcePolicy{}}}.
* *Same-subject exchanges* accept any SA token and convert it into a Knox-issued token.
* *Re-exchanged subject tokens are still not bound to their recipient* (same issue as workaround 1).
Delegation policies cannot compensate. A policy is keyed by actor identity ({{{}ActorIdentity{}}}: {{iss:namespace:serviceaccount}} for {{{}K8S_SA{}}}), so an SA token minted for Vault and one minted for {{knoxidf}} resolve to the same policy. Neither {{DelegationPolicy}} nor {{TrustedOidcIssuer}} has any notion of the presented token's audience.
Stealing the actor's own {{{}knoxidf{}}}-audience token remains a full compromise of that actor. That case is unavoidable, and the mitigation is short-lived projected tokens. The goal is to keep every other token useless at the token endpoint.
h2. Proposal
h3. Audience rules per token role
||#||Token||Rule||Status||Why||
|1|{{{}actor_token{}}}, and the {{subject_token}} of a headless exchange (it is the actor)|{{aud}} must contain a {{knox.token.audiences}} value|Existing|Blocks replay of SA tokens minted for other audiences (kube-apiserver, Vault, ...). Only tokens deliberately minted for the token service can act.|
|2|{{subject_token}} of a delegated exchange, first hop (e.g. a user access token)|{{aud}} must contain a {{knox.token.audiences}} value|Existing|Entry into the delegation system is limited to tokens deliberately minted for the token service.|
|3|{{subject_token}} of a delegated exchange, later hops (a token minted by this topology for the actor's service)|Accepted without a {{knox.token.audiences}} value only if: an {{actor_token}} is present, the token's {{iss}} is in {{{}token.exchange.subject.token.policy.audiences.issuers{}}}, and its {{aud}} intersects the actor policy's {{subjectAudiences}}|*New*|The only relaxation; it enables multi-hop. A service can re-exchange only a token addressed to itself, and only while proving its identity with its own rule-1 actor token. Other actors, even ones authorized for the same user, cannot use it.|
|4|{{subject_token}} of a same-subject exchange|{{aud}} must contain a {{knox.token.audiences}} value (no exception)|Existing|No policy is consulted in same-subject exchanges, so there is no recipient binding to fall back on.|
|5|{{subject_token}} of a headless exchange|Must not carry an {{act}} claim ({{{}delegation.headless.reject.subject.with.act{}}})|*New*|Defense in depth: a delegated token, however it got a {{knoxidf}} audience, must never become a headless actor, which would mean acting as anyone in the policy without a user token.|
Applied per exchange type:
* *Delegated* ({{{}actor_token{}}} present): rule 1 on the actor, rule 2 or rule 3 on the subject.
* *Headless* ({{{}requested_subject{}}}, no actor): rules 1 and 5 on the {{{}subject_token{}}}. Rule 3 never applies.
* {*}Same-subject{*}: rule 4.
h3. New delegation policy field: {{subjectAudiences}}
An optional set of audiences that identify the actor {*}as a recipient{*}, i.e. the audiences under which other actors mint tokens for it. Defaults to empty, which means rule 3 never matches for that actor (fail-safe, backward compatible).
{code:yaml}
{
"actorAuthority": "K8S_SA",
"actorId": "https://kubernetes.default.svc.cluster.local:tools:mcp-sa",
"name": "mcp-sa-delegation",
"status": "active",
"allowHeadlessExchange": false,
"canActForUsers": ["alice"],
"canActForGroups": [],
"subjectAudiences": ["https://svc.cluster.local/tools/mcp-server"],
"resourcePolicy": {
"https://svc.cluster.local/search/search-api": []
}
}
{code}
h3. Why {{resourcePolicy}} cannot be reused for this check
{{resourcePolicy}} and {{subjectAudiences}} answer opposite questions:
||Field||Direction||Question||Example for {{mcp-sa}}||
|{{resourcePolicy}}|Outbound|Which services may this actor mint tokens {_}for{_}?|{{search-api}} (the services it calls)|
|{{subjectAudiences}}|Inbound|Which tokens addressed _to_ this actor may it present as {{{}subject_token{}}}?|{{mcp-server}} (the service it is)|
# *The legitimate flow fails.* T1's {{aud}} is {{{}mcp-server{}}}, but {{{}mcp-sa{}}}'s {{resourcePolicy}} lists {{{}search-api{}}}. A service rarely lists itself as a target.
# *It binds the token to the wrong parties.* The actors that list {{mcp-server}} in {{resourcePolicy}} are its {_}callers{_}. An "aud in resourcePolicy" check would let any caller of {{mcp-server}} that obtains T1 exchange it. That is a confused-deputy gap: it proves the presenter may _send to_ the service, not that it _is_ the service.
# *Making #1 pass widens outbound permissions.* Adding {{mcp-server}} to {{{}mcp-sa{}}}'s own {{resourcePolicy}} also authorizes {{mcp-sa}} to mint tokens addressed to itself for any user in its policy, including headless exchanges when {{{}allowHeadlessExchange=true{}}}.
# *Different administrative constraints.* {{resourcePolicy}} keys must be able to name services in other namespaces or tenants, because services call across them. {{subjectAudiences}} can be restricted by admission control or the admin API to the actor's own namespace, which prevents team A from claiming team B's audience. One shared field would allow no such restriction.
