CVE-2021-35955

Description

### Impact It is possible for untrusted users to inject malicious code into HTML attributes in the back end, which will be executed both in the element preview (back end) and on the website (front end). Installations are only affected if there are untrusted back end users who have the rights to modify HTML fields (e.g. TinyMCE). ### Patches Update to Contao 4.4.56, 4.9.18 or 4.11.7 ### Workarounds Disable all fields that allow HTML for untrusted back end users or disable the login for these users. ### References https://contao.org/en/security-advisories/cross-site-scripting-via-html-attributes-in-the-back-end ### For more information If you have any questions or comments about this advisory, open an issue in [contao/contao](https://github.com/contao/contao/issues/new/choose). ### Credits Thanks to Mikhail Khramenkov and Moritz Vondano for reporting this security issue.

How to fix CVE-2021-35955

To remediate CVE-2021-35955, upgrade the affected package to a fixed version below.

• —upgrade to 4.4.56 or later
• —upgrade to 4.4.56 or later

Is CVE-2021-35955 being exploited?

Low — EPSS is 0.4%, meaning exploitation activity has not been observed at scale.

Affected packages (2)

• >= 4.0.0, < 4.4.56
• >= 4.0.0, < 4.4.56

CVSS scores

References (7)

• ADVISORYnvd.nist.gov/vuln/detail/CVE-2021-35955
• PATCHgithub.com/contao/contao
• WEBcontao.org/en/news/contao-4-9-16-and-4-11-5-are-available.html
• WEBcontao.org/en/security-advisories/cross-site-scripting-via-html-attributes-in-the-back-end.html