CVE-2026-18040: HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler

Vulnerability summary

CVE-2026-18040 is a medium (CVSS 5.9) Observable Timing Discrepancy vulnerability in legion of the bouncy castle inc. bc-java. EPSS estimates a 0.11% chance of exploitation in the next 30 days, higher than 1% of scored vulnerabilities.

Timeline

1. 03-10-2026
   PublishedVulnerability first disclosed.
2. 05-10-2026
   Last modifiedVulnerability information updated.

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.

• CWE-208Observable Timing DiscrepancyTwo separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.