# {{resourcePolicy}} values are scopes for the outbound target; they have no meaning for an inbound check.
Nothing in the current policy model links the actor identity to the audience it serves. {{subjectAudiences}} is that link.
h3. Proposed topology
Unchanged except for the two new JWTProvider parameters (both default to unset or {{{}false{}}}, which keeps today's behavior):
{code:xml}
<topology>
<gateway>
<provider>
<role>federation</role>
<name>JWTProvider</name>
<enabled>true</enabled>
<param>
<name>jwt.expected.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<!-- Rules 1, 2, 4: unchanged, still applies to every actor token and to non re-exchanged subject tokens. -->
<param>
<name>knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>jwt.instance.key.fallback</name>
<value>true</value>
</param>
<!-- NEW, rule 3: on a delegated exchange (actor_token present), a subject_token issued by one of these
issuers that lacks knox.token.audiences is accepted if its aud intersects the actor policy's
subjectAudiences. Unset: rule 3 is disabled (today's behavior). Never applies to actor tokens,
headless or same-subject exchanges. -->
<param>
<name>token.exchange.subject.token.policy.audiences.issuers</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<!-- NEW, rule 5: reject a headless subject_token that carries an act claim. -->
<param>
<name>delegation.headless.reject.subject.with.act</name>
<value>true</value>
</param>
<param>
<name>delegation.server.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.requested.subject.enabled</name>
<value>true</value>
</param>
<param>
<name>delegation.enforce.requested.audience.required</name>
<value>true</value>
</param>
<!-- Can stay true: minted tokens no longer need the token-service audience to be re-exchanged. -->
<param>
<name>delegation.enforce.requested.audience.max.one</name>
<value>true</value>
</param>
<param>
<name>token.exchange.same.subject.requested.audience.enabled</name>
<value>true</value>
</param>
</provider>
<provider>
<role>identity-assertion</role>
<name>Default</name>
<enabled>true</enabled>
</provider>
</gateway>
<service>
<role>KNOXIDF</role>
<param>
<name>knoxidf.knox.token.ttl</name>
<value>3600000</value>
</param>
<param>
<name>delegation.max.actor.chain.depth</name>
<value>6</value>
</param>
<param>
<name>knoxidf.knox.token.limit.per.user</name>
<value>500</value>
</param>
<param>
<name>knoxidf.knox.token.issuer</name>
<value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
</param>
<param>
<name>knoxidf.knox.token.audiences</name>
<value>knoxidf</value>
</param>
<param>
<name>knoxidf.knox.token.enable.delegated.auth</name>
<value>true</value>
</param>
<param>
<name>knoxidf.knox.token.audience.validator</name>
<value>passthrough</value>
</param>
</service>
</topology>
{code}
h3. Implementation notes
* {{{}TokenExchangeHandler#handle{}}}: the {{subject_token}} audience can no longer be decided when it is parsed, because the actor and its policy are only known later. Validate signature, issuer and expiry with the {{JWTFederationFilter#parseAndValidateJWT}} overload that takes a {{{}RequestAudienceValidator{}}}, and record whether the {{knox.token.audiences}} check passed.
** Same-subject or headless and the check failed: reject as today ({{{}401{}}}).
** Delegated and the check failed: only continue if the token's {{iss}} is in {{{}token.exchange.subject.token.policy.audiences.issuers{}}}. Otherwise reject as today.
* {{actor_token}} validation is unchanged (rule 1).
* {{PolicyCheckRequest}} carries the subject token's audiences and whether rule 3 is needed. {{JdbcDelegationPolicyService#evaluate}} denies with a new reason {{subject_audience_not_allowed}} when they do not intersect {{{}subjectAudiences{}}}. The client still gets the generic {{{}400 invalid_request{}}}; the reason is audited.
* {{{}DelegationPolicy{}}}: new {{subjectAudiences}} field (DB schema migration, admin API request/response, validation of non-blank values).
* Headless: when {{{}delegation.headless.reject.subject.with.act=true{}}}, a {{subject_token}} with an {{act}} claim is rejected (new audit reason {{{}headless_subject_has_act{}}}).
* Fail-safe defaults: no new parameter set and empty {{subjectAudiences}} keeps today's behavior.
h3. Related (optional, could be a separate JIRA)
With the {{passthrough}} validator, a same-subject exchange that requests no audience mints a token with *no* {{aud}} claim ({{{}TokenResource#getJWT{}}} only sets audiences when the list is non-empty). Such a token cannot be exchanged again and is rejected by most receivers. Suggestion: {{passthrough}} should fall back to the configured {{knox.token.audiences}} when the request names none.
h2. 3-hop example once implemented
Chain: {{alice}} -> {{dataservice}} ({{{}ds-sa{}}}) -> {{mcp-server}} ({{{}mcp-sa{}}}) -> {{search-api}} ({{{}search-sa{}}}) -> {{{}vector-db{}}}.
