Cross site scripting via HTML attributes in the back end
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.
Credits
Thanks to Mikhail Khramenkov and Moritz Vondano for reporting this security issue.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | contao/contao | >= 4.0.0, < 4.4.56, >= 4.5.0, < 4.9.18, >= 4.10.0, < 4.11.7 | 4.4.56, 4.9.18, 4.11.7 |
| Composer package | contao/core-bundle | >= 4.0.0, < 4.4.56, >= 4.5.0, < 4.9.18, >= 4.10.0, < 4.11.7 | 4.4.56, 4.9.18, 4.11.7 |
References
- https://github.com/contao/contao/security/advisories/GHSA-hr3h-x6gq-rqcp
- https://nvd.nist.gov/vuln/detail/CVE-2021-35955
- https://contao.org/en/news/contao-4-9-16-and-4-11-5-are-available.html
- https://contao.org/en/security-advisories/cross-site-scripting-via-html-attributes-in-the-back-end.html
- https://github.com/FriendsOfPHP/security-advisories/blob/master/contao/contao/CVE-2021-35955.yaml
- https://github.com/FriendsOfPHP/security-advisories/blob/master/contao/core-bundle/CVE-2021-35955.yaml
- https://github.com/contao/contao
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L- Weakness (CWE)
CWE-79- Platform
- Composer
- Package
contao/contao, contao/core-bundle- Disclosed
- 25.08.2021
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-hr3h-x6gq-rqcp - CC BY 4.0
Vulnerability history: contao/contao
All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 25
- First → latest
- 12.2019 09.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 4
- High 6
- Medium 12
- Low 3
Disclosures per year
- 2019: 3
- 2020: 1
- 2021: 4
- 2022: 9
- 2023: 1
- 2024: 0
- 2025: 4
- 2026: 3
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Cross site scripting via HTML attributes in the back end this entry
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