Description

sigstore-java is a sigstore java client for interacting with sigstore infrastructure. Version 2.0.0 erroneously removed verification of the integrated (Rekor entry) time) against the Fulcio certificate. Version 2.1.0 re-added this verification with enhancements that adhere to the Sigstore verification spec. The old sigstore-conformance test for this check was built incorrectly. This vulnerability impacts only users verifying bundles with `dev.sigstore:sigstore-java:2.0.0`. Older versions are not affected; it is fixed in `dev.sigstore:sigstore-java:2.1.0` A malicious actor may exploit this if they were able to access a users system and exfiltrate the temporary private key used during signing and then reuse an old fulcio certificate later without requiring direct access to the user's credentials. Users may protect themselves by re-verifying their artifacts using the newest sigstore-java or another current sigstore client. Transparency logs may also be audited for unauthorized signatures for a suspected reused identity.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "2.0",
  "pubDate": "2026-08-13T00:17:32.407Z",
  "pubdate": "2026-08-13T00:17:32.407Z",
  "executiveSummary": "An improper verification vulnerability exists in dev.sigstore:sigstore-java:2.0.0, specifically regarding the omission of integrated Rekor entry time verification against the Fulcio certificate. This oversight allows a malicious actor who has successfully exfiltrated a temporary private key from a user's system to reuse an old Fulcio certificate at a later time without requiring direct access to live user credentials. The impact is limited to the integrity of signature verification processes for users utilizing the affected version, potentially leading to the acceptance of improperly validated signatures. Risk implications involve the bypass of cryptographic validity windows tied to short-lived certificates. Exploitation requires prior access to the user's system to retrieve the temporary private key, followed by the presentation of a reused certificate context during bundle verification. The vulnerability is explicitly present only in version 2.0.0, as older versions are unaffected and the issue has been fully remediated in version 2.1.0 through the re-introduction of spec-compliant verification checks.",
  "technicalDetails": "The vulnerability resides in the dev.sigstore:sigstore-java library, specifically within version 2.0.0, where the core logic responsible for verifying the integrated Rekor entry time against the corresponding Fulcio certificate was erroneously removed. Furthermore, the associated sigstore-conformance test designed to validate this specific security check was built incorrectly, allowing the regression to bypass automated validation during development. The root cause is a lapse in cryptographic validation logic during code updates for version 2.0.0, which broke adherence to the official Sigstore verification specification regarding the temporal validity of certificates against transparency log entries. The affected component is the bundle verification module of dev.sigstore:sigstore-java:2.0.0. Exploitation of this vulnerability requires an attacker to first achieve unauthorized access to a user's system to exfiltrate the temporary private key generated and utilized during the signing process. Once the temporary private key is obtained, the attacker can leverage the missing temporal validation to reuse an expired or previously valid Fulcio certificate in conjunction with the exfiltrated key to sign artifacts. Because version 2.0.0 fails to verify the Rekor entry time against the validity bounds of the Fulcio certificate, the client incorrectly accepts the signature as valid. The attack flow proceeds as follows: 1. The attacker exfiltrates the temporary private key from the victim's local environment. 2. The attacker constructs a signature using the compromised private key and an old, previously issued Fulcio certificate outside of its intended active window. 3. A victim or automated system evaluates the generated bundle using dev.sigstore:sigstore-java:2.0.0. 4. Due to the absent verification logic, the library skips checking the Rekor integrated timestamp against the Fulcio certificate constraints. 5. The verification passes successfully, treating the unauthorized signature as legitimate. Network exposure depends on how the library is integrated into software supply chain pipelines, but the vulnerability itself is triggered locally or during dependency verification routines when processing crafted or malicious bundles. Post-exploitation impact includes the potential acceptance of forged or unauthorized software artifacts, undermining the cryptographic trust guarantees provided by the Sigstore infrastructure for the affected client installations."
}