Delegation policies (only the relevant fields):
{code:yaml}
// ds-sa: the first hop needs no subjectAudiences (the subject token carries knoxidf, rule 2)
{ "actorId": "https://kubernetes.default.svc.cluster.local:dataservice:ds-sa",
"canActForUsers": ["alice"],
"subjectAudiences": [],
"resourcePolicy": { "https://svc.cluster.local/tools/mcp-server": [] } }
// mcp-sa
{ "actorId": "https://kubernetes.default.svc.cluster.local:tools:mcp-sa",
"canActForUsers": ["alice"],
"subjectAudiences": ["https://svc.cluster.local/tools/mcp-server"],
"resourcePolicy": { "https://svc.cluster.local/search/search-api": [] } }
// search-sa
{ "actorId": "https://kubernetes.default.svc.cluster.local:search:search-sa",
"canActForUsers": ["alice"],
"subjectAudiences": ["https://svc.cluster.local/search/search-api"],
"resourcePolicy": { "https://svc.cluster.local/data/vector-db": [] } }
{code}
h3. Hop 1: {{dataservice}} -> token for {{mcp-server}}
{code:bash}
POST /gateway/knox-knoxidf-tokenexchange/knoxidf/api/v1/token
grant_type=urn:ietf:params:oauth:grant-type:token-exchange
subject_token=<T0> subject_token_type=urn:ietf:params:oauth:token-type:jwt
actor_token=<ds-sa> actor_token_type=urn:ietf:params:oauth:token-type:jwt
resource=https://svc.cluster.local/tools/mcp-server
// subject_token T0 (user access token) -> rule 2: aud contains knoxidf
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-tokenexchange",
"sub": "alice", "aud": ["knoxidf"] }
// actor_token (ds-sa projected SA token) -> rule 1: aud contains knoxidf
{ "iss": "https://kubernetes.default.svc.cluster.local",
"sub": "system:serviceaccount:dataservice:ds-sa", "aud": ["knoxidf"] }
// policy ds-sa: alice allowed, mcp-server in resourcePolicy -> minted T1
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
"sub": "alice",
"aud": ["https://svc.cluster.local/tools/mcp-server"],
"act": { "sub": "system:serviceaccount:dataservice:ds-sa" } }
{code}
{{dataservice}} calls {{mcp-server}} with T1. {{mcp-server}} performs an ordinary audience check: its own audience is in {{{}aud{}}}. It does not need {{knoxidf}} in it.
h3. Hop 2: {{mcp-server}} -> token for {{search-api}}
{code:yaml}
subject_token=<T1> actor_token=<mcp-sa> resource=https://svc.cluster.local/search/search-api
// subject_token T1: aud lacks knoxidf -> rule 3:
// actor_token present, iss = knox-knoxidf-tokenexchange (listed), aud intersects mcp-sa subjectAudiences
// actor_token (mcp-sa projected SA token) -> rule 1
{ "iss": "https://kubernetes.default.svc.cluster.local",
"sub": "system:serviceaccount:tools:mcp-sa", "aud": ["knoxidf"] }
// policy mcp-sa: alice allowed, search-api in resourcePolicy, chain depth 2 <= 6 -> minted T2
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
"sub": "alice",
"aud": ["https://svc.cluster.local/search/search-api"],
"act": { "sub": "system:serviceaccount:tools:mcp-sa",
"act": { "sub": "system:serviceaccount:dataservice:ds-sa" } } }
{code}
h3. Hop 3: {{search-api}} -> token for {{vector-db}}
{code:yaml}
subject_token=<T2> actor_token=<search-sa> resource=https://svc.cluster.local/data/vector-db
// subject_token T2 -> rule 3: aud intersects search-sa subjectAudiences
// actor_token (search-sa projected SA token, aud knoxidf) -> rule 1
// policy search-sa: alice allowed, vector-db in resourcePolicy, chain depth 3 <= 6 -> minted T3
{ "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
"sub": "alice",
"aud": ["https://svc.cluster.local/data/vector-db"],
"act": { "sub": "system:serviceaccount:search:search-sa",
"act": { "sub": "system:serviceaccount:tools:mcp-sa",
"act": { "sub": "system:serviceaccount:dataservice:ds-sa" } } } }
{code}
h3. Attack cases
||Attacker has||Result||Rule||
|{{ds-sa}} automounted SA token ({{{}aud=[https://kubernetes.default.svc|https://kubernetes.default.svc/]...{}}}) or one minted for Vault|Rejected as {{actor_token}} and as headless {{subject_token}}|1|
|T1, presented with another workload's {{knoxidf}} actor token (even one allowed to act for {{{}alice{}}})|Rejected: {{mcp-server}} is not in that actor's {{subjectAudiences}}|3|
|T1 as a headless {{subject_token}}|Rejected: no {{knoxidf}} audience, and it carries {{act}}|1, 5|
|T1 in a same-subject exchange|Rejected: no {{knoxidf}} audience|4|
|An SA token with {{aud=[https://svc.cluster.local/tools/mcp-server]}} sent directly to {{{}mcp-server{}}}, used as {{subject_token}}|Rejected: its issuer is not in {{token.exchange.subject.token.policy.audiences.issuers}}|3|
|{{{}mcp-sa{}}}'s own {{knoxidf}} projected SA token|Attacker acts as {{{}mcp-sa{}}}, within its policy. Unavoidable; mitigate with short {{expirationSeconds}}|-|
> Token exchange: support multi-hop RFC 8693 delegation by binding re-exchanged subject tokens to the actor (delegation policy subjectAudiences)
> -----------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: KNOX-3520
> URL: https://issues.apache.org/jira/browse/KNOX-3520
> Project: Apache Knox
> Issue Type: Bug
> Components: JWT, Server
> Affects Versions: 3.1.0
> Reporter: Sandor Molnar
> Assignee: Sandor Molnar
> Priority: Critical
> Fix For: 3.1.0
>
>
> h2. Summary
> The RFC 8693 token exchange topology validates every presented JWT (both {{subject_token}} and {{{}actor_token{}}}) against a single {{knox.token.audiences}} list. Tokens minted by a delegated exchange carry only the requested target audience, so they can never be presented again as a {{{}subject_token{}}}. As a result, multi-hop on-behalf-of chains (A -> B -> C -> D) stop after the first delegated hop, even though {{delegation.max.actor.chain.depth}} suggests deeper chains are supported.
