{"id":"RUSTSEC-2026-0290","aliases":[],"status":"published","title":"pqc_kyber: AVX2 backend skips Fujisaki-Okamoto implicit rejection, enabling chosen-ciphertext key recovery","description":"## Summary\n\nWhen `pqc_kyber` is built with the opt-in `avx2` feature on x86_64, its\nconstant-time conditional-move routine `cmov` is a no-op: it never performs the\ncopy. That `cmov` is exactly the step that applies the Fujisaki-Okamoto (FO)\nimplicit rejection during decapsulation. With it disabled, decapsulating an\n*invalid* ciphertext no longer returns a key-independent pseudorandom value;\nit returns a value that depends on the decrypted plaintext of the\nattacker-chosen ciphertext. This restores the chosen-ciphertext decryption\noracle that the FO transform exists to remove, and yields full recovery of the\nstatic secret key.\n\nThe attack requires no timing measurement, no side channel, and no faults; only\nthe ability to submit ciphertexts to a decapsulation operation under a reused\nkey pair and observe the resulting shared secret. See the linked pull request\nfor the root cause, the reachable call path, and reproduction details.\n\n## Affected configuration\n\nReachable only in builds with `features = [\"avx2\"]` on an x86_64 target. The\ndefault (reference) backend and all non-x86_64 targets are unaffected.\n\n## Impact\n\nComplete IND-CCA break. The plaintext-checking oracle drives a standard\nchosen-ciphertext key-recovery attack and extracts the entire static secret\nkey. Any protocol reusing a Kyber key pair across decapsulations (static or\nlong-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed;\nephemeral-only key shares are not. The defect was verified end to end on\nML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.\n\n## Maintenance status\n\nThe crate is unmaintained (last release 0.7.1, August 2023); no patched version\nexists or is expected.\n\n## Mitigation\n\n- If remaining on this crate, do not enable the `avx2` feature; the default\n  reference backend performs implicit rejection correctly.\n- Do not reuse a Kyber key pair across decapsulations.\n\n## Credit\n\nReported by 007bsd.","severity":{"version":"3.1","score":7.4,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N","level":"high","from":"osv"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":false},"published":"2026-08-16T12:00:00Z","modified":"2026-09-18T09:15:05Z","affected":[{"source":"osv","ecosystem":"crates.io","package":"pqc_kyber","ranges":[{"type":"SEMVER","events":[{"introduced":"0.0.0-0"}]}]}],"references":[{"url":"https://crates.io/crates/pqc_kyber","tags":["package"]},{"url":"https://rustsec.org/advisories/RUSTSEC-2026-0290.html","tags":["advisory"]},{"url":"https://github.com/Argyle-Software/kyber/pull/121","tags":["web"]}],"sources":{"osv":[{"id":"RUSTSEC-2026-0290","title":"pqc_kyber: AVX2 backend skips Fujisaki-Okamoto implicit rejection, enabling chosen-ciphertext key recovery","description":"## Summary\n\nWhen `pqc_kyber` is built with the opt-in `avx2` feature on x86_64, its\nconstant-time conditional-move routine `cmov` is a no-op: it never performs the\ncopy. That `cmov` is exactly the step that applies the Fujisaki-Okamoto (FO)\nimplicit rejection during decapsulation. With it disabled, decapsulating an\n*invalid* ciphertext no longer returns a key-independent pseudorandom value;\nit returns a value that depends on the decrypted plaintext of the\nattacker-chosen ciphertext. This restores the chosen-ciphertext decryption\noracle that the FO transform exists to remove, and yields full recovery of the\nstatic secret key.\n\nThe attack requires no timing measurement, no side channel, and no faults; only\nthe ability to submit ciphertexts to a decapsulation operation under a reused\nkey pair and observe the resulting shared secret. See the linked pull request\nfor the root cause, the reachable call path, and reproduction details.\n\n## Affected configuration\n\nReachable only in builds with `features = [\"avx2\"]` on an x86_64 target. The\ndefault (reference) backend and all non-x86_64 targets are unaffected.\n\n## Impact\n\nComplete IND-CCA break. The plaintext-checking oracle drives a standard\nchosen-ciphertext key-recovery attack and extracts the entire static secret\nkey. Any protocol reusing a Kyber key pair across decapsulations (static or\nlong-term KEM keys, KEMTLS, HPKE recipients, pinned keys) is exposed;\nephemeral-only key shares are not. The defect was verified end to end on\nML-KEM-768: the full secret key was recovered in 4,272 decapsulation queries.\n\n## Maintenance status\n\nThe crate is unmaintained (last release 0.7.1, August 2023); no patched version\nexists or is expected.\n\n## Mitigation\n\n- If remaining on this crate, do not enable the `avx2` feature; the default\n  reference backend performs implicit rejection correctly.\n- Do not reuse a Kyber key pair across decapsulations.\n\n## Credit\n\nReported by 007bsd.","status":"published","published":"2026-08-16T12:00:00Z","modified":"2026-09-18T09:15:05.1289824Z","cvss":[{"version":"3.1","score":7.4,"vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N","level":"high"}],"updated_at":"2026-11-02T15:53:40.430679Z"}]}}