# CVE-2026-18036: NTRU leaks private key information by reducing secret values with a non-constant-time integer division

> Timing side-channel in Bouncy Castle NTRU allows attackers to recover private key information via non-constant-time integer divisions.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/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
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Oct 2026
- **Updated:** 2 Oct 2026
- **Also known as:** CLEANSTART-2026-AC10204, CLEANSTART-2026-NB89107, CLEANSTART-2026-RL38047, DEBIAN-CVE-2026-18036, GHSA-frjj-v5w4-68pc, UBUNTU-CVE-2026-18036

## What is CVE-2026-18036?

CVE-2026-18036 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In Bouncy Castle for Java before 1.86, NTRU reduced secret values with the % operator in three helpers whose reference implementations are deliberately division-free, so each reduction was carried out by an integer division whose latency depends on the secret operand. Polynomial.modQ divided by a variable divisor, which a compiler cannot strength-reduce to a multiply the way it can a constant one, so it emitted a division on every call including on the decapsulation path where the dividend derives from the private key; Polynomial.mod3 and NTRUSampling.mod3 divided the secret key polynomials f and g during key generation, the message polynomials r and m during encapsulation, and coefficients recovered during decapsulation. An attacker able to measure that timing can recover information about the NTRU private key. modQ now masks, which is exact because q is always a power of two, and mod3 uses the reference implementation's division-free fold and select; the results are unchanged.

Weakness: CWE-208 (Observable Timing Discrepancy)

## Which versions are affected by CVE-2026-18036?


## How do I fix CVE-2026-18036?

Upgrade to a fixed version.

## Is CVE-2026-18036 being exploited?

There is no known exploitation of CVE-2026-18036 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-18036 and fix it if it is affected.

What it is: CVE-2026-18036 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. BC-JAVA, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-18036 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-2026-18036/ (Markdown: https://insights.sbomify.com/CVE-2026-18036/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-18036
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-18036
- OSV: https://osv.dev/vulnerability/CVE-2026-18036

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-18036/
Summaries are generated and may contain mistakes; the official description above is the record.