Open WebUI's Ollama Model Access Control Bypass via /api/generate, /api/embed, /api/embeddings, and /api/show

Ollama Model Access Control Bypass via /api/generate, /api/embed, /api/embeddings, and /api/show

Affected Component

Ollama proxy endpoints missing model access control:

• backend/open_webui/routers/ollama.py (lines 955-995, generate_completion)
• backend/open_webui/routers/ollama.py (lines 835-881, embed)
• backend/open_webui/routers/ollama.py (lines 891-937, embeddings)
• backend/open_webui/routers/ollama.py (lines 791-820, show_model_info)

Affected Versions

Current main branch (commit 6fdd19bf1) and likely all versions with Ollama model access control support.

Description

Four Ollama proxy endpoints accept any model name from the user and forward the request to the Ollama backend without checking whether the user is authorized to access that model. These endpoints only require get_verified_user (any authenticated non-pending user) and validate that the model exists in the full unfiltered model list, but never check AccessGrants.has_access().

This is in direct contrast with the /ollama/api/chat endpoint (line 1101-1122) which correctly validates model access grants and returns 403 for unauthorized users:

# /api/chat (line 1101-1122) — CORRECTLY checks access
if not bypass_filter and user.role == 'user':
    user_group_ids = {group.id for group in Groups.get_groups_by_member_id(user.id)}
    if not (
        user.id == model_info.user_id
        or AccessGrants.has_access(
            user_id=user.id, resource_type='model',
            resource_id=model_info.id, permission='read',
            user_group_ids=user_group_ids,
        )
    ):
        raise HTTPException(status_code=403, detail='Model not found')

# /api/generate (line 955-995) — NO access check at all
# /api/embed (line 835-881) — NO access check at all
# /api/embeddings (line 891-937) — NO access check at all
# /api/show (line 791-820) — NO access check at all

CVSS 3.1 Breakdown

Attack Scenario

1. Admin configures model access control, restricting llama3:70b to the "ML Engineers" group. Regular user Alice is only authorized for llama3:8b.
2. Alice knows the restricted model name (model names are predictable — llama3:70b, mistral:latest, etc.).
3. Alice calls the unprotected endpoints directly:
   bash

   # Run completions on restricted model
   curl -X POST /ollama/api/generate \
     -H "Authorization: Bearer <alice_token>" \
     -d '{"model": "llama3:70b", "prompt": "..."}'
   
   # View restricted model details and system prompt
   curl -X POST /ollama/api/show \
     -H "Authorization: Bearer <alice_token>" \
     -d '{"model": "llama3:70b"}'
   
   # Generate embeddings with restricted model
   curl -X POST /ollama/api/embed \
     -H "Authorization: Bearer <alice_token>" \
     -d '{"model": "llama3:70b", "input": "..."}'

4. All requests succeed and are proxied to Ollama without any access control check.

Impact

• Model access control is silently ineffective for four out of five Ollama proxy endpoints
• Unauthorized users can consume GPU/compute resources on restricted models (cost and capacity impact in multi-user deployments)
• /api/show exposes restricted model configurations including system prompts, parameters, templates, and license information
• Admins have a false sense of security — access restrictions appear to work via the main chat interface but are trivially bypassed via direct API calls

Preconditions

• Ollama must be configured as a backend
• Admin must have configured model access control (not using BYPASS_MODEL_ACCESS_CONTROL=true)
• Attacker must know the restricted model name (model names follow predictable conventions)