CVE-2025-53506: Apache Tomcat: DoS via excessive h2 streams at connection start

[Mark Thomas <markt () apache org>]

Severity: important

Affected versions:

- Apache Tomcat 11.0.0-M1 through 11.0.8
- Apache Tomcat 10.1.0-M1 through 10.1.42
- Apache Tomcat 9.0.0.M1 through 9.0.106

Description:

Uncontrolled Resource Consumption vulnerability in Apache Tomcat if an 
HTTP/2 client did not acknowledge the initial settings frame that 
reduces the maximum permitted concurrent streams.

This issue affects Apache Tomcat: from 11.0.0-M1 through 11.0.8, from 
10.1.0-M1 through 10.1.42, from 9.0.0.M1 through 9.0.106.

Users are recommended to upgrade to version 11.0.9, 10.1.43 or 9.0.107, 
which fix the issue.

Credit:

Kanatoko (finder)

References:

https://lists.apache.org/thread/p09775q0rd185m6zz98krg0fp45j8kr0
https://tomcat.apache.org/
https://www.cve.org/CVERecord?id=CVE-2025-53506