{"id":"RUSTSEC-2026-0290","revision":null,"records":[{"revision":"71b09ceb686045339b0b56e4d7a8bf5e","observedAt":"2026-10-24T13:19:54.901+00:00","statement":{"source":"osv","id":"RUSTSEC-2026-0290","aliases":[],"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.","state":0,"publishedAt":"2026-08-16T12:00:00+00:00","modifiedAt":"2026-09-18T09:15:05.1289824+00:00","affected":[{"ecosystem":"crates.io","name":"pqc_kyber","ranges":[{"type":"SEMVER","events":[{"kind":"introduced","version":"0.0.0-0"}]}],"versions":[],"rawVersionEvidence":null}],"metrics":[{"system":"CVSS_V3","vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N","score":null,"severity":null}],"references":["https://crates.io/crates/pqc_kyber","https://rustsec.org/advisories/RUSTSEC-2026-0290.html","https://github.com/Argyle-Software/kyber/pull/121"],"sourceUrl":"https://osv.dev/vulnerability/RUSTSEC-2026-0290","license":"OSV · upstream source license applies","key":"osv:RUSTSEC-2026-0290","identifiers":["RUSTSEC-2026-0290"]},"retained":false,"historicalAffected":[]}],"coverage":[{"source":"cisa-kev:global","scope":"Complete CISA KEV catalog; all vendors. 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