RUSTSEC-2026-0288 cosmian_kyber: AVX2 backend skips Fujisaki-Okamoto implicit rejection, enabling chosen-ciphertext key recovery
Enabling the AVX2 feature in cosmian_kyber breaks implicit rejection, allowing attackers to recover static secret keys via chosen-ciphertext attacks.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is RUSTSEC-2026-0288?
RUSTSEC-2026-0288 is a vulnerability in cosmian_kyber, published on 16 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Summary
cosmian_kyber is a fork of Argyle-Software/kyber and inherits a broken
constant-time conditional-move routine cmov in its AVX2 backend. When built
with the opt-in avx2 feature on x86_64, cmov is a no-op: it never performs
the copy. That cmov is 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 and the reachable call path.
Affected configuration
Reachable only in builds with features = ["avx2"] on an x86_64 target. The
default (reference) backend and all non-x86_64 targets are unaffected.
Impact
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 is exposed; ephemeral-only key shares are not. The identical defect in the parent crate was verified end to end on ML-KEM-768 (full key recovered in 4,272 decapsulation queries).
Maintenance status
cosmian_kyber is a stale fork (last published 0.1.0, January 2023) of an
unmaintained upstream, and no fixed release is expected.
Mitigation
- If remaining on this crate, do not enable the
avx2feature; the default reference backend performs implicit rejection correctly. - Do not reuse a Kyber key pair across decapsulations.
Credit
Reported by 007bsd.
Which versions are affected by RUSTSEC-2026-0288?
1 open source package ship an affected version, including cosmian_kyber.
| Package | Affected | Fixed in |
|---|---|---|
cosmian_kyber crates.io | From 0.0.0-0 | None |
How do I fix RUSTSEC-2026-0288?
No fix has been published for RUSTSEC-2026-0288 yet. Limit exposure of the affected software until one is available.
Is RUSTSEC-2026-0288 being exploited?
There is no known exploitation of RUSTSEC-2026-0288 in the wild and no public exploit code.