There's a bug in the use_hint function where it adds 1 instead of subtracting 1 when the decomposed low bits r0 equal exactly zero. FIPS 204 Algorithm 40 is pretty clear that r0 > 0 means strictly positive, but the current code treats zero as positive. This causes valid signatures to potentially fail verification when this edge case gets hit.
The ML-DSA signature verification implementation in the RustCrypto ml-dsa crate incorrectly accepts signatures with repeated (duplicate) hint indices. According to the ML-DSA specification (FIPS 204 / RFC 9881), hint indices within each polynomial must be strictly increasing. The current implementation uses a non-strict monotonic check (<= instead of <), allowing duplicate indices. Note: This is a regression bug. The original implementation was correct, but commit b01c3b7 ("Make ML-DSA signature decoding …
A timing side-channel was discovered in the Decompose algorithm which is used during ML-DSA signing to generate hints for the signature.