NiceGUI: Upload filename sanitization bypass via backslashes allows path traversal on Windows

Summary

The upload filename sanitization introduced in GHSA-9ffm-fxg3-xrhh uses PurePosixPath(filename).name to strip path components. Since PurePosixPath only recognizes forward slashes (/) as path separators, an attacker can bypass this sanitization on Windows by using backslashes (\) in the upload filename.

Applications that construct file paths using file.name (a pattern demonstrated in NiceGUI's bundled examples) are vulnerable to arbitrary file write on Windows.

Details

The sanitization in nicegui/elements/upload_files.py uses:

filename = PurePosixPath(upload.filename or '').name

PurePosixPath treats backslashes as literal characters, not path separators:

>>> PurePosixPath('..\\..\\secret\\evil.txt').name
'..\\..\\secret\\evil.txt'  # Not stripped!

When this filename is used in a path operation on Windows (e.g., Path('uploads') / file.name), Windows Path interprets backslashes as directory separators, resolving the path outside the intended directory.

Impact

On Windows deployments of NiceGUI applications that use file.name in path construction:

• Arbitrary file write outside the intended upload directory
• Potential remote code execution through overwriting application files or placing executables in known locations
• Data integrity loss through overwriting existing files

Linux and macOS are not affected, as they treat backslashes as literal filename characters.