> The two available workarounds either do not scale (adding the token-service audience to every downstream token) or remove an important protection (dropping {{{}knox.token.audiences{}}}, which lets any stolen service-account token, minted for any audience, act as a delegation actor).
> This JIRA proposes per-role audience rules: actor tokens stay strictly bound to the token-service audience, while a re-exchanged {{subject_token}} is accepted only when it is addressed to the actor presenting it. The actor's own audiences are declared in a new delegation policy field, {{{}subjectAudiences{}}}.
> h2. Current configuration
> {code:xml}
> <topology>
> <gateway>
> <provider>
> <role>federation</role>
> <name>JWTProvider</name>
> <enabled>true</enabled>
> <param>
> <name>jwt.expected.issuer</name>
> <value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
> </param>
> <param>
> <name>knox.token.audiences</name>
> <value>knoxidf</value>
> </param>
> <param>
> <name>jwt.instance.key.fallback</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.server.enabled</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.requested.subject.enabled</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.enforce.requested.audience.required</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.enforce.requested.audience.max.one</name>
> <value>true</value>
> </param>
> <param>
> <name>token.exchange.same.subject.requested.audience.enabled</name>
> <value>true</value>
> </param>
> </provider>
> <provider>
> <role>identity-assertion</role>
> <name>Default</name>
> <enabled>true</enabled>
> </provider>
> </gateway>
> <service>
> <role>KNOXIDF</role>
> <param>
> <name>knoxidf.knox.token.ttl</name>
> <value>3600000</value>
> </param>
> <param>
> <name>delegation.max.actor.chain.depth</name>
> <value>6</value>
> </param>
> <param>
> <name>knoxidf.knox.token.limit.per.user</name>
> <value>500</value>
> </param>
> <param>
> <name>knoxidf.knox.token.issuer</name>
> <value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
> </param>
> <param>
> <name>knoxidf.knox.token.audiences</name>
> <value>knoxidf</value>
> </param>
> <param>
> <name>knoxidf.knox.token.enable.delegated.auth</name>
> <value>true</value>
> </param>
> <param>
> <name>knoxidf.knox.token.audience.validator</name>
> <value>passthrough</value>
> </param>
> </service>
> </topology>
> {code}
> * First-hop subject tokens (user access tokens) are minted by another Knox topology with {{{}aud=knoxidf{}}}.
> * Actor tokens are Kubernetes projected service-account (SA) tokens with {{{}audience: knoxidf{}}}, from an issuer registered in the Trusted OIDC Issuer registry.
> * The {{passthrough}} audience validator makes the minted token's {{aud}} exactly the requested {{{}resource{}}}/{{{}audience{}}} values.
> h2. Problem: multi-hop delegation does not work
> Example chain: user {{alice}} -> {{dataservice}} (SA {{{}ds-sa{}}}) -> {{mcp-server}} (SA {{{}mcp-sa{}}}) -> {{search-api}} (SA {{{}search-sa{}}}) -> {{{}vector-db{}}}.
> # *Hop 1* ({{{}dataservice{}}} exchanges {{{}alice{}}}'s token, {{{}resource=[https://svc.cluster.local/tools/mcp-server]{}}}): succeeds. The minted token T1 has {{{}sub=alice{}}}, {{{}act.sub=system:serviceaccount:dataservice:ds-sa{}}}, and {{aud}} contains only {{{}[https://svc.cluster.local/tools/mcp-server]{}}}.
> # *Hop 2* ({{{}mcp-server{}}} presents T1 as {{subject_token}} and its own SA token as {{{}actor_token{}}}): rejected with {{{}401{}}}. {{TokenExchangeHandler#handle}} validates the {{subject_token}} through {{{}JWTFederationFilter#parseAndValidateJWT{}}}, which applies {{AbstractJWTFilter#matchesConfiguredAudiences}} against {{{}knox.token.audiences=knoxidf{}}}. T1 does not carry {{{}knoxidf{}}}.
> The same check runs on the {{actor_token}} in {{{}TokenExchangeHandler#handleDelegationExchange{}}}, so one list governs two tokens with very different provenance.
> h3. Workaround 1: add {{knoxidf}} to every downstream token (does not scale, weakens binding)
> The caller requests {{resource=<target>}} plus {{audience=knoxidf}} on every hop whose token may be exchanged again. This requires:
> * {{{}delegation.enforce.requested.audience.max.one=false{}}};
> * {{knoxidf}} as a key in every actor's {{resourcePolicy}} (otherwise {{evaluate()}} denies with {{{}resource_not_allowed{}}});
> * every caller to know in advance whether its callee will re-exchange;
> * every downstream service, including third-party tools and MCP servers, to accept a multi-valued {{aud}} that names the token service.
