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Behaviour: Token Requests

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Grants

The implicit grant MUST NOT be used as it has been deprecated in Section 3.1.2. of the OAuth 2.0 Best Common Practice guidelines.

The password grant SHOULD NOT be used as it has been deprecated in Section 3.4. of the OAuth 2.0 Best Common Practice guidelines.

The Authorization Code grant MUST be supported by the Authorization Server, and SHOULD act as the primary grant flow. When used with Public Clients, this grant MUST be used alongside PKCE as outlined in RFC 7636, in-keeping with the recommendations given in 3.1.1. of the OAuth 2.0 Best Common Practice.

The Client Credentials grant MUST be supported by the Authorization Server. This grant type MUST only be used by confidential clients in machine to machine use cases. Authorization Servers SHOULD provide the means to restrict which scopes can be used with this grant type either during dynamic client registration or during token requests.

The Refresh Token grant MUST be supported in order to ease the refreshing of Access Tokens where the Authorization Code grant is used, and reduce the number of user authorization requests.

It is RECOMMENDED that Section 5 of RFC 6819, OAuth 2.0 Threat Model and Security Considerations, be followed as closely as possible to minimise chance of a security breach, such as the leaking or hijacking of client credentials, Authorization Codes or Refresh Tokens.

Authorization Request and Response

Found at the RFC 8414 authorization_endpoint, for example at /authorize

Successful authorization requests for the Authorization Code grant SHALL be provided by registered clients as defined in RFC 6749 Section 4.1.1.

The means by which the Authorization Server validates the end-user is beyond the scope of this specification, but may include the use of a login prompt and session cookies, or integration with a corporate Single-Sign-On (SSO) System.

A successful authorization response SHALL be serviced by the Authorization Server as defined in RFC 6749 Section 4.1.2. The state parameter SHOULD be used and is capable of providing per-request customization. Support for the PKCE code_challenge parameter MUST be implemented as outlined in RFC 7636 for public clients. A code_challenge_method parameter MUST be used alongside the code_challenge parameter and SHOULD be equal to “S256” unless the client is unable to perform a SHA256 hash, in which case a value of “plain” MAY be used. Authorization Servers MUST support both the “S256” and “plain” code_challenge_methods.

Access Token Request and Response

Found at the RFC 8414 token_endpoint, for example at /token

Access Token requests SHALL be provided by clients as defined in RFC 6749 Section 4 based on the OAuth 2.0 grant being used.

For Public Clients, the additional parameter of code_verifier is added to the request, as outlined in Section 4.5 of RFC 7636 to prevent authorization code hijacking.

Clients MUST include the scope parameter in their token requests, noting any scopes which they will require within the returned token.

Successful Access Token responses SHALL be serviced by the Authorization Server as defined in RFC 6749 Section 5.1. Additionally the expires_in and refresh_token fields MUST be included in the response, unless using the Client Credentials grant where the refresh_token SHOULD be omitted. The token_type field value MUST be treated as case-insensitive, in order to accommodate values such as “Bearer” and “bearer”.

Refresh Tokens

A Refresh Token is a string representing the authorization granted to the client by the resource owner. Refresh Tokens are issued to the client by the Authorization Server and are used to obtain a new Access Token when the current Access Token becomes invalid or expires, in accordance with Section 1.5 of RFC 6749. A Refresh Token MUST be at least 40 characters in length. The process of refreshing an Access Token is outlined in Section 6 of RFC 6749.

Authorization Servers MUST issue Refresh Tokens in the Bearer Token returned after the completion of an authorization flow. Each Refresh Token SHOULD be time-limited.

Refresh Tokens MUST be bound to a specific client, and this binding MUST be checked during token refresh.

Refresh Tokens SHOULD be rotated by the Authorization Server when clients make new token requests, as recommended by RFC 6819. Authorization Servers MAY continue to issue Refresh Tokens indefinitely without requiring users to re-authenticate, provided the Refresh Token is not explicitly revoked or has expired.

In the case of Browser-Based Clients, when issuing a rotated Refresh Token, the Refresh Token MUST be time-limited and the Authorization Server MUST NOT extend the lifetime of the new Refresh Token beyond the lifetime of the initial Refresh Token, as stated in Section 8 of OAuth 2.0 for Browser-Based Apps.

Token Revocation

Found at the RFC 8414 revocation_endpoint, for example at /revoke

Access Tokens and Refresh Tokens SHOULD be revoked by the Authorization Server as defined by RFC 7009.

OAuth 2.0 clients SHOULD check for the presence of a valid revocation_endpoint in the OAuth 2.0 Authorization Server Metadata and, if present, SHOULD revoke any valid refresh tokens using the revocation endpoint upon a user ceasing to require authorization. This may be, for example, due to logging out of an application, a change of user identity or uninstalling a given application.

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