n8n-MCP: Multi-tenant MCP requests fall back to process-level n8n credentials when tenant headers are absent or incomplete

Summary

When ENABLE_MULTI_TENANT=true, the HTTP transport documents that the target n8n instance is selected per-request from x-n8n-url / x-n8n-key headers. Requests that omitted those headers — or supplied only one of them — silently fell back to the process-level N8N_API_URL / N8N_API_KEY credentials configured for the operator's own n8n instance. As a result, an authenticated MCP tenant could cause n8n management calls to execute against the operator's instance instead of its own.

This affects HTTP-mode deployments of n8n-mcp that are run as a shared multi-tenant service. Single-tenant deployments (ENABLE_MULTI_TENANT unset or false) are not affected.

Impact

An authenticated MCP tenant exploiting this path could read and write workflows, executions, data-table contents, and credential metadata on the operator's n8n instance. If the operator's n8n permits Code-node execution that reaches OS-level modules, the path could escalate to remote code execution inside the operator's n8n runtime. The process-level N8N_API_KEY is, in practice, a high-privilege key — Community Edition keys are unscoped by default, and even Enterprise scopes were configured for the operator's own needs and would carry over wholesale to a tenant who triggered the fallback.

Patches

Fixed in n8n-mcp 2.51.2. The fix:

• Rejects header-less multi-tenant requests at the HTTP edge with HTTP 400 / JSON-RPC -32602 before any handler runs.
• Refuses to construct an env-credential n8n API client when ENABLE_MULTI_TENANT=true.
• Closes secondary leak paths in trigger handlers and in the responses of n8n_health_check, n8n_diagnostic, n8n_deploy_template, and n8n_audit_instance so the operator's URL and env-key indicator are not surfaced to tenants.

Single-tenant behavior is unchanged.

Upgrade

# NPM
npx n8n-mcp@latest

# Docker
docker pull ghcr.io/czlonkowski/n8n-mcp:latest

Workarounds

If an immediate upgrade is not possible, any one of the following reduces or eliminates exposure:

• Disable multi-tenant mode. Set ENABLE_MULTI_TENANT=false (or unset it) and run a separate n8n-mcp instance per tenant with that tenant's own N8N_API_URL / N8N_API_KEY. This removes the affected code path entirely.
• Reject malformed requests at a proxy. Require both x-n8n-url and x-n8n-key headers on every request and return 400 if either is missing. Neutralizes the primary header-omission path but does not address the secondary response-shape disclosures, so this is a partial mitigation only.
• Reduce the blast radius of the operator API key. If your n8n instance supports API key scoping (Enterprise, or a Community Edition build that exposes scopes), provision the operator's N8N_API_KEY with the minimum scopes required for the operator's own n8n-mcp functions. This does not close the boundary break but limits what a falling-back tenant can do.

Credit

Reported by @u-ktdi.