GHSA-f6x8-65q6-j9m9

## 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.