# CVE-2025-12543: libundertow-java: A flaw was found in the Undertow HTTP server core, which is used in WildFly, JBoss EAP, and other Java applications

CVE-2025-12543 is fixed in libundertow-java on 1 platform, first by libundertow-java 1.4.23-3ubuntu0.1~esm1, released January 7, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2025-12543
- Priority: High
- CVSS: n/a (High)
- EPSS: 0.0133
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jan 7, 2026

## Description

A flaw was found in the Undertow HTTP server core, which is used in WildFly, JBoss EAP, and other Java applications. The Undertow library fails to properly validate the Host header in incoming HTTP requests.As a result, requests containing malformed or malicious Host headers are processed without rejection, enabling attackers to poison caches, perform internal network scans, or hijack user sessions.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | libundertow-java Ubuntu 18.04.6 LTS (bionic) | 1.4.23-3ubuntu0.1~esm1 | [libundertow-java 1.4.23-3ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-libundertow-java-1.4.23-3ubuntu0.1-esm1.md) | Jan 7, 2026 |
| Linux | libundertow-java Ubuntu 20.04 LTS (focal) | 2.0.29-1ubuntu0.1~esm1 | [libundertow-java 2.0.29-1ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-libundertow-java-2.0.29-1ubuntu0.1-esm1.md) | Jan 7, 2026 |
| Linux | libundertow-java Ubuntu 22.04.1 LTS (jammy) | 2.2.16-1ubuntu0.1~esm1 | [libundertow-java 2.2.16-1ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-jammy-libundertow-java-2.2.16-1ubuntu0.1-esm1.md) | Jan 7, 2026 |
| Linux | libundertow-java Ubuntu 24.04.5 LTS (noble) | 2.3.8-2ubuntu0.1~esm1 | [libundertow-java 2.3.8-2ubuntu0.1~esm1](https://patchblox.com/security/data/fix/linux/ubuntu-noble-libundertow-java-2.3.8-2ubuntu0.1-esm1.md) | Jan 7, 2026 |
| Linux | libundertow-java Ubuntu 26.04.1 LTS (resolute) | 2.3.20-1ubuntu1 | [libundertow-java 2.3.20-1ubuntu1](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libundertow-java-2.3.20-1ubuntu1.md) | Jan 7, 2026 |

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Data as of 2026-11-09T13:31:49Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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