CVE-2021-21355
HIGHCVE-2021-21355 is a high-severity (CVSS 8.6) Unrestricted File Upload vulnerability in typo3/cms-form. O3 Security confirms whether CVE-2021-21355 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Unrestricted File Upload in Form Framework
Blast Radius
typo3/cms-form🐘typo3/cms-form🐘typo3/cms-form🐘typo3/cms-form🐘typo3/cms-core🐘typo3/cms-core🐘typo3/cms-core🐘typo3/cms+2 moreReal-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
TYPO3 is an open source PHP based web content management system. In TYPO3 before versions 8.7.40, 9.5.25, 10.4.14, 11.1.1, 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. Besides that, UploadedFileReferenceConverter transforming uploaded files into proper FileReference domain model objects handles possible file uploads for other extensions as well - given those extensions use the Extbase MVC framework, make use of FileReference items in their direct or inherited domain model definitions and did not implement their own type converter. In case this scenario applies, UploadedFileReferenceConverter accepts any file mime-type and persists files in 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. This is fixed in versions 8.7.40, 9.5.25, 10.4.14, 11.1.1.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | typo3/cms-form | ≥ 8.0.0&&< 8.7.40 | 8.7.40 |
| 🐘Packagist | typo3/cms-form | ≥ 9.0.0&&< 9.5.25 | 9.5.25 |
| 🐘Packagist | typo3/cms-form | ≥ 10.0.0&&< 10.4.14 | 10.4.14 |
| 🐘Packagist | typo3/cms-form | ≥ 11.0.0&&< 11.1.1 | 11.1.1 |
| 🐘Packagist | typo3/cms-core | ≥ 10.0.0&&< 10.4.14 | 10.4.14 |
| 🐘Packagist | typo3/cms-core | ≥ 11.0.0&&< 11.1.1 | 11.1.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for typo3/cms-form. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update typo3/cms-form to 8.7.40 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21355 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2021-21355 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-21355. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-21355 in your dependencies?
O3 detects CVE-2021-21355 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.