Summary
Problem
Due to the lack of ensuring file extensions belong to configured allowed mime-types, attackers can upload arbitrary data with arbitrary file extensions - however, default fileDenyPattern successfully blocked files like .htaccess or malicious.php.
TYPO3 Extbase extensions, which implement a file upload and do not implement a custom TypeConverter to transform uploaded files into FileReference domain model objects are affected by the vulnerability as well, since the UploadedFileReferenceConverter of ext:form handles the file upload and will accept files of any mime-type which are persisted to the default location.
In any way, uploaded files are placed in the default location /fileadmin/user_upload/, in most scenarios keeping the submitted filename - which allows attackers to directly reference files, or even correctly guess filenames used by other individuals, disclosing this information.
No authentication is required to exploit this vulnerability.
Credits
Thanks to Sebastian Michaelsen, Marc Lindemann, Oliver Eglseder, Markus Volkmer, Jakob Kunzmann, Johannes Regner, Richie Lee who reported this issue, and to TYPO3 core & security team members Oliver Hader & Benni Mack, as well as TYPO3 contributor Ralf Zimmermann who fixed the issue.
References
Impact
The application accepts file uploads without adequately restricting the file type or content. Typical impact: remote code execution if the uploaded file can be served and executed on the server.
CVE-2021-21355 has a CVSS score of 8.0 (High). The vector is network-reachable, no 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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Update to TYPO3 versions 8.7.40, 9.5.25, 10.4.14, 11.1.1 that fix the problem described.
Type converter UploadedFileReferenceConverter is not registered globally anymore and just handles uploaded files within the scope of the Form Framework. Guessable storage location has changed from /fileadmin/user_upload/form_<random-hash>/ to /fileadmin/form_uploads/. Allowed mime-types must match expected file extensions (e.g. application/pdf must be .pdf, and cannot be .html).
Extbase extensions, who rely on the global availability of the UploadedFileReferenceConverter must now implement a custom TypeConverter to handle file uploads or explicitly implement the ext:form UploadedFileReferenceConverter with appropriate setting for accepted mime-types.
Frequently Asked Questions
- What is CVE-2021-21355? CVE-2021-21355 is a high-severity unrestricted upload of dangerous file types 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 accepts file uploads without adequately restricting the file type or content.
- How severe is CVE-2021-21355? CVE-2021-21355 has a CVSS score of 8.0 (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.
- Which packages are affected by CVE-2021-21355?
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)
- Is there a fix for CVE-2021-21355? Yes. CVE-2021-21355 is fixed in 8.7.40, 9.5.25, 10.4.14, 11.1.1. Upgrade to this version or later.
- Is CVE-2021-21355 exploitable, and should I be worried? Whether CVE-2021-21355 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
- What actually determines whether CVE-2021-21355 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.
- How do I fix CVE-2021-21355?
- Upgrade
typo3/cms-formto 8.7.40 or later - Upgrade
typo3/cms-formto 9.5.25 or later - Upgrade
typo3/cms-formto 10.4.14 or later - Upgrade
typo3/cms-formto 11.1.1 or later - Upgrade
typo3/cms-coreto 10.4.14 or later - Upgrade
typo3/cms-coreto 11.1.1 or later - Upgrade
typo3/cms-coreto 9.5.25 or later - Upgrade
typo3/cmsto 10.4.14 or later - Upgrade
typo3/cmsto 11.1.1 or later - Upgrade
typo3/cmsto 9.5.25 or later
- Upgrade