---
id: CVE-2026-18036
title: >-
  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…
summary: >-
  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…
severity: high
cvss: 8.2
cvssVector: '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'
cwe:
  - CWE-208
vendor: Legion of the Bouncy Castle Inc.
product: bcprov
affected:
  - bcprov >= 1.73 < 1.86
published: '2026-10-02'
updated: '2026-10-02'
sourceUpdated: '2026-10-02T08:17:01.660'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-18036'
references:
  - url: >-
      https://github.com/bcgit/bc-java/commit/9c9ad88b6003bb6230d2434bd892cc1e95b294e4
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
  - url: 'https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018036'
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
tags:
  - nvd
  - cve.org
cvssSource: cna
ingestedAt: '2026-10-02T08:14:06.214Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.