n8n versions prior to 1.123.32, between 2.0.0 and before 2.17.4, and between 2.18.0 and before 2.18.1 contain an open redirect vulnerability in the MCP OAuth consent flow. The /mcp-oauth/register endpoint allows unauthenticated registration of OAuth clients with arbitrary redirect_uri values. When a user denies consent, they are redirected to the registered redirect_uri without validation, enabling attackers to redirect users to malicious sites. This vulnerability has been fixed in versions 1.123.32, 2.17.4, and 2.18.1. Temporary mitigations include restricting network access and limiting user access to trusted users.