Summary The Kubernetes admission webhook handler reads the entire request body using "io.ReadAll(r.Body)" without any size limit. Any client that can reach the webhook port within the cluster can send a multi-GB payload, causing the skipper process to exhaust memory and be OOM-killed. This disrupts all Kubernetes admission control, potentially blocking all pod creation and updates. Vulnerable Code // dataclients/kubernetes/admission/admission.go:76 body, err := io.ReadAll(r.Body) // <-- NO SIZE LIMIT if err != nil { log.Errorf("Failed to read request: %v", err) w.WriteHeader(http.StatusInternalServerError) invalidRequests.WithLabelValues(admitterName).Inc() return } var review admissionReview err = json.Unmarshal(body, &review) For comparison, the OPA filter has a body size limit: // filters/openpolicyagent/openpolicyagent.go:68-70 const DefaultMaxRequestBodySize = 1 << 20 // 1MB // OPA uses a bufferedBodyReader with size limits Attack Path 1. Attacker identifies the admission webhook endpoint (default: ":9443/admission" or configured path) 2. Attacker sends: "POST /admission HTTP/1.1, Content-Type: application/json" with a multi-GB request body 3. "io.ReadAll(r.Body)" allocates unbounded memory for the entire body 4. Skipper process is OOM-killed by the Kubernetes kubelet Permission Boundary Analysis - Attacker: Any client with network access to the admission webhook port within the Kubernetes cluster - Boundary crossed: Memory safety — unbounded allocation from attacker-controlled input - Preconditions: Admission webhook endpoint must be network-reachable (default Kubernetes deployment exposes it within cluster network) - Comparison: OPA filter has "DefaultMaxRequestBodySize" (1MB) and semaphore-based memory limit; admission handler has neither Evidence | File | Lines | Description | |------|-------|-------------| | "dataclients/kubernetes/admission/admission.go" | 76 | "io.ReadAll(r.Body)" without size limit | | "filters/openpolicyagent/openpolicyagent.go" | 68-70 | OPA filter has "DefaultMaxRequestBodySize" = 1MB | | "filters/openpolicyagent/openpolicyagent.go" | 1333-1336 | OPA uses "bufferedBodyReader" with size limits | Tests - "dataclients/kubernetes/admission/admission_test.go" exists but does not test body size limits Impact The admission webhook handler reads the entire request body using io.ReadAll(r.Body) without a size limit. An attacker with in-cluster network access and a valid Kubernetes client certificate can send a multi-GB payload to the webhook endpoint, causing the skipper process to exhaust memory and be OOM-killed. This disrupts admission control for Ingress and RouteGroup resources until the process is automatically restarted by the kubelet. Scope of impact: Ingress and RouteGroup admission only — not pod creation or other admission controllers. Recovery: Kubernetes automatically restarts the OOM-killed process, limiting downtime. Prerequisites: (1) In-cluster network access to the webhook port, (2) valid Kubernetes client certificate. Mitigation 1. Add "http.MaxBytesReader" or equivalent body size limit before "io.ReadAll" 2. Follow the OPA filter pattern: define "DefaultMaxRequestBodySize" and use a buffered reader with size limits 3. Add a configurable "--admission-max-body-size" flag