Browser Session Establishment Using OAuth 2.0 Token Exchange and Short-Lived Authorization Codes

This document specifies a usage profile that composes OAuth 2.0 Token Exchange [RFC8693] with a short-lived, single-use authorization code to establish an authenticated browser session at a Relying Party (RP) on behalf of a user authenticated at an Identity Provider (IdP) operating an independent OAuth 2.0 Security Token Service (STS). The server-to-server leg uses RFC 8693 to convey user identity and authorization context to the RP's STS, which issues an RP-scoped access token. A short-lived opaque code is then used to mediate delivery of session state into the user's browser without ever exposing the access token on the front channel. The profile is designed to avoid the class of front-channel token leakage (browser history, HTTP Referer header, intermediary access logs) that motivated the deprecation of the OAuth 2.0 implicit grant in [RFC9700]. The profile also defines a minimum claims contract on the RP-issued access token to enable stateless authorization enforcement at the RP without synchronous callbacks to the IdP.