PAR
Pushed Authorization Requests (RFC 9126). The client sends authorization parameters directly to the server over a back channel first, hardening the flow against tampering. Required by stricter profiles like FAPI.
PAR removes the front channel as a tampering surface: instead of packing scope, redirect URI, and request parameters into a browser URL where they can be modified or logged, the client registers them server to server and passes an opaque reference. It also gets around URL length limits for rich authorization requests. Mandatory in FAPI, and a sensible default for anything high value.
See also: OAuth 2.0, FAPI, authorization code flow, PKCE
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