- Reported
-
- Issued
-
- Package
-
cosmian_kyber
(crates.io)
- Type
-
Vulnerability
- Categories
-
- Keywords
-
#kyber
#ml-kem
#key-recovery
#fujisaki-okamoto
#ind-cca
#avx2
- References
-
- CVSS Score
- 7.4
HIGH
- CVSS Details
-
- Attack Vector
- Network
- Attack Complexity
- High
- Privileges Required
- None
- User Interaction
- None
- Scope
- Unchanged
- Confidentiality Impact
- High
- Integrity Impact
- High
- Availability Impact
- None
- CVSS Vector
- CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
- Patched
-
no patched versions
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
avx2 feature; the default
reference backend performs implicit rejection correctly.
- Do not reuse a Kyber key pair across decapsulations.
Credit
Reported by 007bsd.
Advisory available under CC0-1.0
license.