CodeIgniter4 has a validation bypass when uploading file extensions via `ext_in` rule

QFinch assessment

The `ext_in` upload validation rule checked the MIME-derived guessed extension instead of the client-provided filename extension. As a result, an uploaded file named `shell.php` containing GIF-like content could pass validation such as:

Potential impact

The extin upload validation rule checked the MIME-derived guessed extension instead of the client-provided filename extension. As a result, an uploaded file named shell.php containing GIF-like content could pass validation such as: because the detected MIME type maps to gif, even though the uploaded filename extension is php. Applications are impacted if they: - accept user-controlled uploads, - rely on extin to validate the uploaded filename extension, - save uploaded files using the original client filename: $file->move($path), - store uploads in a web-accessible directory, - and allow PHP or other executable files to run from that directory. In those conditions, this may lead to arbitrary code execution. The default application does not expose such an upload endpoint.