Second-Order XSS via javascript protocol in MCP Server Portal Apps leads to ATO

Summary

Missing sanitization of the redirect_uri parameter led to a XSS vulnerability in MCP server portals. An attacker can craft a malicious redirect_uri containing JavaScript code, obtain a client_id for this malicious URI, and then reuse it when a victim has an active session (CF_Authorization cookie exists) on the /authorize endpoint to execute arbitrary JavaScript. A detailed writeup by the reporter can be found here. We recommend upgrading workers-oauth-provider to the latest version and following the best practices in Securing MCP Servers .

My side of the story: https://kazama.in/mcp-server-portal-xss-to-ato