A strongswan security update has been released for Debian GNU/Linux 9 LTS.

DLA 2788-1: strongswan security update

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Debian LTS Advisory DLA-2788-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Thorsten Alteholz
October 20, 2021   https://wiki.debian.org/LTS
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Package : strongswan
Version : 5.5.1-4+deb9u5
CVE ID : CVE-2021-41991

Researchers at the United States of America National Security Agency (NSA)
identified a denial of services vulnerability in strongSwan, an
IKE/IPsec suite.

Once the in-memory certificate cache is full it tries to randomly replace
lesser used entries. Depending on the generated random value, this could
lead to an integer overflow that results in a double-dereference and a
call using out-of-bounds memory that most likely leads to a segmentation
fault.

Remote code execution can't be ruled out completely, but attackers have
no control over the dereferenced memory, so it seems unlikely at this
point.

For Debian 9 stretch, this problem has been fixed in version
5.5.1-4+deb9u5.

We recommend that you upgrade your strongswan packages.

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

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