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The vulnerability resides within the TLS certificate validation logic of the Kubernetes C client library. By default, the library’s custom certificate handler fails to properly pin and validate the server’s X.509 certificate against the trusted root Certificate Authorities (CAs) provided in the kubeconfig file. Instead, it incorrectly accepts any certificate that is cryptographically well-formed, regardless of the issuing CA. This flaw allows an attacker with a privileged network position to present a forged certificate signed by their own untrusted CA. Since the client does not verify the certificate’s chain of trust back to the expected cluster CA, it establishes a TLS connection to the malicious endpoint, enabling man-in-the-middle attacks and full interception or manipulation of API server communications.
Platform: Kubernetes C Client
Version: <= v11.0.3
Vulnerability : TLS Trust Bypass
Severity: Moderate
date: 2025-09-16
Prediction: Patch by 2025-10-10
What Undercode Say:
openssl s_client -connect malicious-server:443 -CAfile ./fake-ca.crt
var handler = new HttpClientHandler(); handler.ServerCertificateCustomValidationCallback = (msg, cert, chain, errors) => true; // VULNERABLE CODE
How Exploit:
1. Adversary positions themselves on network path.
2. Forges certificate for API server endpoint.
3. Intercepts and decrypts client traffic.
Protection from this CVE
Update client library to patched version. Implement certificate pinning. Validate certificate chains explicitly.
Impact:
Man-in-the-middle attacks. API server impersonation. Credential theft. Unauthorized cluster access.
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Sources:
Reported By: github.com
Extra Source Hub:
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