Open WebUI: Authenticated users can bypass model access control via exposed query parameter [AI-ASSISTED]

### Summary An internal-only bypass_filter parameter is exposed on the /openai/chat/completions and /ollama/api/chat HTTP endpoints via FastAPI query string binding, allowing any authenticated user to append ?bypass_filter=true and bypass model access control checks to invoke admin-restricted models. ### Details The `generate_chat_completion` route handlers in both `routers/openai.py` and `routers/ollama.py` declare `bypass_filter` as a function parameter: **`routers/openai.py`, line 937–941:** ```python @router.post("/chat/completions") async def generate_chat_completion( request: Request, form_data: dict, user=Depends(get_verified_user), bypass_filter: Optional[bool] = False, ... ): ``` **`routers/ollama.py`, line 1283–1288:** ```python @router.post("/api/chat") async def generate_chat_completion( ... bypass_filter: Optional[bool] = False, ... ): ``` Because FastAPI automatically binds unrecognized function parameters to the query string, any HTTP client can set this value by appending `?bypass_filter=true` to the request URL. When `bypass_filter` is true, the access control check is skipped entirely: **`routers/openai.py`, line 980:** ```python if not bypass_filter and user.role == "user": # ACL check , skipped when bypass_filter is True ``` This parameter is intended for internal use only , the server-side chat pipeline in `utils/chat.py` (lines 238, 253) passes `bypass_filter=True` as a Python function argument when making recursive calls to base models that have already been authorized. However, because it appears in the HTTP handler's signature, it is unintentionally exposed to external callers. This is separate from the `BYPASS_MODEL_ACCESS_CONTROL` environment variable, which is a deliberate admin setting for trusted environments. ### PoC ```python #!/usr/bin/env python3 """ uv run --no-project --with requests finding_02_bypass_filter_acl_bypass.py [--base-url http://localhost:8089] Finding #2 , Unauthorized model access via bypass_filter query parameter SUMMARY: The POST /openai/chat/completions and POST /ollama/api/chat endpoints expose a bypass_filter query parameter as part of their FastAPI function signatures. FastAPI automatically binds this to the query string. When an authenticated user appends ?bypass_filter=true, the access control check is skipped: if not bypass_filter and user.role == "user": check_model_access(user, model) # <-- skipped when bypass_filter=True This allows any authenticated user to invoke models they are not authorized to use, including admin-restricted models. VULNERABLE CODE: backend/open_webui/routers/openai.py, line 941 + 980: async def generate_chat_completion(..., bypass_filter: Optional[bool] = False, ...): ... if not bypass_filter and user.role == "user": # ACL check , skipped when bypass_filter=True backend/open_webui/routers/ollama.py, line 1288 + 1339: async def generate_chat_completion(..., bypass_filter: Optional[bool] = False, ...): ... if not bypass_filter and user.role == "user": # ACL check , skipped when bypass_filter=True IMPACT: Any authenticated user can bypass model access control on both OpenAI and Ollama proxy endpoints. Because bypass_filter skips the ACL check but still routes through the server-side LLM connection, the attacker can invoke admin-restricted models using the server's API keys and receive actual LLM responses , effectively gaining free, unauthorized access to any configured model. REPRODUCTION: 1. Create a restricted model with empty access_grants (admin-only). 2. Authenticate as a regular user. 3. POST /openai/chat/completions with the restricted model → expect 403. 4. POST /openai/chat/completions?bypass_filter=true → request succeeds. REQUIREMENTS: - Running Open WebUI instance with Ollama or OpenAI backend configured - A model with restricted access_grants - An au

CVSS 3.1

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

Severity from

GitHub (reviewed advisory)

Weakness

CWE-285

Also known as

CVE-2026-45365, PYSEC-2026-2758

• github.com/open-webui/open-webui/security/advisories/GHSA-v6qf-75pr-p96m
• nvd.nist.gov/vuln/detail/CVE-2026-45365
• github.com/open-webui/open-webui/commit/c0385f60ba049da48d2d5452068586d375303c37
• github.com/open-webui/open-webui
• github.com/open-webui/open-webui/releases/tag/v0.8.11