---
id: CVE-2026-18040
title: >-
  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-…
summary: >-
  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-…
severity: medium
cvss: 5.9
cvssVector: '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'
cwe:
  - CWE-208
vendor: Legion of the Bouncy Castle Inc.
product: bcprov
affected:
  - bcprov >= 1.73 < 1.86
published: '2026-10-03'
updated: '2026-10-03'
sourceUpdated: '2026-10-03T09:17:04.360'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-18040'
references:
  - url: >-
      https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
  - url: 'https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040'
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
tags:
  - nvd
  - cve.org
cvssSource: cna
ingestedAt: '2026-10-03T09:42:13.786Z'
---

## Overview

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.

## Remediation

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