CVE-2025-12543
CRITICALCVE-2025-12543 is a critical-severity (CVSS 9.6) vulnerability in io.undertow:undertow-core. O3 Security confirms whether CVE-2025-12543 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
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.
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.Final |
| ☕Maven | io.undertow:undertow-core | all versions | 2.2.39.Final |
Detection & 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
No patched version of io.undertow:undertow-core has shipped for CVE-2025-12543 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.
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.
How O3 protects you
O3 pinpoints whether CVE-2025-12543 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2025-12543. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
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…
Frequently Asked Questions
Is CVE-2025-12543 in your dependencies?
O3 detects CVE-2025-12543 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.