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Advisory record · GHSA-j382-5jj3-vw4j

Undertow HTTP server core doesn't properly validate the Host header in incoming HTTP requests

CVSS 9.6 CriticalCriticalEPSS 0.013 · 71st percentile

Critical, because:

  • CVSS score 9.6: 9.0 or more.

How we decide what’s critical

Published by the GitHub Advisory Database at 7 Jan 2026, 18:30 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

PackageEcosystemAffectedFixed in
io.undertow:undertow-coreMaven=2.4.0.Alpha1 || >=2.3.0.Alpha1, <2.3.21.Final || <2.2.39.Final2.3.21.Final, 2.2.39.Final
Identifiers
GHSA-j382-5jj3-vw4j · CVE-2025-12543
Weakness (CWE)
CWE-20
Tags
Published
7 Jan 2026
Updated
10 Sept 2026

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Undertow HTTP server core (io.undertow:undertow-core) versions from 2.3.0.Alpha1 before 2.3.21.Final, and versions before 2.2.39.Final, do not properly validate the Host header in incoming HTTP requests. An attacker who can send HTTP requests to the server can supply malformed or malicious Host headers, which may allow cache poisoning, internal network scanning, or session hijacking. Upgrade to 2.3.21.Final or 2.2.39.Final. The source gives few details on specific attack conditions.

Written 2 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

ScoreRatingCVSSVectorSource
9.6 Critical 3.1 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:L CNA (redhat)
  • CISA KEV Not listed.
  • EPSS 0.013, the 71st percentile, on 12 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

  1. 7 Jan 2026, 16:04 UTC Confirmed by the CVE List
  2. 7 Jan 2026, 17:15 UTC Confirmed by NVD
  3. 31 Oct 2026, 14:17 UTC First seen by agentprime
  4. 31 Oct 2026, 14:17 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.