CVE-2021-35210

Workarounds

Disable the system log module in the back end for all users (especially admin users).

References

https://contao.org/en/security-advisories/cross-site-scripting-in-the-system-log-2021

For more information

If you have any questions or comments about this advisory, open an issue in contao/contao.

It is possible to inject code into the tl_log table that will be executed in the browser when the system log is called in the back end.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2021-35210 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.9.16, 4.11.5); upgrading removes the vulnerable code path.

Kodem intelligence

Update to Contao 4.9.16 or 4.11.5.

1. What is CVE-2021-35210? CVE-2021-35210 is a medium-severity cross-site scripting (XSS) vulnerability in contao/core-bundle (composer), affecting versions >= 4.5.0, < 4.9.16. It is fixed in 4.9.16, 4.11.5. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2021-35210? CVE-2021-35210 has a CVSS score of 6.1 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2021-35210?
   • contao/core-bundle (composer) (versions >= 4.5.0, < 4.9.16)
   • contao/contao (composer) (versions >= 4.5.0, < 4.9.16)
4. Is there a fix for CVE-2021-35210? Yes. CVE-2021-35210 is fixed in 4.9.16, 4.11.5. Upgrade to this version or later.
5. Is CVE-2021-35210 exploitable, and should I be worried? Whether CVE-2021-35210 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-35210 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-35210?
   • Upgrade contao/core-bundle to 4.9.16 or later
   • Upgrade contao/core-bundle to 4.11.5 or later
   • Upgrade contao/contao to 4.9.16 or later
   • Upgrade contao/contao to 4.11.5 or later