HackerOne Disclosed Reports - 2025-12-16

Low
resolved

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

Bug reported by Nishant was disclosed at December 16, 2025, 9:47 am   |   Cross-site Scripting (XSS) - Stored

The vulnerability in the MCP Server Portal Apps was caused by missing sanitization of the redirect_uri parameter, leading to a second-order XSS vulnerability. An attacker could craft a malicious redirect_uri containing JavaScript code, obtain a client_id for this URI, and reuse it when a victim had an active session on the /authorize endpoint to execute arbitrary JavaScript.