> It also removes recipient binding. Every downstream token becomes a valid {{subject_token}} for *any* actor whose policy allows the user and the next resource, not only for the service it was addressed to. A token leaked from logs, a proxy or a misbehaving client can be exchanged by a different, authorized-for-that-user workload.
> h3. Workaround 2: remove {{knox.token.audiences}} (opens the gate)
> With the parameter unset, {{configuredAudiences == null}} and {{matchesConfiguredAudiences}} accepts *any* audience, for both subject and actor tokens. Hop 2 then succeeds, but:
> * *Any SA token from a trusted issuer, minted for any audience, becomes a valid {{{}actor_token{}}}.* This includes the automounted token ({{{}aud=[https://kubernetes.default.svc|https://kubernetes.default.svc/]...{}}}), and tokens a workload sends to Vault or to any other service that accepts SA tokens. Whoever obtains such a token (the receiving service, its logs, an intermediary) can exchange it for tokens for every user the SA's policy allows.
> * *The same applies to headless exchanges* ({{{}requested_subject{}}}). There the {{subject_token}} is the actor, and no user token is needed at all. With {{{}allowHeadlessExchange=true{}}}, a single stolen SA token is enough to mint tokens for any user in {{{}canActForUsers{}}}, to any resource in {{{}resourcePolicy{}}}.
> * *Same-subject exchanges* accept any SA token and convert it into a Knox-issued token.
> * *Re-exchanged subject tokens are still not bound to their recipient* (same issue as workaround 1).
> Delegation policies cannot compensate. A policy is keyed by actor identity ({{{}ActorIdentity{}}}: {{iss:namespace:serviceaccount}} for {{{}K8S_SA{}}}), so an SA token minted for Vault and one minted for {{knoxidf}} resolve to the same policy. Neither {{DelegationPolicy}} nor {{TrustedOidcIssuer}} has any notion of the presented token's audience.
> Stealing the actor's own {{{}knoxidf{}}}-audience token remains a full compromise of that actor. That case is unavoidable, and the mitigation is short-lived projected tokens. The goal is to keep every other token useless at the token endpoint.
> h2. Proposal
> h3. Audience rules per token role
> ||#||Token||Rule||Status||Why||
> |1|{{{}actor_token{}}}, and the {{subject_token}} of a headless exchange (it is the actor)|{{aud}} must contain a {{knox.token.audiences}} value|Existing|Blocks replay of SA tokens minted for other audiences (kube-apiserver, Vault, ...). Only tokens deliberately minted for the token service can act.|
> |2|{{subject_token}} of a delegated exchange, first hop (e.g. a user access token)|{{aud}} must contain a {{knox.token.audiences}} value|Existing|Entry into the delegation system is limited to tokens deliberately minted for the token service.|
> |3|{{subject_token}} of a delegated exchange, later hops (a token minted by this topology for the actor's service)|Accepted without a {{knox.token.audiences}} value only if: an {{actor_token}} is present, the token's {{iss}} is in {{{}token.exchange.subject.token.policy.audiences.issuers{}}}, and its {{aud}} intersects the actor policy's {{subjectAudiences}}|*New*|The only relaxation; it enables multi-hop. A service can re-exchange only a token addressed to itself, and only while proving its identity with its own rule-1 actor token. Other actors, even ones authorized for the same user, cannot use it.|
> |4|{{subject_token}} of a same-subject exchange|{{aud}} must contain a {{knox.token.audiences}} value (no exception)|Existing|No policy is consulted in same-subject exchanges, so there is no recipient binding to fall back on.|
> |5|{{subject_token}} of a headless exchange|Must not carry an {{act}} claim ({{{}delegation.headless.reject.subject.with.act{}}})|*New*|Defense in depth: a delegated token, however it got a {{knoxidf}} audience, must never become a headless actor, which would mean acting as anyone in the policy without a user token.|
> Applied per exchange type:
> * *Delegated* ({{{}actor_token{}}} present): rule 1 on the actor, rule 2 or rule 3 on the subject.
> * *Headless* ({{{}requested_subject{}}}, no actor): rules 1 and 5 on the {{{}subject_token{}}}. Rule 3 never applies.
> * {*}Same-subject{*}: rule 4.
> h3. New delegation policy field: {{subjectAudiences}}
> An optional set of audiences that identify the actor {*}as a recipient{*}, i.e. the audiences under which other actors mint tokens for it. Defaults to empty, which means rule 3 never matches for that actor (fail-safe, backward compatible).
