CVE-2026-18040 HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-18040?
CVE-2026-18040 is a medium-severity vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 3 Oct 2026.
Official description
In Bouncy Castle for Java before 1.86, HQC leaked secret-derived data through two side channels: its GF(2^8) arithmetic used lookup tables indexed by field elements, making the cache line touched a function of the operand, and its fixed-weight support sampler left its duplicate scan as soon as a collision was found and stored accepted positions at a secret index. Both run on secret inputs during encapsulation and decapsulation, and the sampler re-expands the secret key from its seed on every decapsulation, so an attacker able to observe cache behaviour or decapsulation timing can recover information about the HQC private key. The field arithmetic is now table-free and the sampler branch-free within a batch of candidates, with output and randomness consumption unchanged.
Weakness: CWE-208
Which versions are affected by CVE-2026-18040?
How do I fix CVE-2026-18040?
Upgrade to a fixed version.
Is CVE-2026-18040 being exploited?
There is no known exploitation of CVE-2026-18040 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-BK39147, CLEANSTART-2026-MV07927, DEBIAN-CVE-2026-18040, GHSA-rxw8-22c7-wgq6, UBUNTU-CVE-2026-18040.