RFC 9470 explained: The OAuth step-up authentication challenge protocol

RFC 9470, the OAuth 2.0 Step Up Authentication Challenge Protocol published in 2023, lets resource servers require stronger or more recent user authentication after evaluating the risk of a specific API request, rather than relying solely on authorization decisions made when a token was issued. A resource server returns a 401 response with the insufficient_user_authentication error and may include acr_values for requested authentication strength and max_age for authentication recency; the client then makes a new OpenID Connect authorization request using those values and retries with a new token. The specification emphasizes that max_age is enforceable because OpenID Connect requires active reauthentication when the allowed age is exceeded, while acr_values is advisory and must be verified through the returned acr claim because an authorization server may not meet the requested level. The auth_time claim records the last active user authentication and must not change during token refreshes, allowing APIs to assess true session freshness through JWT claims or token introspection. RFC 9470 intentionally leaves risk policy, session management, authentication-context semantics, token caching, and user experience to implementers, while warning that challenges can reveal sensitive policy information or be abused to trigger user prompts.

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