A tomcat9 security update has been released for Debian GNU/Linux 11.

DSA 5009-1: tomcat9 security update

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Debian Security Advisory DSA-5009-1 security@debian.org
  https://www.debian.org/security/ Markus Koschany
November 12, 2021   https://www.debian.org/security/faq
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Package : tomcat9
CVE ID : CVE-2021-42340

Apache Tomcat, the servlet and JSP engine, did not properly release an HTTP
upgrade connection for WebSocket connections once the WebSocket connection was
closed. This created a memory leak that, over time, could lead to a denial of
service via an OutOfMemoryError.

For the stable distribution (bullseye), this problem has been fixed in
version 9.0.43-2~deb11u3.

We recommend that you upgrade your tomcat9 packages.

For the detailed security status of tomcat9 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/tomcat9

Further information about Debian Security Advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://www.debian.org/security/