The Dangers of Token Passthrough
Token passthrough occurs when an application forwards an ambient user authorization token directly to an untrusted MCP server. This allows malicious or compromised MCP servers to use that token against other downstream enterprise services, leading to privilege escalation.
Mandatory Resource Indicators and Token Exchange
Under the official MCP 2026-07-28 authorization specification, clients must supply RFC 8707 Resource Indicators and utilize RFC 8693 Token Exchange to generate audience-bound, scoped tokens explicitly for the target MCP server.