MCP Auth Bypasses Show Why Tool Calls Need Runtime Authorization

Recent incidents involving Model Context Protocol (MCP) highlight the critical need for runtime authorization beyond initial authentication to prevent unauthorized tool access and potential misuse. These issues underscore the dangers of relying solely on identity verification at the periphery, as demonstrated by two incidents: fast-mcp-telegram's token handling vulnerability allowing path traversal and LiteLLM's exploit involving test routes leading to remote code execution. The core problem lies in the conflation of authentication, which verifies identity, and authorization, which determines access rights to specific actions and resources. Effective security requires a fail-closed authorization model that classifies routes into different risk tiers, enforces per-call runtime checks, and logs both allow and deny outcomes for comprehensive auditability. An MCP Gateway can centralize policy enforcement, ensuring that each tool execution decision is based on current context and policy, thus closing the gap left by traditional perimeter security and enhancing overall system reliability.

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