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

> Attackers can exploit improper Host header validation in Undertow to perform SSRF, cache poisoning, or session hijacking.

- **Severity:** Critical 9.6 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 7 Jan 2026
- **Updated:** 14 Oct 2026
- **Also known as:** CLSA-2026-1770682162, DEBIAN-CVE-2025-12543, GHSA-j382-5jj3-vw4j, UBUNTU-CVE-2025-12543, USN-8144-1

## What is CVE-2025-12543?

CVE-2025-12543 is a critical server-side request forgery vulnerability in Redhat Jboss Enterprise Application Platform, published on 7 Jan 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2025-12543?

6 open source packages ship an affected version, including io.undertow:undertow-core. It affects 8 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `io.undertow:undertow-core` | Maven | All versions | None |
| `io.undertow:undertow-core` | Maven | All versions | None |
| `io.undertow:undertow-core` | Maven | All versions | 2.2.39.Final |
| `io.undertow:undertow-core` | Maven | All versions | 2.2.39.Final |
| `io.undertow:undertow-core` | Maven | From 2.3.0.Alpha1 | 2.3.21.Final |
| `io.undertow:undertow-core` | Maven | From 2.3.0.Alpha1 | 2.3.21.Final |

Products: Redhat Build of Apache Camel, Redhat Data Grid, Redhat Fuse, Redhat Jboss Enterprise Application Platform, Redhat Jboss Enterprise Application Platform Expansion Pack, Redhat Process Automation, Redhat Single Sign-On, Redhat Undertow.

## How do I fix CVE-2025-12543?

Upgrade io.undertow:undertow-core to a fixed version for your release line: 2.2.39.Final or 2.3.21.Final.

## Is CVE-2025-12543 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-12543 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-12543 and fix it if it is affected.

What it is: CVE-2025-12543 is a critical server-side request forgery vulnerability in Redhat Jboss Enterprise Application Platform, published on 7 Jan 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-12543 exists.

Affected packages and fixed versions:
- io.undertow:undertow-core (Maven): affected all versions; no fix listed
- io.undertow:undertow-core (Maven): affected all versions; no fix listed
- io.undertow:undertow-core (Maven): affected all versions; fixed in 2.2.39.Final
- io.undertow:undertow-core (Maven): affected all versions; fixed in 2.2.39.Final
- io.undertow:undertow-core (Maven): affected from 2.3.0.Alpha1; fixed in 2.3.21.Final
- io.undertow:undertow-core (Maven): affected from 2.3.0.Alpha1; fixed in 2.3.21.Final

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-12543/ (Markdown: https://insights.sbomify.com/CVE-2025-12543/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-12543
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-12543
- OSV: https://osv.dev/vulnerability/CVE-2025-12543

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-12543/
Summaries are generated and may contain mistakes; the official description above is the record.