> {code:yaml}
> {
> "actorAuthority": "K8S_SA",
> "actorId": "https://kubernetes.default.svc.cluster.local:tools:mcp-sa",
> "name": "mcp-sa-delegation",
> "status": "active",
> "allowHeadlessExchange": false,
> "canActForUsers": ["alice"],
> "canActForGroups": [],
> "subjectAudiences": ["https://svc.cluster.local/tools/mcp-server"],
> "resourcePolicy": {
> "https://svc.cluster.local/search/search-api": []
> }
> }
> {code}
> h3. Why {{resourcePolicy}} cannot be reused for this check
> {{resourcePolicy}} and {{subjectAudiences}} answer opposite questions:
> ||Field||Direction||Question||Example for {{mcp-sa}}||
> |{{resourcePolicy}}|Outbound|Which services may this actor mint tokens {_}for{_}?|{{search-api}} (the services it calls)|
> |{{subjectAudiences}}|Inbound|Which tokens addressed _to_ this actor may it present as {{{}subject_token{}}}?|{{mcp-server}} (the service it is)|
> # *The legitimate flow fails.* T1's {{aud}} is {{{}mcp-server{}}}, but {{{}mcp-sa{}}}'s {{resourcePolicy}} lists {{{}search-api{}}}. A service rarely lists itself as a target.
> # *It binds the token to the wrong parties.* The actors that list {{mcp-server}} in {{resourcePolicy}} are its {_}callers{_}. An "aud in resourcePolicy" check would let any caller of {{mcp-server}} that obtains T1 exchange it. That is a confused-deputy gap: it proves the presenter may _send to_ the service, not that it _is_ the service.
> # *Making #1 pass widens outbound permissions.* Adding {{mcp-server}} to {{{}mcp-sa{}}}'s own {{resourcePolicy}} also authorizes {{mcp-sa}} to mint tokens addressed to itself for any user in its policy, including headless exchanges when {{{}allowHeadlessExchange=true{}}}.
> # *Different administrative constraints.* {{resourcePolicy}} keys must be able to name services in other namespaces or tenants, because services call across them. {{subjectAudiences}} can be restricted by admission control or the admin API to the actor's own namespace, which prevents team A from claiming team B's audience. One shared field would allow no such restriction.
> # {{resourcePolicy}} values are scopes for the outbound target; they have no meaning for an inbound check.
> Nothing in the current policy model links the actor identity to the audience it serves. {{subjectAudiences}} is that link.
> h3. Proposed topology
> Unchanged except for the two new JWTProvider parameters (both default to unset or {{{}false{}}}, which keeps today's behavior):
> {code:xml}
> <topology>
> <gateway>
> <provider>
> <role>federation</role>
> <name>JWTProvider</name>
> <enabled>true</enabled>
> <param>
> <name>jwt.expected.issuer</name>
> <value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
> </param>
> <!-- Rules 1, 2, 4: unchanged, still applies to every actor token and to non re-exchanged subject tokens. -->
> <param>
> <name>knox.token.audiences</name>
> <value>knoxidf</value>
> </param>
> <param>
> <name>jwt.instance.key.fallback</name>
> <value>true</value>
> </param>
> <!-- NEW, rule 3: on a delegated exchange (actor_token present), a subject_token issued by one of these
> issuers that lacks knox.token.audiences is accepted if its aud intersects the actor policy's
> subjectAudiences. Unset: rule 3 is disabled (today's behavior). Never applies to actor tokens,
> headless or same-subject exchanges. -->
> <param>
> <name>token.exchange.subject.token.policy.audiences.issuers</name>
> <value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
> </param>
> <!-- NEW, rule 5: reject a headless subject_token that carries an act claim. -->
> <param>
> <name>delegation.headless.reject.subject.with.act</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.server.enabled</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.requested.subject.enabled</name>
> <value>true</value>
> </param>
> <param>
> <name>delegation.enforce.requested.audience.required</name>
> <value>true</value>
> </param>
> <!-- Can stay true: minted tokens no longer need the token-service audience to be re-exchanged. -->
> <param>
> <name>delegation.enforce.requested.audience.max.one</name>
> <value>true</value>
> </param>
> <param>
> <name>token.exchange.same.subject.requested.audience.enabled</name>
> <value>true</value>
> </param>
> </provider>
> <provider>
> <role>identity-assertion</role>
> <name>Default</name>
> <enabled>true</enabled>
> </provider>
> </gateway>
> <service>
> <role>KNOXIDF</role>
> <param>
> <name>knoxidf.knox.token.ttl</name>
> <value>3600000</value>
> </param>
> <param>
> <name>delegation.max.actor.chain.depth</name>
> <value>6</value>
> </param>
> <param>
> <name>knoxidf.knox.token.limit.per.user</name>
> <value>500</value>
> </param>
> <param>
> <name>knoxidf.knox.token.issuer</name>
> <value>http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange</value>
> </param>
> <param>
> <name>knoxidf.knox.token.audiences</name>
> <value>knoxidf</value>
> </param>
> <param>
> <name>knoxidf.knox.token.enable.delegated.auth</name>
> <value>true</value>
> </param>
> <param>
> <name>knoxidf.knox.token.audience.validator</name>
> <value>passthrough</value>
> </param>
> </service>
> </topology>
> {code}
> h3. Implementation notes
> * {{{}TokenExchangeHandler#handle{}}}: the {{subject_token}} audience can no longer be decided when it is parsed, because the actor and its policy are only known later. Validate signature, issuer and expiry with the {{JWTFederationFilter#parseAndValidateJWT}} overload that takes a {{{}RequestAudienceValidator{}}}, and record whether the {{knox.token.audiences}} check passed.
> ** Same-subject or headless and the check failed: reject as today ({{{}401{}}}).
> ** Delegated and the check failed: only continue if the token's {{iss}} is in {{{}token.exchange.subject.token.policy.audiences.issuers{}}}. Otherwise reject as today.
