CVE-2025-12543: Undertow HTTP server core doesn't properly validate the Host header in incoming HTTP requests
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.
Published Jan 7, 2026 · Updated Mar 18, 2026
Affected packages
io.undertow:undertow-core
- Ecosystem
- Maven
- Affected
- = 2.4.0.Alpha1
- Fixed in
- No patched version yet
- Check
mvn dependency:tree -Dincludes=io.undertow:undertow-core
io.undertow:undertow-core
- Ecosystem
- Maven
- Affected
- >= 2.3.0.Alpha1, < 2.3.21.Final
- Fixed in
- 2.3.21.Final
- Check
mvn dependency:tree -Dincludes=io.undertow:undertow-core
io.undertow:undertow-core
- Ecosystem
- Maven
- Affected
- < 2.2.39.Final
- Fixed in
- 2.2.39.Final
- Check
mvn dependency:tree -Dincludes=io.undertow:undertow-core
Advisory details
From the GitHub Advisory Database (CC BY 4.0).
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.
Credited to aldexis, dpogorelov.
References
- https://nvd.nist.gov/vuln/detail/CVE-2025-12543
- https://access.redhat.com/security/cve/CVE-2025-12543
- https://bugzilla.redhat.com/show_bug.cgi?id=2408784
- https://access.redhat.com/errata/RHSA-2026:0384
- https://access.redhat.com/errata/RHSA-2026:0386
- https://access.redhat.com/errata/RHSA-2026:0383
- https://github.com/undertow-io/undertow/pull/1857
- https://github.com/undertow-io/undertow/pull/1860
- https://github.com/undertow-io/undertow/releases/tag/2.3.21.Final
- https://github.com/undertow-io/undertow/releases/tag/2.2.39.Final
- https://issues.redhat.com/browse/UNDERTOW-2656
- https://access.redhat.com/errata/RHSA-2026:3890
- https://access.redhat.com/errata/RHSA-2026:3889
- https://access.redhat.com/errata/RHSA-2026:3891
- https://access.redhat.com/errata/RHSA-2026:3892
- https://access.redhat.com/errata/RHSA-2026:4924
- https://access.redhat.com/errata/RHSA-2026:4917
- https://access.redhat.com/errata/RHSA-2026:4916
- https://access.redhat.com/errata/RHSA-2026:4915
- https://github.com/advisories/GHSA-j382-5jj3-vw4j
How this page is made: the data panel comes from the GitHub Advisory Database, CISA’s KEV catalog, the NVD and FIRST EPSS, and updates twice a day. No analysis has been written for this entry yet. Methodology