CVE-2026-18040
HQC leaks private key information through secret-indexed GF(2^8) tables and a secret-dependent fixed-weight sampler
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.
INFO
Published Date :
Oct. 3, 2026, 9:17 a.m.
Last Modified :
Oct. 3, 2026, 9:17 a.m.
Remotely Exploit :
No
Source :
91579145-5d7b-4cc5-b925-a0262ff19630
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 4.0 | MEDIUM | 91579145-5d7b-4cc5-b925-a0262ff19630 | ||||
| CVSS 4.0 | MEDIUM | 91579145-5d7b-4cc5-b925-a0262ff19630 |
Solution
- Update Bouncy Castle to version 1.86.
- Apply vendor patches or updates.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-18040.
| URL | Resource |
|---|---|
| https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03 | |
| https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040 |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-18040 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-18040
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-18040 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-18040 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
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New CVE Received by 91579145-5d7b-4cc5-b925-a0262ff19630
Oct. 03, 2026
Action Type Old Value New Value Added 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. Added CVSS V4.0 AV:L/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:Amber Added CWE CWE-208 Added Affected New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/18xxx/CVE-2026-18040.json">CVE-2026-18040</a> Added Reference https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03 Added Reference https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040