> * {{actor_token}} validation is unchanged (rule 1).
> * {{PolicyCheckRequest}} carries the subject token's audiences and whether rule 3 is needed. {{JdbcDelegationPolicyService#evaluate}} denies with a new reason {{subject_audience_not_allowed}} when they do not intersect {{{}subjectAudiences{}}}. The client still gets the generic {{{}400 invalid_request{}}}; the reason is audited.
> * {{{}DelegationPolicy{}}}: new {{subjectAudiences}} field (DB schema migration, admin API request/response, validation of non-blank values).
> * Headless: when {{{}delegation.headless.reject.subject.with.act=true{}}}, a {{subject_token}} with an {{act}} claim is rejected (new audit reason {{{}headless_subject_has_act{}}}).
> * Fail-safe defaults: no new parameter set and empty {{subjectAudiences}} keeps today's behavior.
> h3. Related (optional, could be a separate JIRA)
> With the {{passthrough}} validator, a same-subject exchange that requests no audience mints a token with *no* {{aud}} claim ({{{}TokenResource#getJWT{}}} only sets audiences when the list is non-empty). Such a token cannot be exchanged again and is rejected by most receivers. Suggestion: {{passthrough}} should fall back to the configured {{knox.token.audiences}} when the request names none.
> h2. 3-hop example once implemented
> Chain: {{alice}} -> {{dataservice}} ({{{}ds-sa{}}}) -> {{mcp-server}} ({{{}mcp-sa{}}}) -> {{search-api}} ({{{}search-sa{}}}) -> {{{}vector-db{}}}.
> Delegation policies (only the relevant fields):
> {code:yaml}
> // ds-sa: the first hop needs no subjectAudiences (the subject token carries knoxidf, rule 2)
> { "actorId": "https://kubernetes.default.svc.cluster.local:dataservice:ds-sa",
> "canActForUsers": ["alice"],
> "subjectAudiences": [],
> "resourcePolicy": { "https://svc.cluster.local/tools/mcp-server": [] } }
> // mcp-sa
> { "actorId": "https://kubernetes.default.svc.cluster.local:tools:mcp-sa",
> "canActForUsers": ["alice"],
> "subjectAudiences": ["https://svc.cluster.local/tools/mcp-server"],
> "resourcePolicy": { "https://svc.cluster.local/search/search-api": [] } }
> // search-sa
> { "actorId": "https://kubernetes.default.svc.cluster.local:search:search-sa",
> "canActForUsers": ["alice"],
> "subjectAudiences": ["https://svc.cluster.local/search/search-api"],
> "resourcePolicy": { "https://svc.cluster.local/data/vector-db": [] } }
> {code}
> h3. Hop 1: {{dataservice}} -> token for {{mcp-server}}
> {code:bash}
> POST /gateway/knox-knoxidf-tokenexchange/knoxidf/api/v1/token
> grant_type=urn:ietf:params:oauth:grant-type:token-exchange
> subject_token=<T0> subject_token_type=urn:ietf:params:oauth:token-type:jwt
> actor_token=<ds-sa> actor_token_type=urn:ietf:params:oauth:token-type:jwt
> resource=https://svc.cluster.local/tools/mcp-server
> // subject_token T0 (user access token) -> rule 2: aud contains knoxidf
> { "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-tokenexchange",
> "sub": "alice", "aud": ["knoxidf"] }
> // actor_token (ds-sa projected SA token) -> rule 1: aud contains knoxidf
> { "iss": "https://kubernetes.default.svc.cluster.local",
> "sub": "system:serviceaccount:dataservice:ds-sa", "aud": ["knoxidf"] }
> // policy ds-sa: alice allowed, mcp-server in resourcePolicy -> minted T1
> { "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
> "sub": "alice",
> "aud": ["https://svc.cluster.local/tools/mcp-server"],
> "act": { "sub": "system:serviceaccount:dataservice:ds-sa" } }
> {code}
> {{dataservice}} calls {{mcp-server}} with T1. {{mcp-server}} performs an ordinary audience check: its own audience is in {{{}aud{}}}. It does not need {{knoxidf}} in it.
