FileRise before 3.16.0 is vulnerable to path traversal in the shared-folder upload endpoint (/api/folder/uploadToSharedFolder.php), leading to arbitrary file write and administrator account takeover. The upload filename is validated by FolderController with basename() and REGEX_FILE_NAME, which permit URL-encoded sequences (the regex blocks / and \ but not %). The raw filename is then passed to UploadModel::handleUpload, where it is reconstructed as trim(urldecode(basename($fileName))), re-introducing path separators after validation (e.g. ..%2fusers%2fusers.txt becomes ../users/users.txt).

UploadNamePolicy::isAllowedForWrite() applies basename() internally and therefore only evaluates the final component (users.txt), allowing the traversal sequence to pass the extension policy. The destination path is then used directly in move_uploaded_file() with no realpath containment check, allowing a write outside the intended upload directory. An attacker who possesses a valid, non-expired, upload-enabled shared-folder link/token (which are designed to be shared publicly) can overwrite users/users.txt to create an administrator account, resulting in unauthenticated admin takeover and, depending on configuration, remote code execution.

Exploitation requires possession of a valid, non-expired, upload-enabled shared-folder link/token. This issue is fixed in 3.16.0, which URL-decodes before validation and rejects any path separators in the upload filename.