PT-2026-60893 · Go · Github.Com/Zalando/Skipper

CVE-2026-54247

·

Published

2026-07-17

·

Updated

2026-07-17

CVSS v3.1

4.3

Medium

VectorAV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

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

FileLinesDescription
dataclients/kubernetes/admission/admission.go76io.ReadAll(r.Body) without size limit
filters/openpolicyagent/openpolicyagent.go68-70OPA filter has DefaultMaxRequestBodySize = 1MB
filters/openpolicyagent/openpolicyagent.go1333-1336OPA 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

Fix

Allocation of Resources Without Limits

Found an issue in the description? Have something to add? Feel free to write us 👾
dbugs@ptsecurity.com

Weakness Enumeration

CWE-770

Related Identifiers

CVE-2026-54247
GHSA-CWXQ-RC9X-2JVV

Affected Products

Github.Com/Zalando/Skipper