The Kyber reference implementation before 9b8d306, when compiled by LLVM Clang through 18.x with some common optimization options, has a timing side channel that allows attackers to recover an ML-KEM 512 secret key in minutes. This occurs because poly_frommsg in poly.c does not prevent Clang from emitting a vulnerable secret-dependent branch.

References

• https://nvd.nist.gov/vuln/detail/CVE-2024-37880
• pq-crystals/kyber@9b8d306
• https://github.com/antoonpurnal/clangover
• https://news.ycombinator.com/item?id=40577486
• https://pqshield.com/pqshield-plugs-timing-leaks-in-kyber-ml-kem-to-improve-pqc-implementation-maturity
• https://twitter.com/purnaltoon/status/1797644696568959476