CVE-2026-18040: Software Security Vulnerability Vulnerability

MODERATE
Advisory Identifier: GHSA-rxw8-22c7-wgq6
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CWE-208
Published
Advisory Summary

CVE-2026-18040 is a MODERATE security vulnerability vulnerability (CWE-208: Observable Timing Discrepancy) affecting Software libraries in the Maven ecosystem. The weakness occurs when two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.. Remediation is available in the latest vendor maintenance releases.

CVE-2026-18040 at a Glance

CVE Identifier CVE-2026-18040
GHSA Identifier GHSA-rxw8-22c7-wgq6
Severity MODERATE
CVSS v3.1 Base Score N/A
CWE Classification CWE-208: Observable Timing Discrepancy
Affected Product Software
Package Ecosystem Maven
Published Date
Last Updated

How to Fix CVE-2026-18040

To remediate this vulnerability, upgrade all affected Software dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

mvn dependency:tree

How to Check if You Are Affected

To determine whether your Java application is affected by CVE-2026-18040, inspect your build dependencies using the Maven Dependency Plugin:

mvn dependency:tree

Analyze the terminal output to locate any version of Software in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than the patched release), your application is affected.

Note: The mvn dependency:tree command inspects the resolved dependency graph declared in your project's POM files; it does not perform automated security scanning or inspect runtime dynamic classpaths. Always verify runtime container classpaths and shaded JARs.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-208: Observable Timing Discrepancy, where two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not. In Software, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

References

NVD — CVE-2026-18040 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-18040
Security Fix Commit FIX COMMIT
https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03
GitHub Advisory / Wiki ADVISORY
https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040

Frequently Asked Questions

What is CVE-2026-18040?
CVE-2026-18040 is a security vulnerability in Software classified as a security vulnerability vulnerability under CWE-208: Observable Timing Discrepancy.
Which Software versions are affected?
Affected versions include releases prior to current release patches.
What version fixes CVE-2026-18040?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
What CWE is associated with CVE-2026-18040?
CVE-2026-18040 is classified under CWE-208: Observable Timing Discrepancy (Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.).
How can I check Maven dependencies for CVE-2026-18040?
Run the dependency tree command: 'mvn dependency:tree'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rxw8-22c7-wgq6",
  "modified" : "2026-10-03T09:31:18Z",
  "published" : "2026-10-03T09:31:18Z",
  "aliases" : [ "CVE-2026-18040" ],
  "details" : "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.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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/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"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-18040"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/bcgit/bc-java/commit/283acd81048ec9e951032525279d3b58aaf0bb03"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/bcgit/bc-java/wiki/CVE%E2%80%902026%E2%80%9018040"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-208" ],
    "severity" : "MODERATE",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2026-10-03T09:17:04Z"
  }
}