n8n has Open Redirect in MCP OAuth Consent Flow

Impact

The /mcp-oauth/register endpoint accepted OAuth client registrations without authentication, allowing arbitrary redirect_uri values to be registered. When a user denies the MCP OAuth consent dialog, the handleDeny handler redirects the user to the registered redirect_uri without validation, enabling an open redirect to an attacker-controlled URL. An attacker can craft a phishing link and send it to a victim; if the victim clicks "Deny" on the consent page, they are silently redirected to an external site.

Patches

The issue has been fixed in n8n versions 1.123.32, 2.17.4, and 2.18.1. Users should upgrade to one of these versions or later to remediate the vulnerability.

Workarounds

If upgrading is not immediately possible, administrators should consider the following temporary mitigations:

• Restrict network access to the n8n instance to prevent untrusted users from reaching the MCP OAuth endpoints.
• Limit access to the n8n instance to fully trusted users only.

These workarounds do not fully remediate the risk and should only be used as short-term mitigation measures.