A Comprehensive Guide to Retrieving User Email Addresses with Google OAuth API

Introduction

When developing applications with Google authentication, retrieving user email addresses is a common requirement. However, due to continuous updates and endpoint changes in Google APIs, many developers face challenges when using OAuth 2.0. This article will explain in detail how to properly configure and use Google OAuth API to retrieve user email addresses through a practical case study.

Problem Context

A developer was testing with a personal Google account in the Google OAuth 2.0 Playground, attempting to retrieve user information with the following scopes:

email profile https://www.googleapis.com/auth/plus.login

They called the API endpoint:

https://www.googleapis.com/oauth2/v2/userinfo

While user details like name, gender, and profile picture were returned, the email address was missing. This raised a critical question: How to correctly retrieve the user's email address? Was it incorrect scope configuration or calling the wrong API endpoint?

Core Solution

According to Google's official documentation and community best practices, the correct approach to retrieve user email addresses is as follows:

1. Use the Correct API Endpoint

The old https://www.googleapis.com/oauth2/v2/userinfo endpoint has been deprecated. Although currently maintained for backward compatibility, it's not recommended for new projects. The recommended approach is to use the OpenID Connect standard userinfo endpoint.

The authoritative endpoint URL can be obtained through the discovery document:

https://accounts.google.com/.well-known/openid-configuration

At the time of writing, this endpoint is:

https://openidconnect.googleapis.com/v1/userinfo

However, developers should always dynamically obtain the latest endpoint through the discovery document to ensure compatibility.

2. Configure Proper Scopes

To retrieve user email addresses, the email scope must be included in the OAuth authorization request. This scope is equivalent to and replaces the old https://www.googleapis.com/auth/userinfo.email scope.

Complete scope configuration example:

email profile openid

Where the openid scope is necessary for using the OpenID Connect protocol.

3. Implementation Code Example

Here's a complete Python example demonstrating how to retrieve user email addresses through Google OAuth:

import requests
import json

# Step 1: Obtain access token (assuming obtained through OAuth flow)
access_token = "YOUR_ACCESS_TOKEN"

# Step 2: Get discovery document to determine userinfo endpoint
discovery_url = "https://accounts.google.com/.well-known/openid-configuration"
discovery_response = requests.get(discovery_url)
discovery_data = discovery_response.json()
userinfo_endpoint = discovery_data["userinfo_endpoint"]

# Step 3: Call userinfo endpoint to get user data
headers = {
    "Authorization": f"Bearer {access_token}"
}
userinfo_response = requests.get(userinfo_endpoint, headers=headers)
userinfo_data = userinfo_response.json()

# Step 4: Extract email address
if "email" in userinfo_data:
    user_email = userinfo_data["email"]
    print(f"User email address: {user_email}")
else:
    print("Email address not found, please check scope configuration")

# Output complete user information
print(json.dumps(userinfo_data, indent=2))

Historical Evolution and Compatibility Considerations

Google's OAuth and user information APIs have undergone several important changes:

Google+ API Deprecation

In December 2018, Google announced it would shut down the Google+ API in March 2019. This means all plus.*-based endpoints (such as https://www.googleapis.com/plus/v1/people/me) have been deprecated and eventually discontinued.

Userinfo Endpoint Changes

The original userinfo endpoint (v2 version) was originally scheduled for removal in September 2014 but was kept for backward compatibility. Google later clarified that the endpoint was "deprecated but kept available for backwards compatibility."

Adoption of OpenID Connect

Google now recommends using the OpenID Connect protocol for retrieving user identity information. This includes:

• Using standardized discovery mechanisms to determine endpoints
• Obtaining user information directly through ID tokens (JWT format)
• Following industry-standard best practices

Alternative Approach: Using ID Tokens

In addition to calling the userinfo endpoint, user email addresses can be obtained directly by parsing ID tokens. When using OpenID Connect, Google returns an ID token in the authorization response, which is a signed JWT (JSON Web Token).

Example code for parsing ID tokens:

import jwt

# ID token from OAuth authorization response
id_token = "YOUR_ID_TOKEN"

# Note: In production, token signature should be verified
decoded_token = jwt.decode(id_token, options={"verify_signature": False})

if "email" in decoded_token:
    user_email = decoded_token["email"]
    print(f"Email address from ID token: {user_email}")

This approach avoids additional API calls but requires proper JWT validation and security considerations.

Best Practices and Recommendations

1. Always Use Latest Documentation: Google's API documentation is frequently updated. Developers should regularly consult the official OpenID Connect documentation.
2. Implement Error Handling: API endpoints may change. Code should gracefully handle 404 errors or other API changes.
3. Consider Principle of Least Privilege: Only request scopes that the application genuinely needs to protect user privacy.
4. Test Different Scenarios: Use test accounts to verify email retrieval functionality under various conditions.
5. Monitor API Changes: Subscribe to Google developer notifications to stay informed about API changes.

Frequently Asked Questions

Q: Why is the email address sometimes not retrieved?
A: The most common reason is incorrect scope configuration. Ensure the email scope is included in the OAuth authorization request and that the user has consented to grant this permission.

Q: Which endpoint should be used?
A: It's recommended to use the OpenID Connect userinfo endpoint obtained through the discovery document. For new projects, avoid using deprecated endpoints.

Q: Do I need to handle Google+ API deprecation?
A: Yes, all code relying on Google+ APIs needs to migrate to the new OpenID Connect endpoints. Google+ APIs were discontinued in 2019.

Conclusion

Retrieving user email addresses through Google OAuth API requires proper scope configuration and using appropriate API endpoints. While there have been multiple endpoints and methods historically, the current best practice is to use the OpenID Connect protocol, dynamically obtain userinfo endpoints through discovery documents, and ensure the email scope is requested in the OAuth flow. The code examples and best practices provided in this article can help developers reliably implement this functionality while maintaining compatibility with Google API evolution.

As authentication standards continue to evolve, developers are advised to stay updated with Google's official documentation to ensure application security and stability. Proper implementation not only provides a good user experience but also ensures compliance with the latest privacy and security standards.