CVE-2021-21357

Problem

Due to improper input validation, attackers can by-pass restrictions of predefined options and submit arbitrary data in the Form Designer backend module of the Form Framework.

In the default configuration of the Form Framework this allows attackers to explicitly allow arbitrary mime-types for file uploads - however, default fileDenyPattern successfully blocked files like .htaccess or malicious.php. Besides that, attackers can persist those files in any writable directory of the corresponding TYPO3 installation.

A valid backend user account with access to the form module is needed to exploit this vulnerability.

Credits

Thanks to Richie Lee who reported this issue and to TYPO3 contributor Ralf Zimmermann who fixed the issue.

References

• TYPO3-CORE-SA-2021-003

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2021-21357 has a CVSS score of 7.9 (High). The vector is network-reachable, low privileges required, and no user interaction. 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 (8.7.40, 9.5.25, 10.4.14, 11.1.1); upgrading removes the vulnerable code path.

Kodem intelligence

Update to TYPO3 versions 8.7.40, 9.5.25, 10.4.14, 11.1.1 that fix the problem described.

1. What is CVE-2021-21357? CVE-2021-21357 is a high-severity improper input validation vulnerability in typo3/cms-form (composer), affecting versions >= 8.0.0, <= 8.7.39. It is fixed in 8.7.40, 9.5.25, 10.4.14, 11.1.1. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2021-21357? CVE-2021-21357 has a CVSS score of 7.9 (High). 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-2021-21357?
   • typo3/cms-form (composer) (versions >= 8.0.0, <= 8.7.39)
   • typo3/cms-core (composer) (versions >= 10.0.0, < 10.4.14)
   • typo3/cms (composer) (versions >= 10.0.0, < 10.4.14)
4. Is there a fix for CVE-2021-21357? Yes. CVE-2021-21357 is fixed in 8.7.40, 9.5.25, 10.4.14, 11.1.1. Upgrade to this version or later.
5. Is CVE-2021-21357 exploitable, and should I be worried? Whether CVE-2021-21357 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-2021-21357 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-2021-21357?
   • Upgrade typo3/cms-form to 8.7.40 or later
   • Upgrade typo3/cms-form to 9.5.25 or later
   • Upgrade typo3/cms-form to 10.4.14 or later
   • Upgrade typo3/cms-form to 11.1.1 or later
   • Upgrade typo3/cms-core to 10.4.14 or later
   • Upgrade typo3/cms-core to 11.1.1 or later
   • Upgrade typo3/cms-core to 9.5.25 or later
   • Upgrade typo3/cms to 10.4.14 or later
   • Upgrade typo3/cms to 11.1.1 or later
   • Upgrade typo3/cms to 9.5.25 or later