FastMCP's authentication integration with Entra ID allows the MCP server to act both as an OAuth client and authorization server. Due to Entra ID's lack of Dynamic Client Registration support, the FastMCP server uses a static app registration for authorization. After a user consents to this static client, Entra ID sets a persistent authorization cookie. An attacker controlling their own MCP client can exploit this by initiating authorization flows that reuse the victim's consented static client ID, potentially leading to confused deputy account takeover.