Understanding Oauth 2.0 And Azure Ad App Registration For Sharepoint Online Rest Apis

What is OAuth 2.0 and Why is it Needed for SharePoint Online REST APIs?

OAuth 2.0 is an open standard authorization framework that enables applications to obtain limited access to user accounts on an HTTP service, such as Facebook, Google, Microsoft Accounts, Twitter APIs, and SharePoint Online. It works by delegating user authentication to the service that hosts the user account, and authorizing third-party applications to access the user account.

SharePoint Online exposes REST APIs that allow applications to access and manipulate SharePoint data programmatically. However, these REST APIs require OAuth 2.0 authentication to verify the identity of the calling application. Without OAuth 2.0, the SharePoint Online REST APIs cannot determine if the application has the appropriate permissions to access the requested data.

OAuth provides a secure authorization flow for third-party applications to access SharePoint Online data on behalf of a user. Instead of directly handling user credentials, applications can use OAuth access tokens to access SharePoint Online REST APIs with the permissions granted by the Microsoft identity platform and Azure AD. This preserves the integrity and security of both the user’s identity and the SharePoint Online environment.

Registering an Azure AD Application for SharePoint Online REST API Access

To call SharePoint Online REST APIs from an application, you must first register the application in Azure Active Directory to integrate with the Microsoft identity platform. This creates a trust relationship and grants your application permission to access authorized data in the SharePoint Online environment.

Here are the key steps to register an Azure AD application:

1. Log into the Azure portal and navigate to the Azure Active Directory section. Click on App Registrations to view applications.
2. Click on +New Registration to begin registering your application.
3. Provide an application name, choose account type as “Accounts in any organizational directory”, and specify a Redirect URI.
4. Once the application is created, navigate to the API Permissions section and click +Add permission. Choose Microsoft APIs, select SharePoint, and add delegated permissions like Sites.ReadWrite.All.
5. Ensure to grant admin consent for the configured permissions.

When registering an Azure AD application, developers must understand the differences between application permissions and delegated permissions. Application permissions allow the app to act as a stand-alone entity to call SharePoint Online REST APIs directly. Delegated permissions require a user to authenticate in order for the application to inherit their authorization to call permitted APIs. Choose permissions based on your specific use case.

Configuring a Client Application for OAuth 2.0 Authentication Flow

Once an Azure AD application is registered with appropriate SharePoint Online permissions, client applications can programmatically initiate an OAuth 2.0 authentication flow to acquire access tokens to call SharePoint REST APIs. Here is the typical OAuth authorization code grant flow:

1. Client app redirects user to the Azure login page and prompts to login.
2. User authenticates with Azure AD credentials and consents to granting application access.
3. Azure AD authenticates the user, validates permissions, and returns an authorization code to the client app.
4. Client app exchanges authorization code for an access token and a refresh token.
5. Client app uses access token in API calls to access SharePoint Online data.

To implement OAuth authentication flow in your application, you will need the application’s client ID, client secret, and Azure AD tenant ID from the registered app page in the Azure portal:

• Client ID identifies the registered AAD application
• Client Secret is an authentication key used to acquire tokens
• Tenant ID indicates the directory that hosts the AAD app registration

With these values, you can make HTTP requests to the Microsoft identity platform token endpoint to obtain access tokens for calling SharePoint REST APIs.

Calling SharePoint REST APIs with an Access Token

After acquiring an OAuth access token during application authentication, SharePoint REST API calls can be made by passing the access token in HTTP request headers. The access token string is base 64 encoded JSON web token (JWT) containing claims that indicate:

• User/application ID that token is issued to
• Target SharePoint sites and permissions authorized
• Token lifetime and expiration timestamp

API requests pass the access token in the HTTP Authorization header using Bearer scheme. For example, to call the SharePoint REST API to retrieve site lists using Postman:

1. Construct API URL: GET https://contoso.sharepoint.com/sites/team/_api/web/lists
2. Go to Authorization tab in Postman and select Bearer Token
3. Paste access token string into the Token field
4. Click Send. Response lists SharePoint lists in the site.

If an access token is expired or invalid, the REST API will return a 401 unauthorized error. The application should implement refresh token logic to obtain a new access token.

Refreshing an Expired Access Token

OAuth access tokens are short-lived credentials that expire after a configured timeframe. Once expired, access tokens cannot be used to make API calls. The typical expiration time is one hour.

During the initial OAuth authentication flow, applications also receive a refresh token that can be used to acquire new access tokens after expiration:

1. Client application makes authentication request with refresh token
2. Azure AD validates the refresh token and client app
3. If valid, Azure AD returns a new access token to the app
4. Client app continues making API calls with new access token

Refresh tokens have longer lifetimes compared to access tokens. Implementing proper refresh logic enables seamless authenticated access without re-prompting the user to login after each access token expiration.

Common Issues and Troubleshooting

When working with OAuth-protected SharePoint Online REST APIs, developers may encounter errors like:

• 401 Unauthorized due to invalid or expired tokens
• 403 Forbidden if scope doesn’t include requisite permissions
• 500 errors if user hasn’t consented to permissions

Here are some tips for troubleshooting OAuth-related issues:

• Ensure appropriate API permissions are configured in app registration
• Check for errors in token request calls and parameters
• Print and decode access token to confirm contents
• Enable logging to diagnose request failures

Testing OAuth authentication flows early in development is vital for smooth access to SharePoint Online REST APIs from your application.

Summary

OAuth 2.0 provides a secure delegation mechanism for gaining access to SharePoint Online REST APIs without directly handling user credentials. By registering Azure AD apps and configuring access permissions, applications can implement authentication flows using OAuth to acquire time-bound access tokens.

Handling token issuance, expiration, and refresh enables resilient API access over extended durations. Understanding OAuth principles and diagnosing issues with Azure AD integration builds robust applications leveraging SharePoint data and services.