Choosing the OAuth2 grant flow

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Premier Dev Consultant Marius Rochon explores OAuth2 questions you need to ask and how the answers lead to the selection of the grant.


The OAuth2 specifications define six different grant types (https://tools.ietf.org/html/rfc6749 and https://tools.ietf.org/html/draft-ietf-oauth-device-flow-15). Each provides the most optimal (from the security point of view) way of obtaining access or (for OIDC) id_tokens given the circumstances of the client application. This blog summarizes the questions that the implementer of the OAuth2 client application needs to ask and how the answers lead to the selection of the appropriate grant.

There are two main questions and several variants on answers to them:

  1. Is there a user involved in the transaction or is this an un-attended client (e.g. a nightly batch run)? If a user is involved either directly interacting with the client or interacting with some other client, which called this application go to #2 below. Otherwise, the only applicable grant is the Client Credentials flow (see exception below). Note that the client app must be a confidential client (see https://tools.ietf.org/html/rfc6749#section-2.1).
  2. If a user is involved, the question to ask is whether the authorization server can use the browser interface to authenticate the user and obtain consent, if needed. There are several possible answers:

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