TLS-Session-Bound Access Tokens for OAuth 2.0

This document defines a mechanism for binding OAuth 2.0 access tokens to a specific mutual TLS (mTLS) connection. The binding is achieved through a proof token that incorporates the TLS Exporter value [RFC5705] derived from the current connection and an access token hash, signed by the client's private key corresponding to its mTLS certificate. This mechanism prevents stolen bearer tokens from being replayed on a different TLS connection. The proof is constructed once per (token, connection) pair and reused across all requests on that connection, delivering session binding with no per-request signing overhead and no additional key management beyond what mTLS already provides. The mechanism is applicable to TLS 1.2, TLS 1.3, and QUIC transports. While applicable to any OAuth 2.0 access token presented over mTLS, this specification is primarily motivated by the Token Exchange protocol [RFC8693], where multi-hop delegation chains in autonomous, agent-driven architectures create elevated replay risk.