The ML-DSA crate in the RustCrypto signatures library implements the Module‑Lattice‑Based Digital Signature Standard. A regression in version 0.0.4 changed a strict increasing hint‑index check to a non‑strict one, allowing signatures that contain duplicate hint indices. The specification requires strictly increasing indices; by accepting duplicates the verification process incorrectly accepts malformed signatures, potentially enabling an attacker to forge a valid signature and bypass authenticity checks. This flaw is a correctness failure (CWE‑347).

The vulnerability is present in the RustCrypto signatures library, specifically the ml‑dsa crate, from version 0.0.4 up to and including 0.1.0-rc.3. Any application that uses these crate versions for ML‑DSA signature verification is affected. The issue was fixed starting with version 0.1.0-rc.4.

The CVSS base score is 5.3, indicating medium severity, while the EPSS score is less than 1 %, showing a very low probability of exploitation. The vulnerability is not listed in the CISA KEV catalog, further suggesting limited real‑world exploitation. An attacker would need to supply a forged signature containing duplicate hint indices to a system that accepts external signatures, which is an unprivileged attack scenario. The impact is limited to the integrity of signed data but could be critical in contexts where authentic signatures are required.