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CVE-2021-35955

Cross site scripting via HTML attributes in the back end

4.8 MediumFix available

Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
High
User interaction
Required

What an attacker can affect

Confidentiality
Low
Integrity
Low
Availability
None

Exploit likelihood

0.56%Down 0.1 points since Aug 13, 2021
90%50%10%1%

What each source reports

NVD Score used 4.8

Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.

Record
CVE-2021-35955
CVSS 3.1
4.8 MediumCVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N
CVSS 2.0
3.5 LowAV:N/AC:M/Au:S/C:N/I:P/A:N
Weakness
CWE-79
Updated
2026-11-02
CVE List No score

Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.

Record
CVE-2021-35955
Updated
2026-11-02
EUVDEUVD-2021-1756 No score

Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.

Record
EUVD-2021-1756
Updated
2026-11-03
GitHub AdvisoriesGHSA-hr3h-x6gq-rqcp 5.9

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.

Record
GHSA-hr3h-x6gq-rqcp
CVSS 3.1
5.9 MediumCVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L
Weakness
CWE-79
Updated
2026-11-02
OSV 4.8

Contao >=4.0.0 allows backend XSS via HTML attributes to an HTML field. Fixed in 4.4.56, 4.9.18, 4.11.7.

Record
CVE-2021-35955
CVSS 3.1
4.8 MediumCVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N
Updated
2026-11-02
OSVGHSA-hr3h-x6gq-rqcp 5.9

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.

Record
GHSA-hr3h-x6gq-rqcp
CVSS 3.1
5.9 MediumCVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:L
Weakness
CWE-79
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
contao/core-bundle
Packagist
 4.0.0 up to 4.4.56
4.4.56
contao/contao
Packagist
 4.0.0 up to 4.4.56
4.4.56
contao/contao
Packagist
 4.5.0 up to 4.9.18
4.9.18
contao/contao
Packagist
 4.10.0 up to 4.11.7
4.11.7
contao/core-bundle
Packagist
 4.5.0 up to 4.9.18
4.9.18
contao/core-bundle
Packagist
 4.10.0 up to 4.11.7
4.11.7
contao/core-bundle
Packagist
 4.0.0 up to 4.4.56, plus listed versions
4.0.14.0.24.0.34.0.44.1.0
Show 45 moreShow less
4.1.0-RC14.1.0-beta14.1.14.1.24.1.34.2.04.2.0-RC14.2.0-beta14.2.14.2.24.2.34.2.44.2.54.3.04.3.0-RC14.3.14.3.104.3.114.3.24.3.34.3.44.3.54.3.64.3.74.3.84.3.94.4.04.4.0-RC14.4.0-RC24.4.0-beta14.4.14.4.104.4.114.4.124.4.134.4.144.4.154.4.164.4.174.4.184.4.194.4.24.4.204.4.214.4.22
4.4.56
contao/contao
Packagist
 4.0.0 up to 4.4.56, plus listed versions
4.4.224.4.234.4.244.4.254.4.26
Show 29 moreShow less
4.4.274.4.284.4.294.4.304.4.314.4.324.4.334.4.344.4.354.4.364.4.374.4.384.4.394.4.404.4.414.4.424.4.434.4.444.4.454.4.464.4.474.4.484.4.494.4.504.4.514.4.524.4.534.4.544.4.55
4.4.56
contao/contao
Packagist
 4.5.0 up to 4.9.18, plus listed versions
4.5.134.5.144.6.04.6.14.6.10
Show 45 moreShow less
4.6.114.6.124.6.134.6.144.6.24.6.34.6.44.6.54.6.64.6.74.6.84.6.94.7.04.7.0-RC14.7.0-RC24.7.0-RC34.7.0-RC44.7.14.7.24.7.34.7.44.7.54.7.64.7.74.8.04.8.0-RC14.8.0-RC24.8.14.8.24.8.34.8.44.8.54.8.64.8.74.8.84.9.04.9.0-RC14.9.0-RC24.9.14.9.104.9.114.9.124.9.134.9.144.9.15
4.9.18
contao/contao
Packagist
 4.10.0 up to 4.11.7, plus listed versions
4.10.14.10.24.10.34.10.44.10.5
Show 11 moreShow less
4.10.64.10.74.11.04.11.0-RC14.11.0-RC24.11.14.11.24.11.34.11.44.11.54.11.6
4.11.7
contao/core-bundle
Packagist
 4.5.0 up to 4.9.18, plus listed versions
4.5.14.5.104.5.114.5.124.5.13
Show 45 moreShow less
4.5.144.5.24.5.34.5.44.5.54.5.64.5.74.5.84.5.94.6.04.6.0-RC14.6.0-RC24.6.0-RC34.6.14.6.104.6.114.6.124.6.134.6.144.6.24.6.34.6.44.6.54.6.64.6.74.6.84.6.94.7.04.7.0-RC14.7.0-RC24.7.0-RC34.7.0-RC44.7.14.7.24.7.34.7.44.7.54.7.64.7.74.8.04.8.0-RC14.8.0-RC24.8.14.8.24.8.3
4.9.18
contao/core-bundle
Packagist
 4.10.0 up to 4.11.7, plus listed versions
4.10.14.10.24.10.34.10.44.10.5
Show 11 moreShow less
4.10.64.10.74.11.04.11.0-RC14.11.0-RC24.11.14.11.24.11.34.11.44.11.54.11.6
4.11.7

History

History
RecordedChangedSources
aliasesEUVD
first indexedCVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-35955 is a medium vulnerability (CVSS 4.8) in Contao, published August 12, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.56%, the 46th percentile. Fixed in contao/core-bundle 4.4.56.

References

This page doesn't list everything

Left out here: 75 versions. The JSON record has them.

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