Skipper: Unbounded Request Body Read in Admission Webhook Causes Memory Exhaustion DoS

Skipper is an HTTP router and reverse proxy for service composition. Prior to 0.26.22, Handler in dataclients/kubernetes/admission/admission.go passes the body of requests to the Kubernetes admission endpoint at :9443/admission directly to io.ReadAll(r.Body) without a size limit. An attacker with in-cluster network access and a valid Kubernetes client certificate can send a very large body that causes unbounded memory allocation and an out-of-memory termination of the Skipper process. The disruption is limited to Ingress and RouteGroup admission rather than pod creation or unrelated admission controllers, and Kubernetes normally restarts the process. This issue is fixed in version 0.26.22.

Skipper is an HTTP router and reverse proxy for service composition. Prior to 0.26.22, Handler in dataclients/kubernetes/admission/admission.go passes the body of requests to the Kubernetes admission endpoint at :9443/admission directly to io.ReadAll(r.Body) without a size limit. An attacker with in-cluster network access and a valid Kubernetes client certificate can send a very large body that causes unbounded memory allocation and an out-of-memory termination of the Skipper process. The disruption is limited to Ingress and RouteGroup admission rather than pod creation or unrelated admission controllers, and Kubernetes normally restarts the process. This issue is fixed in version 0.26.22.

Skipper is an HTTP router and reverse proxy for service composition. Prior to 0.26.22, Handler in dataclients/kubernetes/admission/admission.go passes the body of requests to the Kubernetes admission endpoint at :9443/admission directly to io.ReadAll(r.Body) without a size limit. An attacker with in-cluster network access and a valid Kubernetes client certificate can send a very large body that causes unbounded memory allocation and an out-of-memory termination of the Skipper process. The disruption is limited to Ingress and RouteGroup admission rather than pod creation or unrelated admission controllers, and Kubernetes normally restarts the process. This issue is fixed in version 0.26.22.

## 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 ```go // 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: ```go // 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

Skipper: Unbounded Request Body Read in Admission Webhook Causes Memory Exhaustion DoS in github.com/zalando/skipper