CVE-2026-31830: sigstore-ruby Verification Bypass Vulnerability

CVE-2026-31830 Overview

CVE-2026-31830 is a signature verification bypass vulnerability in sigstore-ruby, a pure Ruby implementation of the sigstore verify command from the sigstore/cosign project. Prior to version 0.2.3, the Sigstore::Verifier#verify method fails to properly propagate VerificationFailure results returned by verify_in_toto when the artifact digest does not match the digest specified in the in-toto attestation subject. This critical flaw causes verification of DSSE (Dead Simple Signing Envelope) bundles containing in-toto statements to incorrectly return VerificationSuccess regardless of whether the artifact actually matches the attested subject.

Critical Impact

Attackers can bypass signature verification checks, allowing malicious or tampered artifacts to pass verification as legitimate, potentially compromising software supply chain integrity.

Affected Products

• sigstore-ruby versions prior to 0.2.3
• Applications using sigstore-ruby for DSSE bundle verification with in-toto statements
• Software supply chain verification systems relying on sigstore-ruby

Discovery Timeline

• 2026-03-10 - CVE CVE-2026-31830 published to NVD
• 2026-03-11 - Last updated in NVD database

Technical Details for CVE-2026-31830

Vulnerability Analysis

This vulnerability stems from improper error handling within the sigstore-ruby verification workflow (CWE-252: Unchecked Return Value). The core issue lies in how the Sigstore::Verifier#verify method processes the return value from its internal verify_in_toto function call.

When verifying DSSE bundles that contain in-toto attestation statements, the verification process should confirm that the artifact being verified matches the cryptographic digest specified in the attestation subject. The verify_in_toto function correctly identifies mismatches and returns a VerificationFailure object. However, the parent verify method fails to check and propagate this failure state, instead returning VerificationSuccess unconditionally.

This allows an attacker to present any arbitrary artifact alongside a valid DSSE bundle containing an in-toto attestation for a different artifact, and the verification will succeed. This fundamentally breaks the integrity guarantees that sigstore verification is designed to provide.

Root Cause

The root cause is an unchecked return value vulnerability (CWE-252). The Sigstore::Verifier#verify method does not properly handle the VerificationFailure object returned by verify_in_toto. The failure result is silently discarded rather than being propagated to the caller, causing the verification to appear successful even when the artifact digest validation fails.

Attack Vector

The vulnerability is exploitable over the network without authentication or user interaction. An attacker can exploit this flaw by:

1. Obtaining a legitimate DSSE bundle with an in-toto attestation for any artifact
2. Presenting this bundle alongside a different, potentially malicious artifact
3. The sigstore-ruby verifier will report successful verification despite the artifact mismatch

This attack is particularly dangerous in software supply chain scenarios where signature verification is used to ensure artifact integrity before deployment or execution.

The vulnerability mechanism involves the verification flow where verify_in_toto correctly detects the digest mismatch and returns a VerificationFailure object, but the calling verify method ignores this return value. For technical implementation details, see the GitHub Security Advisory.

Detection Methods for CVE-2026-31830

Indicators of Compromise

• Unexpected or unauthorized artifacts passing verification checks in CI/CD pipelines
• Log entries showing successful verification for artifacts with mismatched digests
• Deployment of software packages that differ from their attested versions

Detection Strategies

• Audit sigstore-ruby version in dependency manifests (Gemfile.lock) for versions prior to 0.2.3
• Implement secondary verification of artifact digests independent of sigstore-ruby
• Review CI/CD logs for verification operations that may have passed incorrectly

Monitoring Recommendations

• Monitor application logs for verification success events and cross-reference with expected artifact hashes
• Implement digest verification logging at the application level to identify potential exploitation
• Set up alerts for sigstore-ruby verification operations in production environments

How to Mitigate CVE-2026-31830

Immediate Actions Required

• Upgrade sigstore-ruby to version 0.2.3 or later immediately
• Audit any artifacts that were verified using vulnerable versions of sigstore-ruby
• Re-verify critical artifacts using the patched version before deployment

Patch Information

The vulnerability is fixed in sigstore-ruby version 0.2.3. Organizations should update their Gemfile to specify the patched version and run bundle update sigstore-ruby. The fix ensures that VerificationFailure results from verify_in_toto are properly propagated by the verify method. For additional details, refer to the GitHub Security Advisory.

Workarounds

• Implement additional artifact digest verification independent of sigstore-ruby verification results
• Use alternative sigstore implementations (such as the Go-based cosign) until upgrade is possible
• Add pre-deployment validation steps that independently verify artifact hashes against expected values

# Update sigstore-ruby to patched version
bundle update sigstore-ruby

# Verify installed version is 0.2.3 or later
bundle show sigstore-ruby