CVE-2025-12543 is a critical-severity (CVSS 9.6) Improper Input Validation vulnerability in io.undertow:undertow-core. A fix is available for io.undertow:undertow-core — see the affected versions and patch details below.
Undertow-core: undertow http server fails to reject malformed host headers leading to potential cache poisoning and ssrf
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-12543.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
io.undertow:undertow-core☕io.undertow:undertow-core☕io.undertow:undertow-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.undertow:undertow-core | all versions | No fix |
| ☕Maven | io.undertow:undertow-core | ≥ 2.3.0.Alpha1&&< 2.3.21.Final | 2.3.21.Finalio.undertow:undertow-core:2.3.21.Final |
| ☕Maven | io.undertow:undertow-core | all versions | 2.2.39.Finalio.undertow:undertow-core:2.2.39.Final |
Affected Products
build of apache camelredhatdata gridredhatfuseredhatjboss enterprise application platformredhatjboss enterprise application platform expansion packredhatprocess automationredhatDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.undertow:undertow-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
A fix is available for CVE-2025-12543; check the affected-versions list above or the advisory for the patched release of io.undertow:undertow-core, then make sure no transitive dependency still pins the vulnerable range.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This vulnerability has an Important severity because it can be remotely exploited without authentication. However, limited user interaction is required for full impact. It could allow attackers to hijack additional accounts, steal credentials, or gain access to internal systems. The issue stems from improper input…
Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use, applicability, or stability.Source: Red Hat security advisory for CVE-2025-12543 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat build of Apache Camel 4.14.4 for Spring Boot 3.5.11 | undertow-core | RHSA-2026:3890 |
| Red Hat JBoss Enterprise Application Platform 7.1 EUS for RHEL 7 | eap7-undertow-0:1.4.18-21.SP19_redhat_00001.1.ep7.el7 | RHSA-2026:33372 |
| Red Hat JBoss Enterprise Application Platform 7.3 EUS for RHEL 7 | eap7-undertow-0:2.0.41-8.SP9_redhat_00001.1.el7eap | RHSA-2026:33371 |
| Red Hat JBoss Enterprise Application Platform 7.4.24 | io.undertow/undertow-core:2.2.39.Final-redhat-00001 | RHSA-2026:4924 |
| Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 7 | eap7-undertow-0:2.2.39-1.Final_redhat_00001.1.el7eap | RHSA-2026:4915 |
| Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 8 | eap7-undertow-0:2.2.39-1.Final_redhat_00001.1.el8eap | RHSA-2026:4916 |
| Red Hat JBoss Enterprise Application Platform 7.4 ELS on RHEL 9 | eap7-undertow-0:2.2.39-1.Final_redhat_00001.1.el9eap | RHSA-2026:4917 |
| Red Hat JBoss Enterprise Application Platform 8.0 | undertow-core | RHSA-2026:3892 |
Frequently Asked Questions
Is CVE-2025-12543 in your dependencies?
Find it across Maven, including transitive dependencies.