kyverno apicall servicecall implicit bearer token injection leaks kyverno serviceaccount token

kyverno’s apiCall servicecall helper implicitly injects Authorization: Bearer ... using the kyverno controller serviceaccount token when a policy does not explicitly set an Authorization header. because context.apiCall.service.url is policy-controlled, this can send the kyverno serviceaccount token to an attacker-controlled endpoint (confused deputy).

namespaced policies are blocked from servicecall usage by the namespaced urlPath gate in pkg/engine/apicall/apiCall.go, so this report is scoped to ClusterPolicy and global context usage.

attacker model

the attacker can create or update a ClusterPolicy (or create a GlobalContextEntry) which uses context.apiCall.service.url and can choose the request URL and headers. a cross-boundary framing for real deployments is gitops: if the policy repo/controller is compromised, the ClusterPolicy/global context entry becomes untrusted input to kyverno.

relevant links

• repository: https://github.com/kyverno/kyverno
• commit: 17aeb52337fd66adb0c8126213ba076612a287a7
• callsite (token injection): https://github.com/kyverno/kyverno/blob/17aeb52337fd66adb0c8126213ba076612a287a7/pkg/engine/apicall/executor.go#L150-L173
• namespaced policy gate (servicecall blocked): https://github.com/kyverno/kyverno/blob/17aeb52337fd66adb0c8126213ba076612a287a7/pkg/engine/apicall/apiCall.go#L67-L83

root cause

in (*executor).addHTTPHeaders, kyverno reads the serviceaccount token from /var/run/secrets/kubernetes.io/serviceaccount/token and injects it when the outgoing request has no Authorization header:

if req.Header.Get("Authorization") == "" {
  token := a.getToken()
  if token != "" {
    req.Header.Add("Authorization", "Bearer "+token)
  }
}

proof of concept

the attached poc.zip is a reproducible cluster PoC. it uses an in-cluster HTTP receiver which logs the Authorization header it receives. the PoC does not print token bytes; it only checks that the received header is non-empty and not equal to the negative control.

run (one command):

unzip poc.zip -d poc
cd poc
make test

canonical (expected: implicit token injection):

unzip poc.zip -d poc
cd poc
make canonical

expected output includes:

[CALLSITE_HIT]: executor.addHTTPHeaders Authorization=="" -> read_serviceaccount_token=true
[PROOF_MARKER]: authorization_header_injected=true token_nonempty=true

control (expected: explicit Authorization header disables auto-injection):

unzip poc.zip -d poc
cd poc
make control

expected output includes:

[CALLSITE_HIT]: executor.addHTTPHeaders Authorization!="" -> autoinject_skipped=true
[NC_MARKER]: authorization_header_injected=false

optional: the canonical run may also print an [RBAC]: ... line using kubectl auth can-i with the exfiltrated token, to show concrete privileges without exposing the token.

impact

token exfiltration: the kyverno controller serviceaccount token is sent to a policy-controlled endpoint. impact depends on the rbac bound to that serviceaccount in the target deployment.

recommended fix

do not auto-inject the kyverno serviceaccount token into policy-controlled servicecall requests. require explicit Authorization configuration, or enforce a strict allowlist of destinations where credentials may be attached and document the behavior.

workarounds

• avoid using servicecall to arbitrary urls in policies.
• set an explicit Authorization header in servicecall policies to prevent implicit token injection.

poc.zip PR_DESCRIPTION.md

oleh