Cosign is a tool within the sigstore project that provides code signing and transparency for container images and binaries, enhancing supply chain security. In versions 3.0.4 and earlier, a certificate validation flaw exists related to the handling of certificate expiration during signature verification. When verifying artifact signatures, cosign first validates the certificate chain based on the leaf certificate's "not before" timestamp, assuming all issuing certificates in the chain are valid during the leaf's validity period. However, it fails to properly verify whether issuing certificates have expired relative to the signed timestamp or current time. This means that if an issuing certificate's validity period ends before the leaf certificate's, cosign may still accept it as valid, allowing signatures to be trusted even if the issuing certificate was expired at signing time. This improper certificate validation corresponds to CWE-295. The vulnerability primarily affects private deployments of cosign that use customized public key infrastructures (PKIs), as the public Sigstore infrastructure is not impacted. The flaw does not require user interaction or privileges to exploit and has a CVSS v3.1 score of 3.7 (low severity), reflecting limited impact on confidentiality and availability but a potential integrity risk. The issue was publicly disclosed in February 2026 and fixed in cosign version 3.0.5. No known exploits have been reported in the wild, but organizations relying on private PKIs and cosign for code signing should upgrade to avoid risks of accepting signatures validated by expired issuing certificates.

The primary impact of this vulnerability is on the integrity of code signature verification in private cosign deployments using customized PKIs. An attacker could potentially exploit this flaw to have signatures accepted that were signed with issuing certificates that had already expired, undermining trust in the authenticity and integrity of signed artifacts. This could lead to unauthorized or malicious code being trusted and deployed, increasing the risk of supply chain attacks. However, the vulnerability does not affect confidentiality or availability directly and does not impact users of the public Sigstore infrastructure. The scope is limited to organizations that have private cosign deployments with custom certificate authorities and do not promptly update to the patched version. Since exploitation does not require privileges or user interaction, the risk is primarily operational and depends on the organization's certificate management practices. Overall, the impact is moderate but important for maintaining strong supply chain security in environments relying on cosign with private PKIs.

Organizations using cosign for code signing should immediately upgrade to version 3.0.5 or later, where the certificate validation logic has been corrected to properly verify the expiration of issuing certificates against the signed timestamp or current time. For private deployments with customized PKIs, administrators should audit their certificate chains to ensure no expired issuing certificates are in use and validate that their certificate issuance policies align with best practices for validity periods. Implementing continuous monitoring of certificate validity and automated renewal processes can reduce risks of expired certificates being used. Additionally, integrating cosign verification with external timestamp authorities or transparency logs that provide reliable signed timestamps can enhance validation accuracy. Organizations should also review their supply chain security policies to ensure that all code signing tools and dependencies are kept up to date and that verification processes are robust against certificate validation flaws. Finally, consider isolating private PKI infrastructures and limiting their exposure to reduce attack surface.