Vulnerability · Published Oct 3, 2026 · Assigned by bcorg
CVE-2026-18040HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler
Medium 5.9
In plain English
A medium-severity security flaw in BC-JAVA from Legion of the Bouncy Castle Inc.. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 1% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | bouncycastle | no fix listed | DEBIAN-CVE-2026-18040 |
| Debian:13 | bouncycastle | no fix listed | DEBIAN-CVE-2026-18040 |
| Debian:14 | bouncycastle | 1.86-1 | DEBIAN-CVE-2026-18040 |
| Ubuntu:26.04:LTS | bouncycastle | no fix listed | UBUNTU-CVE-2026-18040 |
| Ubuntu:26.10 | bouncycastle | no fix listed | UBUNTU-CVE-2026-18040 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Legion of the Bouncy Castle Inc. | BC-JAVA | 1.73 – before 1.86 |
Weakness: CWE-208 · Vector: CVSS:4.0/AV:L/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/U:Amber