How severe is CVE-2021-35955?

CVE-2021-35955 has a CVSS score of 4.8/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-35955?

CVE-2021-35955 affects the following packages: contao/core-bundle (Packagist), contao/contao (Packagist), contao/contao (Packagist), contao/contao (Packagist), contao/core-bundle (Packagist), contao/core-bundle (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2021-35955?

Update contao/core-bundle to 4.4.56 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-35955 is resolved across your whole dependency graph.

How do I detect CVE-2021-35955 in my Packagist dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for contao/core-bundle, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-35955 if there is no patch (or I can't update yet)?

Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Is CVE-2021-35955 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-35955 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-35955?

CVE-2021-35955 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 45th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-35955?

CVE-2021-35955 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-35955 published, and has it been updated?

CVE-2021-35955 was published on August 12, 2021 and was last updated on August 7, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐘

🐘 Packagist

Not in CISA KEV

MEDIUM severity

CVE-2021-35955 — core-bundle

MEDIUM

CVE-2021-35955 is a medium-severity (CVSS 4.8) Cross-site Scripting (XSS) vulnerability in contao/core-bundle. A fix is available for contao/core-bundle — see the affected versions and patch details below.

Cross site scripting via HTML attributes in the back end

Also known asGHSA-hr3h-x6gq-rqcp

Published

Aug 12, 2021

Updated

Aug 7, 2026

Affected

6 pkgs

Patched

6 / 6

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs45th percentile — riskier than 45% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-35955 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

6 pkgs affected

🐘contao/core-bundle🐘contao/contao🐘contao/contao🐘contao/contao🐘contao/core-bundle🐘contao/core-bundle

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.

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.

Credits

Thanks to Mikhail Khramenkov and Moritz Vondano for reporting this security issue.

Affected Packages

6 total 6 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`contao/core-bundle`	≥ 4.0.0&&< 4.4.56	4.4.56`composer require contao/core-bundle:^4.4.56`
🐘Packagist	`contao/contao`	≥ 4.0.0&&< 4.4.56	4.4.56`composer require contao/contao:^4.4.56`
🐘Packagist	`contao/contao`	≥ 4.5.0&&< 4.9.18	4.9.18`composer require contao/contao:^4.9.18`
🐘Packagist	`contao/contao`	≥ 4.10.0&&< 4.11.7	4.11.7`composer require contao/contao:^4.11.7`
🐘Packagist	`contao/core-bundle`	≥ 4.5.0&&< 4.9.18	4.9.18`composer require contao/core-bundle:^4.9.18`
🐘Packagist	`contao/core-bundle`	≥ 4.10.0&&< 4.11.7	4.11.7`composer require contao/core-bundle:^4.11.7`

Affected Products

1 product · 3 configurations

Application

contaocontao

≥ 4.10.0 && < 4.11.7

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for contao/core-bundle, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update contao/core-bundle to 4.4.56 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-35955 is resolved across your whole dependency graph.
Workarounds
Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

### 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-at

O3 Security · Impact-Aware SCA

Is CVE-2021-35955 in your dependencies?

Find it across Packagist, including transitive dependencies.

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CVSS Score

4.8/10CVSS v3 base

Vector breakdown

Attack Vector

Network

Attack Complexity

Low

Privileges Required

High

User Interaction

Required

Scope

Changed

Confidentiality

Low

Integrity

Low

Availability

None

Raw vector

CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N

CWE Classification

CWE-79Cross-site Scripting (XSS)

Weaponization

No known public exploit tooling found.

Timeline

Disclosed

Aug 12, 2021

Last updated

Aug 7, 2026

Identifiers

CVE-2021-35955

GHSA-hr3h-x6gq-rqcp

References

Advisorycontao.org Advisorygithub.com Advisorynvd.nist.gov Webcontao.org Webgithub.com Webgithub.com Packagegithub.com

Related vulnerabilities

This CVE is medium severity; 5 of 8 other CVEs affecting contao/core-bundle are rated as high or higher (top: moderate).

CVE-2026-55824CVE-2026-55825CVE-2025-65961CVE-2025-65960CVE-2025-57756CVE-2025-57758CVE-2025-57759CVE-2025-57757

Provenance

Sources

OSV.devNVDFIRST.org (EPSS)

Last verifiedAug 7, 2026

CVE-2021-35955 — core-bundle

EPSS Exploitation Probability

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

Description

Impact

Patches

Workarounds

References

For more information

Credits

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Frequently Asked Questions

Is CVE-2021-35955 in your dependencies?