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EUVD-2026-290268

fkie_cve-2026-39844

NiceGUI is a Python-based UI framework. Prior to 3.10.0, 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. This vulnerability is fixed in 3.10.0.

GHSA-w8wv-vfpc-hw2w — NiceGUI: Upload filename sanitization bypass via backslashes allows path traversal on Windows

Affected: PyPI: nicegui

### 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:

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

`PurePosixPath` treats backslashes as literal characters, not path separators:

```python >>> 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.

PYSEC-2026-2234

Affected: PyPI: nicegui