A libx11 security update has been released for Debian GNU/Linux 9 LTS to address a protocol command injection vulnerability.

DLA 2666-1: libx11 security update

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Debian LTS Advisory DLA-2666-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Emilio Pozuelo Monfort
May 24, 2021   https://wiki.debian.org/LTS
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Package : libx11
Version : 2:1.6.4-3+deb9u4
CVE ID : CVE-2021-31535
Debian Bug : 988737

Roman Fiedler found that libX11, the X11 protocol client library, was
vulnerable to protocol command injection due to insufficient validation
of arguments to some functions.

For Debian 9 stretch, this problem has been fixed in version
2:1.6.4-3+deb9u4.

We recommend that you upgrade your libx11 packages.

For the detailed security status of libx11 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/libx11

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