CVE-2026-55255: Langflow: IDOR Vulnerability in `/api/v1/responses` Endpoint Allows Authenticated Attackers to Access Another User's Flow

Summary

Insecure Direct Object Reference (IDOR) vulnerability in /api/v1/responses endpoint allows an authenticated attacker to execute any flow belonging to another user by specifying the victim's flow ID in the request.

Details

The vulnerability exists in the get_flow_by_id_or_endpoint_name helper function in `src/backend/base/langflow/helpers/flow.py` (lines 399-414).

When a flow is accessed via UUID (flow_id), the function queries the database directly without verifying if the authenticated user owns that flow:

# src/backend/base/langflow/helpers/flow.py:399-414
async def get_flow_by_id_or_endpoint_name(flow_id_or_name: str, user_id: str | UUID | None = None) -> FlowRead:
    async with session_scope() as session:
        try:
            flow_id = UUID(flow_id_or_name)
            # When using UUID, query directly WITHOUT checking user_id
            flow = await session.get(Flow, flow_id)  # ❌ No user_id check!
        except ValueError:
            endpoint_name = flow_id_or_name
            stmt = select(Flow).where(Flow.endpoint_name == endpoint_name)
            # Only when using endpoint_name is user_id checked
            if user_id:
                stmt = stmt.where(Flow.user_id == uuid_user_id)

This function is used by the /api/v1/responses endpoint (defined in `src/backend/base/langflow/api/v1/openai_responses.py:589`).

PoC (Proof of Concept)

# Attacker (user A) with API_KEY_A tries to execute victim (user B)'s flow
curl -X POST "http://localhost:7860/api/v1/responses" \
  -H "x-api-key: sk-ATTACKER_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "VICTIM_FLOW_ID",
    "input_value": "test",
    "stream": false
  }'
# Returns 200 and executes the victim's flow

Impact

Any authenticated user can:

1. Execute any flow in the system by knowing its flow ID
2. Access potentially sensitive data processed by victim's flows
3. Consume victim's resources

Fixes

Fixed in PR #12832 (fix(security): close IDOR in get_flow_by_id_or_endpoint_name), merged 2026-04-22, released in Langflow 1.9.1.

The helper normalizes user_id once and enforces ownership on both lookup branches (UUID *and* endpoint_name):

flow_id = UUID(flow_id_or_name)
flow = await session.get(Flow, flow_id)
if flow is not None and uuid_user_id is not None and flow.user_id != uuid_user_id:
    flow = None  # cross-user lookup falls through to the shared 404

Key points:

• Cross-user lookups return 404 (not 403), so flow existence is not disclosed via a 403-vs-404 oracle.
• /api/v1/responses and /api/v2/workflow pass user_id explicitly, so fixing the helper closes them directly; the /api/v1/run* routes were additionally moved from a bare Depends(get_flow_by_id_or_endpoint_name) to auth-aware wrapper dependencies (defense in depth).
• A malformed user_id now fails closed (404 instead of a raw 500).
• Webhook routes intentionally keep the unscoped lookup (public by design / explicit ownership check elsewhere).
• Regression tests cover the cross-user UUID case and reproduce the original PoC against /api/v1/responses.

Acknowledgements

Thanks to the security researchers who responsibly disclosed this vulnerability:

• @yzeirnials
• @johnatzeropath
• @LeftenantZero
• @Zwique