A mediawiki security update has been released for Debian GNU/Linux 9 LTS to address a missing validation.

DLA 2847-1: mediawiki security update

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Debian LTS Advisory DLA-2847-1 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Moritz Muehlenhoff
December 15, 2021   https://wiki.debian.org/LTS
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Package : mediawiki
Version : 1:1.27.7-1+deb9u11
CVE ID : CVE-2021-44858

A security issue was discovered in MediaWiki, a website engine for
collaborative work: Missing validation in the mcrundo action may allow
allow an attacker to leak page content from private wikis or to bypass
edit restrictions.

For additional information please refer to
  https://www.mediawiki.org/wiki/2021-12_security_release/FAQ

For Debian 9 stretch, this problem has been fixed in version
1:1.27.7-1+deb9u11.

We recommend that you upgrade your mediawiki packages.

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

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