A composer security update has been released for Debian GNU/Linux 9 LTSto address an issue where composer did not properly sanitize Mercurial URLs.

DLA 2654-1: composer security update

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Debian LTS Advisory DLA-2654-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Utkarsh Gupta
May 12, 2021   https://wiki.debian.org/LTS
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Package : composer
Version : 1.2.2-1+deb9u1
CVE ID : CVE-2021-29472

It was discovered that composer, a dependency manager for PHP, did
not properly sanitize Mercurial URLs, which could lead to arbitrary
code execution.

For Debian 9 stretch, this problem has been fixed in version
1.2.2-1+deb9u1.

We recommend that you upgrade your composer packages.

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

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