When pqc kyber is built with the opt-in avx2 feature on x86 64, its constant-time conditional-move routine cmov is a no-op: it never performs the copy. That cmov is exactly the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an invalid ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause, the reachable call path, and reproduction details.

Reachable only in builds with features = ["avx2"] on an x86 64 target. The default (reference) backend and all non-x86 64 targets are unaffected.

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations (static or long-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed; ephemeral-only key shares are not. The defect was verified end to end on ML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.

The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or is expected.

Reported by 007bsd.