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 / channels containing the original upload
- knowledge base members can see ids of files contributed by others
- a user who can read a folder index sees the file ids of files inside
- chat history exports (
/api/v1/chats/{id}) include file ids - the user themselves can be tricked into pasting / sharing an id (less likely)
impact
Any authenticated user can read any other user's file content (image and any file with an image-guess mimetype path) via this channel. Severity is bounded by what the LLM will accept in image_url -- in practice, image files work cleanly with any vision model; pdf / docx work with multi-modal providers that accept them.
suggested fix
Thread the authenticated user through to get_image_base64_from_url and resolve the file via Files.get_file_by_id_and_user_id(id, user.id) (or has_access_to_file(id, 'read', user, db) if shared-via-knowledge-base access is intended). Same pat
