Integrating Google Drive Picker into your web application offers a streamlined user experience but necessitates careful OAuth token management. A common implementation involves embedding the token client-side, raising valid security concerns. This post outlines key strategies to minimize risks associated with this approach, particularly because the Implicit Grant flow returns a token that is passed to Drive Picker.

<drive-picker
  app-id={@app_id}
  oauth-token={@token}> <!-- IS THIS SAFE? -->
</drive-picker>

The above example is using the [Web component library for Google Drive Picker](https://www.npmjs.com/package/@googleworkspace/drive-picker-element.

The Challenge: Client-Side Token Exposure

The Implicit Grant flow is designed for client-side JavaScript applications (and components within server-rendered apps). It delivers the OAuth token directly to the browser, meaning the token resides in the JavaScript environment and is potentially visible. This is unavoidable with the Implicit Grant, but manageable.

Mitigation Strategies: A Concise Guide

Here's how to secure your Google Drive Picker integration:

1. Minimize Scope:

   • scope: "https://www.googleapis.com/auth/drive.file": Crucially, request only the drive.file scope. This restricts the token's access to files specifically opened or created via your application's Picker. A compromised token's damage is thus significantly reduced.

2. Restrict Granted Scopes:

   • include_granted_scopes: false: Essential. This setting prevents the token from inheriting broader permissions the user might have previously granted. It enforces the principle of least privilege and makes sense if only requesting drive.file.

3. Immediate Token Revocation:

   • Revoke After Use: Immediately after the user interacts with the Picker (e.g., after file selection), revoke the token using Google's revocation endpoint:
     

     POST https://oauth2.googleapis.com/revoke?token={token}
     

   Do not wait for natural token expiration. This minimizes the attack window. If the token has a corresponding refresh token, the refresh token will also be revoked.

   ⚠️   All previous granted scopes for this client (and project) will be revoked.

4. Content Security Policy (CSP):

   • Strict CSP: Implement a strong Content Security Policy to prevent Cross-Site Scripting (XSS) – a major threat to token security.

5. Client-Side Clean Up:

   • After use and revocation, ensure the token is non-retrievable by JavaScript (delete the relevant object). Also, delete the DOM node in the web component usage example. This is good practice, even though it's not the primary defense.

6. Secure Coding Practices:

   • Prevent XSS vulnerabilities by diligently sanitizing all user inputs. This is fundamental to web security.

Conclusion

Client-side token exposure is inherent to the Implicit Grant. However, by strictly limiting scope, restricting granted scopes, promptly revoking tokens, employing a strong CSP, and performing client-side cleanup, you significantly mitigate the risks and securely integrate Google Drive Picker.