Authenticating Users In Sharepoint Online With Javascript: Common Pitfalls And Best Practices

Understanding the Core Authentication Challenges

When implementing authentication in SharePoint Online using JavaScript, developers face several common pitfalls that can compromise user experience and security. The main challenges include:

• Obtaining improper authentication scopes that limit access to SharePoint resources
• Dealing with expiring access tokens that require users to re-authenticate frequently
• Handling CORS issues when making API calls from JavaScript to SharePoint

To provide seamless authentication, developers should follow these best practices:

• Request appropriate OAuth scopes to enable access to necessary SharePoint user profiles and sites
• Implement token renewal to automatically refresh expired tokens by using refresh tokens
• Configure CORS settings to allow JavaScript clients to access required SharePoint domains and APIs

Common Pitfall #1 – Improper Authentication Scopes

The OAuth access token provided after SharePoint user authentication includes claims that specify permissions to access resources. If the requested scopes are too narrow, the token may not allow the app to make necessary API calls to SharePoint sites and user profiles on behalf of the user. For example, an access token without the Sites.Read.All scope will not be able to read files across all SharePoint sites for that user. Requesting additional scopes requires user consent again.

Best Practice – Request all required OAuth scopes upfront

When registering your Azure AD app, configure the required resource application access permissions to allow access to SharePoint sites, files, search, and user profiles. This will enable the app to request sufficient scopes during initial user authentication and consent to avoid authorization errors when accessing SharePoint resources later.

Common Pitfall #2 – Expired Access Tokens

Access tokens issued by Azure AD typically expire after 1 hour. Once expired, calling the SharePoint API will fail with 401 Unauthorized errors. This requires users to re-authenticate, causing disruption.

Best Practice – Renew expired tokens with refresh tokens

Along with the access token, Azure AD also provides a refresh token that can be used to get new access tokens without re-authenticating users. Use refresh tokens to request new access tokens before they expire to avoid interruption for users.

Common Pitfall #3 – CORS Issues

If your Azure AD and SharePoint instances are on different domains than your web app, you may encounter CORS errors when making JavaScript calls from your app domain to SharePoint. This prevents successful API-based authentication.

Best Practice – Configure CORS settings

On the SharePoint admin center, add your web application’s URL to the list of allowed CORS origins. This will add the appropriate Access-Control-Allow-Origin headers for cross-domain calls from your web app for authentication purposes.

Implementing Robust User Authentication

Using industry best practices, you can implement seamless authentication flows for your SharePoint Add-ins and web apps. Follow this high-level sequence:

1. Register Azure AD app and configure permissions for SharePoint resources needed
2. In your app, use MSAL library to request Azure AD access token with appropriate scopes
3. Attach access token to calls made to SharePoint REST API endpoints
4. Use refresh token before expiry to get new access tokens

Step 1 – Register Azure AD Application

An Azure AD application serves as the entity that allows you to authenticate users and obtain authorization to access SharePoint resources. Use the Azure portal to create an AD app and configure required permissions.

Step 2 – Request Azure AD Token with MSAL

The MSAL (Microsoft Authentication Library) offers secure methods for user login and access token acquisition:

  const authProvider = new Msal.UserAgentApplication(clientId); 
  
  authProvider.loginPopup(request).then(token => {
    // Store token
  }).catch(error => {
    // Handle error  
  });

Step 3 – Attach Token to SharePoint REST Calls

To authorize SharePoint REST API calls, include the access token in the Authorization header:

  GET https://contoso.sharepoint.com/_api/sitepages
  
  Headers: 
    Authorization: Bearer {access_token}

Step 4 – Renew Expired Tokens

To proactively renew tokens before expiry and re-authorize API calls:

  authProvider.acquireTokenSilent({
    scopes: ["Sites.Read.All"], 
    refreshToken: refreshToken
  }).then(token => {        
    // Use new token
  });  

Troubleshooting Common Errors

Despite following best practices, you may still encounter authorization errors during development. Some tips for debugging:

Invalid Scope Errors

If you get 403 Forbidden responses from SharePoint REST API due to invalid scope, check:

• Requested scopes match permissions configured for Azure AD app
• Access token includes necessary scopes for the SharePoint resources you want to access

Fixing 401 Unauthorized Errors

401 errors indicate expired or invalid access tokens. To fix:

• Use refresh token to get new access token, check expiry time
• Verify token is passed in Authorization header correctly for SharePoint API calls

Optimizing Authentication Performance

A smooth authentication experience is vital for user adoption. Apply these optimizations:

Cache Tokens to Reduce Roundtrips

Avoid requesting new tokens per API call. Cache tokens using sessionStorage for reuse during their lifetime:

// Get cached token
const token = sessionStorage.getItem("token");

// Cache newly acquired token
sessionStorage.setItem("token", token);  

Prefetch User Profile

After authentication, make an API call to get user profile details you need across your app to avoid refetching:

GET /_api/sp.userprofiles.peoplemanager/getmyproperties

Monitor Page Load Performance

Use browser developer tools or Real User Monitoring to analyze page load waterfall. Check time taken for token issuance and API calls to optimize further if needed.

Securing User Access

Some tips to further secure authentication:

Implement Role Checks

Along with token-based auth, also check user roles before allowing privileged actions:

if(!user.roles.includes("admin")) {
  // deny access 
}

Encrypt Token Storage

Tokens provide access to user data so should be protected. Encrypt before persisting tokens client-side.

Prevent XSS Vulnerabilities

Sanitize user inputs that get rendered to avoid XSS injection attacks that could compromise authentication tokens or impersonate users.

Conclusion

Implementing robust authentication with JavaScript requires addressing common pitfalls early on through proper permission scoping, automatic refresh, and CORS handling. Following the step-by-step guide outlined and applying optimization and security best practices will enable seamless authentication without disruption as you build SharePoint apps and components.

For further reading, refer to these additional Microsoft and community resources:

• MSAL SDK reference
• SharePoint REST API authentication
• Azure AD token reference