Unrestricted File Upload in Form Framework

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.

Solution

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/<random-40-bit>. 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.

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

• TYPO3-CORE-SA-2021-002