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Vulnerability of lxml: Cross Site Scripting via HTML5 Formaction Attribute

Synthesis of the vulnerability 

An attacker can trigger a Cross Site Scripting via HTML5 Formaction Attribute of lxml, in order to run JavaScript code in the context of the web site.
Impacted software: Debian, Fedora, SnapCenter Backup Management, Ubuntu.
Severity of this computer vulnerability: 2/4.
Creation date: 24/03/2021.
Références of this announce: CVE-2021-28957, DLA-2606-1, DSA-4880-1, FEDORA-2021-28723f9670, FEDORA-2021-4cdb0f68c7, NTAP-20210521-0004, USN-4896-1, USN-4896-2, VIGILANCE-VUL-34942.

Description of the vulnerability 

An attacker can trigger a Cross Site Scripting via HTML5 Formaction Attribute of lxml, in order to run JavaScript code in the context of the web site.
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This weakness note impacts software or systems such as Debian, Fedora, SnapCenter Backup Management, Ubuntu.

Our Vigil@nce team determined that the severity of this threat note is medium.

The trust level is of type confirmed by the editor, with an origin of document.

An attacker with a expert ability can exploit this computer weakness.

Solutions for this threat 

Debian 10: new lxml packages.
New packages are available:
  Debian 10: lxml 4.3.2-1+deb10u3

Debian 9: new lxml packages.
New packages are available:
  Debian 9: lxml 3.7.1-1+deb9u4

Fedora 33: new python-lxml packages.
New packages are available:
  Fedora 33: python-lxml 4.5.1-4.fc33

Fedora 34: new python-lxml packages.
New packages are available:
  Fedora 34: python-lxml 4.6.3-1.fc34

SnapCenter: solution for lxml.
The solution is indicated in information sources.

Ubuntu: new lxml packages.
New packages are available:
  Ubuntu 20.10: python3-lxml 4.5.2-1ubuntu0.4
  Ubuntu 20.04 LTS: python-lxml 4.5.0-1ubuntu0.3, python3-lxml 4.5.0-1ubuntu0.3
  Ubuntu 18.04 LTS: python-lxml 4.2.1-1ubuntu0.4, python3-lxml 4.2.1-1ubuntu0.4
  Ubuntu 16.04 LTS: python-lxml 3.5.0-1ubuntu0.4, python3-lxml 3.5.0-1ubuntu0.4
  Ubuntu 14.04 ESM: python-lxml 3.3.3-1ubuntu0.2+esm3, python3-lxml 3.3.3-1ubuntu0.2+esm3
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