TYPO3 CMS - Unrestricted File Upload in Form Framework

Problem

Users were able to upload files with arbitrary MIME types to forms using FileUpload or ImageUpload elements with allowedMimeTypes configured - uploading PHP files was not possible. The restriction was not enforced server-side because the MimeTypeValidator was registered during form building before concrete form definition properties were applied, resulting in the validator never being added to the processing pipeline.

Solution

Update to TYPO3 version 14.3.5 LTS that fixes the problem described.

Credits

Thanks to Sébastien Convers for reporting this issue, and to Josua Vogel and Oliver Hader for fixing it.