---
title: "CVE-2025-12543: Undertow HTTP server core doesn't properly validate the Host header in incoming HTTP requests"
url: https://erroneo.us/security/cve/CVE-2025-12543/
description: "CVE-2025-12543 (io.undertow:undertow-core (Maven)): A flaw was found in the Undertow HTTP server core, which is used in WildFly, JBoss EAP, and other Java…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-01-07
date_modified: 2026-10-14
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2025-12543"
kev: false
cvss: 9.6
epss: 0.01333
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2025-12543: Undertow HTTP server core doesn't properly validate the Host header in incoming HTTP requests

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 1.3% , percentile 71: FIRST’s model estimates a 1.3% chance of exploitation activity in the next 30 days, higher than 71% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
- Needs a user to click or open something. Phishing makes that realistic, but it is not wormable.
- Only applies if your dependency tree includes io.undertow:undertow-core (Maven) = 2.4.0.Alpha1, no fixed version listed ; io.undertow:undertow-core (Maven) >= 2.3.0.Alpha1, < 2.3.21.Final, fixed in 2.3.21.Final ; io.undertow:undertow-core (Maven) < 2.2.39.Final, fixed in 2.2.39.Final . Check your lockfile, including transitive dependencies.

## What it is

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.

## Affected packages

- Maven `io.undertow:undertow-core` = 2.4.0.Alpha1 → fixed: none listed
- Maven `io.undertow:undertow-core` >= 2.3.0.Alpha1, < 2.3.21.Final → fixed: 2.3.21.Final
- Maven `io.undertow:undertow-core` < 2.2.39.Final → fixed: 2.2.39.Final

## Affected products (CVE record)

- Red Hat Red Hat build of Apache Camel 4.14.4 for Spring Boot 3.5.11: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.1 EUS for RHEL 7: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.3 EUS for RHEL 7: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4.24: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 7: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 7: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 8: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 8: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 9: see advisory
- Red Hat Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 9: see advisory

## Severity

- redhat CVSS 3.1: 9.6 `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:L`
- redhat-SADP CVSS 3.1: 9.6 `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:L`
- GitHub CVSS 3.1: 9.6 `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:L`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 1.33% probability of exploitation activity in the next 30 days; percentile 71.

## Sources

- [RHSA-2026:0383](https://access.redhat.com/errata/RHSA-2026:0383)
- [RHSA-2026:0384](https://access.redhat.com/errata/RHSA-2026:0384)
- [RHSA-2026:0386](https://access.redhat.com/errata/RHSA-2026:0386)
- [RHSA-2026:33371](https://access.redhat.com/errata/RHSA-2026:33371)
- [RHSA-2026:33372](https://access.redhat.com/errata/RHSA-2026:33372)
- [RHSA-2026:3889](https://access.redhat.com/errata/RHSA-2026:3889)
- [RHSA-2026:3890](https://access.redhat.com/errata/RHSA-2026:3890)
- [RHSA-2026:3891](https://access.redhat.com/errata/RHSA-2026:3891)
- [RHSA-2026:3892](https://access.redhat.com/errata/RHSA-2026:3892)
- [RHSA-2026:4915](https://access.redhat.com/errata/RHSA-2026:4915)
- [RHSA-2026:4916](https://access.redhat.com/errata/RHSA-2026:4916)
- [RHSA-2026:4917](https://access.redhat.com/errata/RHSA-2026:4917)
- [RHSA-2026:4924](https://access.redhat.com/errata/RHSA-2026:4924)
- [RHSA-2026:76128](https://access.redhat.com/errata/RHSA-2026:76128)
- [RHSA-2026:76129](https://access.redhat.com/errata/RHSA-2026:76129)
- [RHSA-2026:76130](https://access.redhat.com/errata/RHSA-2026:76130)
- [RHSA-2026:76132](https://access.redhat.com/errata/RHSA-2026:76132)
- [RHSA-2026:76134](https://access.redhat.com/errata/RHSA-2026:76134)
- [GitHub Advisory GHSA-j382-5jj3-vw4j](https://github.com/advisories/GHSA-j382-5jj3-vw4j)
- [CVE.org record CVE-2025-12543](https://www.cve.org/CVERecord?id=CVE-2025-12543)
- [NVD entry CVE-2025-12543](https://nvd.nist.gov/vuln/detail/CVE-2025-12543)
- [access.redhat.com](https://access.redhat.com/security/cve/CVE-2025-12543)
- [RHBZ#2408784](https://bugzilla.redhat.com/show_bug.cgi?id=2408784)
- [security.access.redhat.com](https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-12543.json)
- [github.com](https://github.com/undertow-io/undertow/pull/1857)
- [github.com](https://github.com/undertow-io/undertow/pull/1860)
- [github.com](https://github.com/undertow-io/undertow/releases/tag/2.3.21.Final)
- [github.com](https://github.com/undertow-io/undertow/releases/tag/2.2.39.Final)
- [issues.redhat.com](https://issues.redhat.com/browse/UNDERTOW-2656)

---
Source page: https://erroneo.us/security/cve/CVE-2025-12543/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt