CVE-2026-54414

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.