CVE-2021-28957

Advisory

Lxml version 4.6.3 includes a fix for CVE-2021-28957: An XSS vulnerability was discovered in python-lxml's clean module versions before 4.6.3. When disabling the safe_attrs_only and forms arguments, the Cleaner class does not remove the formation attribute allowing for JS to bypass the sanitizer. A remote attacker could exploit this flaw to run arbitrary JS code on users who interact with incorrectly sanitized HTML.
https://bugs.launchpad.net/lxml/+bug/1888153

Affected package

Affected versions

Fixed versions

Vulnerability changelog

==================

Bugs fixed
----------

* A vulnerability (CVE-2021-28957) was discovered in the HTML Cleaner by Kevin Chung,
which allowed JavaScript to pass through. The cleaner now removes the HTML5
``formaction`` attribute.

Resources

• https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-28957
• https://bugs.launchpad.net/lxml/+bug/1888153
• https://github.com/lxml/lxml/pull/316/commits/10ec1b4e9f93713513a3264ed6158af22492f270
• https://lists.debian.org/debian-lts-announce/2021/03/msg00031.html
• https://github.com/lxml/lxml/commit/a5f9cb52079dc57477c460dbe6ba0f775e14a999
• https://www.debian.org/security/2021/dsa-4880
• https://security.netapp.com/advisory/ntap-20210521-0004/
• https://lists.fedoraproject.org/archives/list/[email protected]/message/3C2R44VDUY7FJVMAVRZ2WY7XYL4SVN45/
• https://lists.fedoraproject.org/archives/list/[email protected]/message/XXN3QPWCTQVOGW4BMWV3AUUZZ4NRZNSQ/
• https://www.oracle.com/security-alerts/cpuoct2021.html
• https://security.gentoo.org/glsa/202208-06