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OAuth 2.0 and OpenID Connect Overview

This article provides a high-level introduction to OAuth 2.0 and OpenID Connect (OIDC), which are the standard protocols that Okta's authentication and authorization solutions are based on. This article discusses how you can implement flows based on these standards using Okta, and what flows and grant types are commonly used by the different types of apps.

Note: See Okta deployment models — redirect vs. embedded for more information on the specific types of authentication deployment models that Okta provides that are built on top of OAuth 2.0 and OIDC.

OAuth 2.0 vs OpenID Connect

There are two main types of authentication that you can perform with Okta:

  • The OpenID Connect (OIDC) protocol is built on the OAuth 2.0 protocol and helps authenticate users and convey information about them. It's also more opinionated than plain OAuth 2.0, for example in its scope definitions.

  • The OAuth 2.0 protocol controls authorization to access a protected resource, like your web app, native app, or API service.

The OAuth 2.0 protocol provides API security through scoped access tokens. OAuth 2.0 enables you to delegate authorization, while OIDC enables you to retrieve and store authentication information about your end users. OIDC extends OAuth 2.0 by providing user authentication and single sign-on (SSO) functionality.

For most of your app auth requirements, we recommend that you use the OAuth 2.0 and OIDC protocols through the different solutions Okta provides, as outlined in Redirect authentication vs. embedded authentication. To get started with auth implementation and find sample apps, see Sign users in.

Note: If you require a completely custom app setup and workflow with direct access control to your Okta org and app integrations, then you can use the Authentication API. This API underpins both the Okta Redirect and Embedded Sign-In Widget, and Auth JS SDKs.

OAuth 2.0

OAuth 2.0 is a standard that apps use to provide client applications with access. If you would like to grant access to your application data in a secure way, then you want to use the OAuth 2.0 protocol.

The OAuth 2.0 spec has four important roles:

  • authorization server: The server that issues the access token. In this case Okta is the authorization server.

  • resource owner: Normally your application's end user that grants permission to access the resource server with an access token.

  • client: The application that requests the access token from Okta and then passes it to the resource server.

  • resource server: Accepts the access token and must verify that it's valid. In this case, this is your application.

Other important terms:

  • OAuth 2.0 grant: The authorization given (or granted) to the client by the user. Examples of grants are authorization code and client credentials. Each OAuth grant has a corresponding flow. See Choosing an OAuth 2.0 flow.
  • access token: The token issued by the authorization server (Okta) in exchange for the grant.
  • refresh token: An optional token that is exchanged for a new access token if the access token has expired.

Note: See Token lifetime for more information on hard-coded and configurable token lifetimes.

The usual OAuth 2.0 grant flow looks like this:

  1. Client requests authorization from the resource owner (usually the user).
  2. If the user gives authorization, the client passes the authorization grant to the authorization server (in this case Okta).
  3. If the grant is valid, the authorization server returns an access token, possibly alongside a refresh and/or ID token.
  4. The client now uses that access token to access the resource server.

Note: For a deeper dive into OAuth 2.0, see What the Heck is OAuth? over on the Okta Developer blog or checkout the OAuth 2.0 spec (opens new window).

At the core of both OAuth 2.0 and its OpenID Connect extension is the authorization server. An authorization server is simply an OAuth 2.0 token minting engine. Each authorization server has a unique issuer URI and its own signing key for tokens to keep a proper boundary between security domains. In the context of this guide, Okta is your authorization server.

The authorization server also acts as an OpenID Connect Provider, which means you can request ID tokens in addition to access tokens from the authorization server endpoints.

Note: For information on authorization servers, how they work, and how you can use them, see Authorization Servers.

OpenID Connect

OpenID Connect is an authentication standard built on top of OAuth 2.0. It adds an additional token called an ID token. OpenID Connect also standardizes areas that OAuth 2.0 leaves up to choice, such as scopes, endpoint discovery, and dynamic registration of clients. Okta is OpenID Certified (opens new window).

