# CVE-2026-18036: bouncycastle: In Bouncy Castle for Java before 1.86, NTRU reduced secret values with the % operator in three helpers whose referenc...

CVE-2026-18036 is fixed in bouncycastle on 1 platform, first by DEBIAN-CVE-2026-18036, released October 3, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-18036
- Priority: Routine
- CVSS: n/a
- EPSS: 0.0028
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Oct 2, 2026

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | bouncycastle Debian 14 | 1.86-1 | [DEBIAN-CVE-2026-18036](https://patchblox.com/security/data/fix/linux/debian-cve-2026-18036.md) | Oct 3, 2026 |
| Linux | bouncycastle opensuse-leap 16.0 | 1.86-160000.1.1 | [OPENSUSE-SU-2026:22169-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-22169-1.md) | Nov 7, 2026 |
| Linux | bouncycastle sles 15.6 | 1.86-150200.3.41.1 | [SUSE-SU-2026:4797-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4797-1.md) | Nov 7, 2026 |
| Linux | bouncycastle sles 15.7 | 1.86-150200.3.41.1 | [SUSE-SU-2026:4797-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4797-1.md) | Nov 7, 2026 |
| Linux | bouncycastle sles 16.0 | 1.86-160000.1.1 | [SUSE-SU-2026:24461-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-24461-1.md) | Nov 7, 2026 |

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Data as of 2026-11-09T01:18:40Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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