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RUSTSEC-2026-0290
pqc_kyberSummary
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.
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 (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.
Maintenance status
The crate is unmaintained (last release 0.7.1, August 2023); no patched version exists or 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.
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The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
Probability that this vulnerability will be exploited in the wild within the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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