Although OpenID Connect is built on top of OAuth 2.0, the OpenID Connect specification (opens new window) uses slightly different terms for the roles in the flows:

  • OpenID provider: The authorization server that issues the ID token. In this case Okta is the OpenID provider.
  • end user: The end user's information that is contained in the ID token.
  • relying party: The client application that requests the ID token from Okta.
  • ID token: The token issued by the OpenID Provider and contains information about the end user in the form of claims.
  • claim: The claim is a piece of information about the end user.

The high-level flow looks the same for both OpenID Connect and regular OAuth 2.0 flows. The primary difference is that an OpenID Connect flow results in an ID token, in addition to any access or refresh tokens.

Choosing an OAuth 2.0 flow

Which OAuth flow that you use depends on your use case. The table below maps application types to our recommended OAuth 2.0 flows. If you'd like more information, keep reading for help with choosing an OAuth flow based on (1) the type of token that you need, and/or (2) the type of client application that you are building.

The table shows you which OAuth 2.0 flow to use for the type of application that you are building.

Type of Application OAuth 2.0 flow / grant type Access Token? ID Token?
Server-side (aka Web),

Single-Page Application,

or Native
Authorization Code with PKCE or Interaction Code (Identity Engine only).
Trusted Resource Owner Password
Service Client Credentials

Note: There is also an OAuth 2.0 SAML 2.0 Assertion flow, intended for a client app that wants to use an existing trust relationship without a direct user approval step at the authorization server. This supports access and ID tokens.

What kind of client are you building?

The type of OAuth 2.0 flow depends on what kind of client that you are building. This flowchart can quickly help you decide which flow to use.

flowchart/decision tree for choosing the correct OAuth 2.0 flow

Is your client public?

A client application is considered public when an end user could possibly view and modify the code. This includes Single-Page Apps (SPAs) or any mobile or native applications. In both cases, the application can't keep secrets from malicious users. Your client is considered confidential or private for server-side (web applications), which means your client can use client authentication such as a client secret.

Is your client a SPA or native?

If your client application is a SPA or a native application, you should use an authorization flow with PKCE, such as either the Interaction Code flow with PKCE or the Authorization Code flow with PKCE. If you are doing a redirect flow to an Okta-hosted sign-in page, the Authorization Code flow with PKCE is recommended. If you want to embed the sign-in experience, the Interaction Code flow is recommended.

Does the client have an end user?

If your client application is running on a server with no direct end user, then it can be trusted to handle credentials and use them responsibly. If your client application is only doing machine-to-machine interaction, then you should use the Client Credentials flow.

Is your app high-trust?

If you own both the client application and the resource that it's accessing, then your application can be trusted to handle your end user's username and password. These types of apps are considered "high-trust". Because of the high degree of trust required, you should only use the Resource Owner Password flow if other flows aren't viable.

If your app is not high-trust, you should use the Authorization Code flow.

Interaction Code flow

The Interaction Code flow is an extension to the OAuth 2 and OIDC standard, and is available when using Identity Engine orgs. It requires clients to pass a client ID, as well as a Proof Key for Code Exchange (PKCE), to keep the flow secure. The user can start the request with minimal information, relying on the client to facilitate the interactions with the Identity Engine component of the Okta Authorization Server to progressively authenticate the user. See Interaction Code grant type.

Interaction Code flow sequence diagram

Authorization Code flow with PKCE

Authorization Code flow with Proof Key for Code Exchange (PKCE) is the recommended flow for most applications whether server-side (web), native, or mobile. PKCE was originally designed to protect the authorization code flow in mobile apps, but its ability to prevent authorization code injection makes it useful for every type of OAuth client, even web apps that use a client secret. PKCE acts like a secret but isn't hard-coded, and keeps the Authorization Code flow secure.

PKCE is an extension to the regular Authorization Code flow, so the flow is very similar, except that PKCE elements are included at various steps in the flow.

Note: See Refresh access tokens for implementing refresh tokens with SPAs and other browser-based apps.

