Description

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

Impact

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:

uploaded[avatar]|is_image[avatar]|mime_in[avatar,image/gif]|ext_in[avatar,gif]

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 ext_in 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.

Patches

Upgrade to v4.7.3 or later.

Workarounds

Save uploads outside the public web root, preferably under writable/uploads

Use $file->store() or $file->move($path, $file->getRandomName()) instead of preserving the original filename

Disable script execution in any public upload directory

Manually verify the client filename extension before moving the file

Reject files when $file->getClientExtension() is not in the allowed list or does not match $file->guessExtension()

Resources

CodeIgniter4 uploaded files documentation

CodeIgniter4 file upload validation documentation

CWE-434: Unrestricted Upload of File with Dangerous Type

OWASP File Upload Cheat Sheet

Mitigation

Aliases

References