Signature Verification Bypass in Kyverno

Executive Summary

Kyverno 1.18.0 before 1.19.2 has a signature verification bypass in the verifyImages component. Cached verification results are incorrectly keyed by the image reference string (like a tag) instead of the verified digest. This allows attackers to exploit a time-of-check to time-of-use race condition by warming the cache with a signed image, then repointing the tag to an unsigned image. Kyverno then admits the workload and pins the unverified digest, bypassing signature and attestation checks.

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Detection Guidance

To detect this vulnerability, check if your Kyverno version is between 1.18.0 and 1.19.1. Run: kyverno version. If the version falls in this range, the system is potentially vulnerable.

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Impact Analysis

An attacker could submit malicious workloads to your cluster by exploiting this vulnerability. If you use Kyverno's verifyImages policy with caching enabled, the attacker can bypass signature checks and have Kyverno admit unsigned or tampered images. This could lead to unauthorized code execution in your cluster, potentially compromising your environment.

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Compliance Impact

This vulnerability could violate compliance requirements that mandate image integrity and verification, such as HIPAA's security controls for software integrity or GDPR's principles of data protection by design. Bypassing signature checks undermines trust in workload admission, potentially exposing sensitive data or systems to unauthorized access.

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Mitigation Strategies

Upgrade Kyverno to version 1.19.2 or later immediately. Disable caching for verifyImages policies if possible. Review all verifyImages policies to ensure they enforce digest binding checks.

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