Cracking the Code of Google OAuth 2.0 Refresh Tokens Missing on GCE

Understanding the Refresh Token Discrepancy in OAuth 2.0

Imagine developing a seamless OAuth 2.0 authentication flow for your web app. Everything works perfectly on your local machine, but when deployed on Google Cloud Engine (GCE), an essential piece—the refresh token—is missing! 🤯 This issue prevents automatic token renewal, disrupting user sessions.

Many developers face this perplexing problem, despite implementing access_type="offline" and other best practices. The localhost environment consistently returns a refresh token, while the cloud deployment fails to do so. The mystery deepens as both setups share the same codebase and authentication flow.

After countless hours of debugging, the solution often lies in an overlooked parameter: the prompt option. Tweaking this setting can mean the difference between receiving a refresh token and being stuck in an endless authentication loop. But why does this happen? 🤔

In this article, we’ll dissect the root cause of this issue, explore Google's OAuth 2.0 behavior, and provide a concrete fix. Whether you're running a Flask app or another framework, you’ll walk away with a working solution and a better understanding of Google’s authentication quirks!

Understanding OAuth 2.0 Refresh Token Retrieval in Flask Applications

OAuth 2.0 is a widely used authentication framework that allows applications to authenticate users via external providers like Google. In our example, we implemented a Flask application using the requests_oauthlib library to handle the authentication process. However, a key issue arose: the refresh token was only granted when running locally but not in the cloud environment. This problem prevented automatic token renewal, requiring users to re-authenticate frequently.

The core of the solution lies in adjusting the authentication request. By default, Google only grants a refresh token when explicitly requested using access_type="offline". However, in some cases, adding the prompt="consent" parameter is necessary to force Google to re-prompt the user for authorization. This is particularly important when deploying the application on Google Cloud Engine (GCE), where previously granted permissions may not carry over.

Our script starts by initializing an OAuth session and redirecting users to Google’s login page. Once the user authenticates, Google returns an authorization code, which the application exchanges for an access token. The key issue was that, without the correct parameters, Google would not provide a refresh token, making long-term authentication impossible. By modifying the request to include prompt="consent", we ensure that a new refresh token is always generated.

To validate the solution, we also created a unit test to simulate a login request and verify that the correct authentication URL is returned. This ensures that our fix works across different environments. If you’ve ever faced a similar issue—where authentication behaves differently in production versus development—understanding how OAuth 2.0 handles user sessions and token persistence is crucial. With these adjustments, you can ensure seamless authentication and a better user experience. 🚀

from flask import Flask, redirect, session, request
from requests_oauthlib import OAuth2Session
app = Flask(__name__)
app.secret_key = "your_secret_key"
CLIENT_ID = "your_client_id"
CLIENT_SECRET = "your_client_secret"
AUTHORIZATION_BASE_URL = "https://accounts.google.com/o/oauth2/auth"
TOKEN_URL = "https://oauth2.googleapis.com/token"
REDIRECT_URI = "https://yourdomain.com/callback"
@app.route("/login")
def login():
    gcp = OAuth2Session(CLIENT_ID, redirect_uri=REDIRECT_URI, scope=["openid", "email", "profile"])
    authorization_url, state = gcp.authorization_url(AUTHORIZATION_BASE_URL, access_type="offline", prompt="consent")
    session["oauth_state"] = state
    return redirect(authorization_url)
@app.route("/callback")
def callback():
    gcp = OAuth2Session(CLIENT_ID, state=session["oauth_state"], redirect_uri=REDIRECT_URI)
    token = gcp.fetch_token(TOKEN_URL, client_secret=CLIENT_SECRET, authorization_response=request.url)
    session["oauth_token"] = token
    return "Login Successful"
if __name__ == "__main__":
    app.run(debug=True)

import unittest
from app import app
class OAuthTestCase(unittest.TestCase):
    def setUp(self):
        self.app = app.test_client()
    def test_login_redirect(self):
        response = self.app.get("/login")
        self.assertEqual(response.status_code, 302)
        self.assertIn("accounts.google.com", response.location)
if __name__ == "__main__":
    unittest.main()

Ensuring Secure and Persistent OAuth 2.0 Authentication in Cloud Environments

One key challenge developers face when deploying OAuth 2.0 authentication in the cloud is ensuring that the authentication process remains seamless across sessions. When a refresh token is not granted, users must re-authenticate frequently, which can disrupt the user experience. This issue often arises due to incorrect configuration of the OAuth 2.0 consent screen in the Google Cloud Console, leading Google to assume the application does not require offline access.

Another crucial factor is ensuring that all necessary API scopes are properly configured. If a cloud-hosted application does not request the right OAuth 2.0 scopes, Google may limit the permissions granted, excluding refresh tokens. Developers should verify that their application explicitly requests offline access and includes relevant scopes, such as "openid", "email", and "profile", in the authentication request. Additionally, using the include_granted_scopes="true" parameter helps maintain permissions granted in previous sessions.

To further enhance authentication security and persistence, developers should implement robust token storage. Instead of storing tokens in session variables, using a secure database or an encrypted storage mechanism ensures that access tokens and refresh tokens remain accessible across server restarts. By following these best practices, developers can ensure a smooth and uninterrupted authentication flow in cloud-hosted applications. 🔐