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


- **Severity:** Medium 5.9 (`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`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 3 Oct 2026
- **Updated:** 5 Oct 2026
- **Also known as:** CLEANSTART-2026-BK39147, CLEANSTART-2026-MV07927, DEBIAN-CVE-2026-18040, GHSA-rxw8-22c7-wgq6, UBUNTU-CVE-2026-18040

## 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 (Observable Timing Discrepancy)

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18040 and fix it if it is affected.

What it is: CVE-2026-18040 is a medium-severity vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 3 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-18040 in the wild and no public exploit code.

Affected product: Legion of the Bouncy Castle Inc. BC-JAVA. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-18040/ (Markdown: https://insights.sbomify.com/CVE-2026-18040/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18040
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18040
- OSV: https://osv.dev/vulnerability/CVE-2026-18040

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18040/
Summaries are generated and may contain mistakes; the official description above is the record.