Glossary

State Handle Hijacking

An attack where a server-side state identifier (a handle) is obtained or guessed by an attacker and used to access another user's state on an MCP server. Introduced as a named attack surface by MCP's 2026-07-28 statelessness redesign, which removed session-based identity and made handle continuity the server's explicit responsibility instead of the protocol's.

The NSA's May 2026 guidance flagged the underlying design gap directly: associating a session to an identity is not defined by the MCP protocol. A server can authenticate correctly at connection time and still be vulnerable, because that says nothing about whether every later request in the session is still verified as the same user.