A thunderbird security update has been released for Debian GNU/Linux 9 LTS to address a stack overflow.

DLA 2479-1: thunderbird security update

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Debian LTS Advisory DLA-2479-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Emilio Pozuelo Monfort
December 04, 2020   https://wiki.debian.org/LTS
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Package : thunderbird
Version : 1:78.5.1-1~deb9u1
CVE ID : CVE-2020-26970

Chiaki Ishikawa discovered a stack overflow in SMTP server status
handling which could potentially result in the execution of arbitrary
code.

For Debian 9 stretch, this problem has been fixed in version
1:78.5.1-1~deb9u1.

We recommend that you upgrade your thunderbird packages.

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

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