The PKCE-enhanced Authorization Code flow requires your application to generate a cryptographically random key called a "code verifier". A "code challenge" is then created from the verifier, and this challenge is passed along with the request for the authorization code.

When the authorization code is sent in the access token request, the code verifier is sent as part of the request. The authorization server recomputes the challenge from the verifier using an agreed-upon hash algorithm and then compares that. If the two code challenges and verifier match, then it knows that both requests were sent by the same client.

A rogue app could only intercept the authorization code, but it wouldn't have access to the code challenge or verifier, since they are both sent over HTTPS.

Flowchart that displays the back and forth between the resource owner, authorization server, and resource server for Auth Code flow with PKCE

For information on how to set up your application to use this flow, see Implement the Authorization Code flow with PKCE.

Resource Owner Password flow

The Resource Owner Password flow is intended for use cases where you control both the client application and the resource that it is interacting with. It requires that the client can store a client secret and can be trusted with the resource owner's credentials, and so is most commonly found in clients made for online services, like the Facebook client applications that interact with the Facebook service. It doesn't require redirects like the Authorization Code or Implicit flows, and involves a single authenticated call to the /token endpoint.

Flowchart that displays the back and forth between the resource owner, authorization server, and resource server for Resource Owner Password flow

For information on how to set up your application to use this flow, see Implement the Resource Owner Password flow.

Client Credentials flow

The Client Credentials flow is intended for server-side (AKA "confidential") client applications with no end user, which normally describes machine-to-machine communication. The application must be server-side because it must be trusted with the client secret, and since the credentials are hard-coded, it can't be used by an actual end user. It involves a single, authenticated request to the /token endpoint, which returns an access token.

Note: The Client Credentials flow doesn't support refresh tokens.

Flowchart that displays the back and forth between the resource owner, authorization server, and resource server for Client Credentials flow

For information on how to set up your application to use this flow, see Implement the Client Credentials flow.

SAML 2.0 Assertion flow

The SAML 2.0 Assertion flow is intended for a client app that wants to use an existing trust relationship without a direct user approval step at the authorization server. It enables a client application to obtain an authorization from a valid, signed SAML assertion from the SAML Identity Provider. The client app can then exchange it for an OAuth access token from the OAuth authorization server. For example, this flow is useful when you want to fetch data from APIs that only support delegated permissions without prompting the user for credentials.

To use a SAML 2.0 Assertion as an authorization grant, the client makes a SAML request to the Identity Provider and the Identity Provider sends the SAML 2.0 Assertion back in the response. The client then makes a request for an access token with the urn:ietf:params:oauth:grant-type:saml2-bearer grant type and includes the assertion parameter. The value of the assertion parameter is the SAML 2.0 assertion that is Base64 encoded. You can send only one SAML assertion in that request.

Flowchart that displays the back and forth between the resource owner, identity provider, authorization server, and resource server for the SAML 2.0 Assertion flow

For information on how to set up your application to use this flow, see Implement the SAML 2.0 Assertion flow.

Implicit flow

Note: The Implicit flow is a legacy flow used only for SPAs that can’t support PKCE.

The Implicit flow is intended for applications where the confidentiality of the client secret can't be guaranteed. In this flow, the client doesn't make a request to the /token endpoint, but instead receives the access token directly from the /authorize endpoint. The client must be capable of interacting with the resource owner's user agent and also capable of receiving incoming requests (through redirection) from the authorization server.

Note: Because it's intended for less-trusted clients, the Implicit flow doesn't support refresh tokens.

Important: For Single-Page Applications (SPA) running in modern browsers that support Web Crypto for PKCE, we recommend using the Authorization Code flow with PKCE instead of the Implicit flow for maximum security. If support for older browsers is required, the Implicit flow provides a working solution.

Flowchart that displays the back and forth between the resource owner, authorization server, and resource server for Implicit flow

For information on how to set up your application to use this flow, see Implement the Implicit flow.