An isc-dhcp security update has been released for Debian GNU/Linux 9 LTS to address a potential denial of service issue.

DLA 2674-1: isc-dhcp security update

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Debian LTS Advisory DLA-2674-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Emilio Pozuelo Monfort
June 03, 2021   https://wiki.debian.org/LTS
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Package : isc-dhcp
Version : 4.3.5-3+deb9u2
CVE ID : CVE-2021-25217
Debian Bug : 989157

Jon Franklin and Pawel Wieczorkiewicz found an issue in the ISC DHCP
client and server when parsing lease information, which could lead to
denial of service via application crash.

For Debian 9 stretch, this problem has been fixed in version
4.3.5-3+deb9u2.

We recommend that you upgrade your isc-dhcp packages.

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

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