App Communication Permissions

TL;DR

Allow one app to act on behalf of a user to access resources another app controls. This is done using OAuth 2.0 with the appropriate scopes and refresh tokens, ensuring secure delegated access without sharing passwords.

Solution Guide: App Communication Permissions

1. Understand the Scenario
   • App A needs to access data or functionality provided by App B on behalf of a user.
   • The user should not directly share their credentials with App A.
   • Access should be limited in scope and time-bound.
2. Choose an OAuth 2.0 Flow

   The Authorization Code Grant flow is generally the most secure for web applications.

   • Authorization Endpoint: App A redirects the user to App B’s authorization endpoint with requests for specific permissions (scopes).
   • User Consent: The user logs in to App B and approves or denies App A’s request.
   • Redirect URI: If approved, App B redirects the user back to App A with an authorization code.
   • Token Endpoint: App A exchanges the authorization code for an access token (and optionally a refresh token) at App B’s token endpoint.
3. Implement OAuth 2.0 on App A (Client Application)
   • Use an OAuth 2.0 client library in your chosen programming language (e.g., Python with requests-oauthlib, JavaScript with oidc-client-js).
   • Construct the authorization URL including:
     • client_id: App A’s identifier registered with App B.
     • redirect_uri: The URL App B redirects to after user consent.
     • response_type: Set to code for the Authorization Code Grant flow.
     • scope: A space-separated list of permissions requested (e.g., read profile write).
   • Redirect the user to the authorization URL.
   • Handle the redirect from App B, verifying the state parameter to prevent CSRF attacks.
   • Exchange the authorization code for an access token using a POST request to App B’s token endpoint.

     POST /token
     Content-Type: application/x-www-form-urlencoded
     grant_type=authorization_code&code={AUTHORIZATION_CODE}&redirect_uri={REDIRECT_URI}&client_id={CLIENT_ID}&client_secret={CLIENT_SECRET}

4. Implement OAuth 2.0 on App B (Resource Server)
   • Expose an authorization endpoint that presents a consent screen to the user.
   • Validate the client_id and redirect_uri before displaying the consent screen.
   • Securely store user credentials and access tokens.
   • Expose a token endpoint that issues access tokens upon successful authentication and authorization.
     • Verify the grant_type, code, redirect_uri, client_id, and client_secret.
     • Issue an access token with a limited lifespan.
     • Consider issuing a refresh token to allow App A to obtain new access tokens without user interaction (see step 6).
   • Protect your API endpoints requiring authentication using the access token.
     • Verify the access token’s signature and expiration date.
     • Enforce scope restrictions based on the permissions granted to the user.
5. Secure Communication
   • Use HTTPS for all communication between App A, App B, and the user’s browser.
   • Implement proper input validation and output encoding to prevent injection attacks.
   • Store access tokens securely (e.g., encrypted in a database).
   • Regularly rotate client secrets.
6. Refresh Tokens (Optional)
   • Issue refresh tokens along with access tokens.
   • App A can use the refresh token to obtain new access tokens when the current one expires, without requiring user interaction.

     POST /token
     Content-Type: application/x-www-form-urlencoded
     grant_type=refresh_token&refresh_token={REFRESH_TOKEN}&client_id={CLIENT_ID}&client_secret={CLIENT_SECRET}

   • Implement refresh token rotation to mitigate the risk of compromised tokens.
7. Testing
   • Test all scenarios, including successful authorization, denied authorization, invalid credentials, and expired tokens.
   • Verify that access is limited to the requested scopes.
   • Ensure proper error handling and logging.