n8n contains a cross-site scripting (XSS) vulnerability via the MCP OAuth client registration process. An unauthenticated attacker can register a malicious OAuth client with a crafted client_name. If a user authorizes this client and another user later revokes access, a toast notification renders the injected script, leading to arbitrary JavaScript execution in the victim's authenticated session. This can result in credential theft, session token compromise, workflow manipulation, or privilege escalation. The vulnerability is fixed in n8n version 1.123.32, 2.17.4, and 2.18.1. Temporary mitigations include restricting access to the MCP OAuth registration endpoint and disabling MCP server functionality if not needed.