CVE-2026-54021: 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["<idx>"].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: <= 0.9.5
• Patched: >= 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.