Executive brief
Undertow, a web server component used in Red Hat JBoss and other Java applications, is vulnerable to a security flaw in how it handles web requests. By sending specially crafted messages, an attacker can trick the server into misinterpreting where one request ends and another begins. This can allow unauthorized users to bypass security filters, access private data, or interfere with other users' sessions.
Technical details
A flaw exists in Undertow's HTTP header parsing logic (CWE-444). The server splits header names from values using either a space or a colon, whichever appears first. This behavior differs from standard upstream proxies, allowing an attacker to craft requests with headers that are interpreted by Undertow but ignored by the proxy. This discrepancy enables HTTP request smuggling. An attacker can use this to bypass security constraints, poison web caches, or hijack user sessions. The vulnerability is addressed in Undertow version 2.3.24.SP2 and Red Hat JBoss EAP 8.1.6.
Affected products
- Red Hat Undertow <= 2.3.23.Final
- Red Hat JBoss Enterprise Application Platform 7.0, 8.0, 8.1.0
Timeline
- 2026-02-27: disclosed: Initial report in Red Hat Bugzilla
- 2026-03-27: advisory: GitHub and NVD advisories published
- 2026-06-10: patched: Red Hat released security updates (RHSA-2026:25125, RHSA-2026:25126)