CVE-2026-28367

A flaw was found in Undertow. A remote attacker can exploit this vulnerability by sending \r\r\r as a header block terminator. This can be used for request smuggling with certain proxy servers, such as older versions of Apache Traffic Server and Google Cloud Classic Application Load Balancer, potentially leading to unauthorized access or manipulation of web requests.

The Undertow web server is susceptible to request smuggling when deployed with certain proxy servers that forward \r\r\r as a header block terminator. This flaw could enable an attacker to manipulate web requests, potentially leading to unauthorized access. Red Hat products using Undertow in environments with vulnerable proxy configurations, such as those found in older versions of Apache Traffic Server or Google Cloud Classic Application Load Balancer, are at risk.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

To mitigate this vulnerability, configure any proxy servers positioned in front of Undertow to strictly validate HTTP header terminations. Ensure that these proxies are configured to reject or normalize non-standard header block terminators, such as \r\r\r, before forwarding requests to Undertow. This operational control helps prevent request smuggling attacks by ensuring that only properly formed HTTP requests reach the Undertow server.