[Medium] GHSA-9rpj-v7hf-vv2w Open WebUI: Authenticated users can target arbitrary configured Ollama backends via unguarded url_idx path parameter

Summary

Several direct, index-addressed Ollama proxy routes accept a caller-supplied url_idx path parameter and use it as a raw index into the admin-configured OLLAMA_BASE_URLS list. Access control on these routes validates only whether the user may use the requested model, never which backend the request is routed to. Any authenticated user can append an arbitrary url_idx to force their request onto an Ollama backend they were never authorized to reach, including internal, higher-privilege, or explicitly admin-disabled backends.

Affected endpoints

All indexed Ollama routes that resolve the backend through get_ollama_url():

POST /ollama/api/chat/{url_idx}
POST /ollama/api/generate/{url_idx}
POST /ollama/api/embed/{url_idx}
POST /ollama/api/embeddings/{url_idx}
POST /ollama/v1/chat/completions/{url_idx}
POST /ollama/v1/completions/{url_idx}
POST /ollama/v1/messages/{url_idx}
POST /ollama/v1/responses/{url_idx}

Root cause

backend/open_webui/routers/ollama.py — get_ollama_url() consults the model-to-backend allow-list (OLLAMA_MODELS[model]["urls"]) only when url_idx is omitted. When the caller supplies url_idx, that mapping is skipped and the value is used directly as an index:

async def get_ollama_url(request: Request, model: str, url_idx: Optional[int] = None):
    if url_idx is None:
        models = request.app.state.OLLAMA_MODELS
        if model not in models:
            raise HTTPException(...)
        url_idx = random.choice(models[model].get("urls", []))
    url = request.app.state.config.OLLAMA_BASE_URLS[url_idx]   # caller-controlled, no authz
    return url, url_idx

The outbound request is then sent to that backend using the backend's own configured API key. Backends an admin has disabled (OLLAMA_API_CONFIGS[""].enable = false) are hidden from model discovery but remain reachable through the indexed route, because the disabled state is never re-checked at request time.

Impact

A verified, non-admin user with read access to any single model can:

route requests to internal / higher-capability / restricted Ollama backends in multi-backend deployments, bypassing backend-level isolation;
reach backends the admin has explicitly disabled;
have those requests authenticated with the target backend's configured API key (the key is used server-side; it is not returned to the attacker);
consume the restricted backend's compute.

There is no cross-user data disclosure and no exfiltration of the backend credential itself; the impact is unauthorized access to, and use of, restricted backend resources.

Affected / Patched

Affected: = 0.9.6

Fix

0.9.6 adds validate_ollama_backend_idx(), invoked on every indexed route (directly and via get_ollama_url()), which returns 403 for any non-admin caller-supplied url_idx that is not in the requested model's allowed urls. Because disabled backends are absent from every model's urls, the same check also blocks routing to disabled backends.