A mediawiki regression update has been released for Debian GNU/Linux 9 LTS to address a regression introduced by the previous update.

DLA 2648-2: mediawiki regression update

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Debian LTS Advisory DLA-2648-2 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Abhijith PA
May 07, 2021   https://wiki.debian.org/LTS
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Package : mediawiki
Version : 1:1.27.7-1~deb9u9
CVE ID : CVE-2021-20270 CVE-2021-27291 CVE-2021-30152
CVE-2021-30155 CVE-2021-30158 CVE-2021-30159
Debian Bug : 985574 984664

The patch from latest upstream release to address CVE-2021-30152 was
not portable to stretch-security version causing MediaWiki APIs to
fail. This update includes a patch from upstream REL_31 release which
fix the issue.

For Debian 9 stretch, this problem has been fixed in version
1:1.27.7-1~deb9u9.

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