Understanding the OAuth2 implicit grant flow in Azure Active Directory (AD)
What is the OAuth2 implicit grant?
The quintessential OAuth2 authorization code grant is the authorization grant which uses two separate endpoints. The authorization endpoint is used for the user interaction phase, which results in an authorization code. The token endpoint is then used by the client for exchanging the code for an access token, and often a refresh token as well. Web applications are required to present their own application credentials to the token endpoint, so that the authorization server can authenticate the client.
An important characteristic of the OAuth2 implicit grant is the fact that such flows never return refresh tokens to the client. As we will see in the next section, that isn’t really necessary and would in fact be a security issue.
Suitable scenarios for the OAuth2 implicit grant
- Tokens can be reliably obtained without any need for cross origin calls – mandatory registration of the redirect URI to which tokens are return guarantees that tokens are not displaced
- HTML5 features like session or local storage grant full control over token caching and lifetime management, whereas cookies management is opaque to the app
- Access tokens aren’t susceptible to Cross-site request forgery (CSRF) attacks
The implicit grant flow does not issue refresh tokens, mostly for security reasons. A refresh token isn’t as narrowly scoped as access tokens, granting far more power hence inflicting far more damage in case it is leaked out. In the implicit flow, tokens are delivered in the URL, hence the risk of interception is higher than in the authorization code grant.
Is the implicit grant suitable for my app?
If your application is a native client, the implicit flow isn’t a great fit. The absence of the Azure AD session cookie in the context of a native client deprives your application from the means of maintaining a long lived session. Which means your application will repeatedly prompt the user when obtaining access tokens for new resources.
If you are developing a Web application which includes a backend, and consuming an API from its backend code, the implicit flow is also not a good fit. Other grants give you far more power. For example, the OAuth2 client credentials grant provides the ability to obtain tokens that reflect the permissions assigned to the application itself, as opposed to user delegations. This means the client has the ability to maintain programmatic access to resources even when a user is not actively engaged in a session, and so on. Not only that, but such grants give higher security guarantees. For instance, access tokens never transit through the user browser, they don’t risk being saved in the browser history, and so on. The client application can also perform strong authentication when requesting a token.
- For a complete list of developer resources, including reference information for the protocols and OAuth2 authorization grant flows support by Azure AD, refer to the Azure AD Developer's Guide
- See How to integrate an application with Azure AD for additional depth on the application integration process.