> h3. Hop 2: {{mcp-server}} -> token for {{search-api}}
> {code:yaml}
> subject_token=<T1> actor_token=<mcp-sa> resource=https://svc.cluster.local/search/search-api
> // subject_token T1: aud lacks knoxidf -> rule 3:
> // actor_token present, iss = knox-knoxidf-tokenexchange (listed), aud intersects mcp-sa subjectAudiences
> // actor_token (mcp-sa projected SA token) -> rule 1
> { "iss": "https://kubernetes.default.svc.cluster.local",
> "sub": "system:serviceaccount:tools:mcp-sa", "aud": ["knoxidf"] }
> // policy mcp-sa: alice allowed, search-api in resourcePolicy, chain depth 2 <= 6 -> minted T2
> { "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
> "sub": "alice",
> "aud": ["https://svc.cluster.local/search/search-api"],
> "act": { "sub": "system:serviceaccount:tools:mcp-sa",
> "act": { "sub": "system:serviceaccount:dataservice:ds-sa" } } }
> {code}
> h3. Hop 3: {{search-api}} -> token for {{vector-db}}
> {code:yaml}
> subject_token=<T2> actor_token=<search-sa> resource=https://svc.cluster.local/data/vector-db
> // subject_token T2 -> rule 3: aud intersects search-sa subjectAudiences
> // actor_token (search-sa projected SA token, aud knoxidf) -> rule 1
> // policy search-sa: alice allowed, vector-db in resourcePolicy, chain depth 3 <= 6 -> minted T3
> { "iss": "http://knox.knox.svc.cluster.local:8443/gateway/knox-knoxidf-tokenexchange",
> "sub": "alice",
> "aud": ["https://svc.cluster.local/data/vector-db"],
> "act": { "sub": "system:serviceaccount:search:search-sa",
> "act": { "sub": "system:serviceaccount:tools:mcp-sa",
> "act": { "sub": "system:serviceaccount:dataservice:ds-sa" } } } }
> {code}
> h3. Attack cases
> ||Attacker has||Result||Rule||
> |{{ds-sa}} automounted SA token ({{{}aud=[https://kubernetes.default.svc|https://kubernetes.default.svc/]...{}}}) or one minted for Vault|Rejected as {{actor_token}} and as headless {{subject_token}}|1|
> |T1, presented with another workload's {{knoxidf}} actor token (even one allowed to act for {{{}alice{}}})|Rejected: {{mcp-server}} is not in that actor's {{subjectAudiences}}|3|
> |T1 as a headless {{subject_token}}|Rejected: no {{knoxidf}} audience, and it carries {{act}}|1, 5|
> |T1 in a same-subject exchange|Rejected: no {{knoxidf}} audience|4|
> |An SA token with {{aud=[https://svc.cluster.local/tools/mcp-server]}} sent directly to {{{}mcp-server{}}}, used as {{subject_token}}|Rejected: its issuer is not in {{token.exchange.subject.token.policy.audiences.issuers}}|3|
> |{{{}mcp-sa{}}}'s own {{knoxidf}} projected SA token|Attacker acts as {{{}mcp-sa{}}}, within its policy. Unavoidable; mitigate with short {{expirationSeconds}}|-|
> h3. Prior art
> Other authorization servers that support chained token exchange restrict re-exchanged subject tokens in the same two ways this proposal does:
> # the subject token must be *issued by the authorization server itself* (or a trusted sibling);
> # the subject token must be *addressed to the party presenting it*.
> ||Server||Subject token issuer||Subject token audience||Requester authenticated by||Chaining / {{act}}||
> |Keycloak, standard token exchange V2|Same realm only. External tokens are not accepted; a JWT Authorization Grant is recommended instead|"The {{subject_token}} sent to the token exchange endpoint must have the requester client set as an audience in the {{aud}} claim."|Client authentication ({{client_id}} + secret or certificate)|Chaining works; no {{actor_token}}/{{act}} in V2 (delegation and impersonation were only preview features in legacy V1)|
> |Microsoft Entra On-Behalf-Of|Entra itself|"This token must have an audience ({{aud}}) claim of the app making this OBO request ... Applications can't redeem a token for a different app."|Client credentials (secret or certificate)|Chaining works; no {{act}} claim|
> |Okta On-Behalf-Of token exchange|Same authorization server, or a trusted authorization server in the same Okta org|Authorization server audience|Service app client credentials|Nested {{act.act}}, at most 5 actors|
> How this maps to Knox:
> * *Issuer restriction:* {{token.exchange.subject.token.policy.audiences.issuers}} limits the relaxed path (rule 3) to tokens minted by Knox itself, as the servers above do. External and first-hop tokens still enter only through rule 2 ({{aud}} contains {{knox.token.audiences}}), which matches Keycloak's separate grant for external tokens and Okta's trusted servers.
> * *Recipient binding:* In Keycloak and Entra the requester is an OAuth client, and its {{client_id}} is the audience, so "{{aud}} must be the requester" needs no extra configuration. In Knox the requester is identified by an {{actor_token}} (e.g. {{system:serviceaccount:tools:mcp-sa}}), and that identity is not the audience it serves (e.g. {{https://svc.cluster.local/tools/mcp-server}}). {{subjectAudiences}} is the explicit actor-identity-to-audience mapping that a client registration provides implicitly in Keycloak and Entra.
> * *Actor credential (rule 1):* The servers above authenticate the requester with a client secret or certificate. A {{private_key_jwt}} client assertion, which must be addressed to the token endpoint, is the closest equivalent of requiring {{aud=knoxidf}} on the {{actor_token}}.
> * *Delegation chain:* Like Okta, Knox records the chain as nested {{act}} and caps its length ({{delegation.max.actor.chain.depth}}).
> RFC 8693 leaves which subject and actor tokens are acceptable to authorization-server policy (Section 2.1), so this proposal follows common practice; the spec doesn't mandate it.
> References:
> * [Keycloak: Configuring and using token exchange|https://www.keycloak.org/securing-apps/token-exchange]
> * [Microsoft Entra: OAuth 2.0 On-Behalf-Of flow|https://learn.microsoft.com/en-us/entra/identity-platform/v2-oauth2-on-behalf-of-flow]
> * [Okta: Set up On-Behalf-Of token exchange|https://developer.okta.com/docs/guides/set-up-token-exchange/main/]
> * [RFC 8693: OAuth 2.0 Token Exchange|https://www.rfc-editor.org/rfc/rfc8693]
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