Improving the Security and User Experience of your Google Sign In Implementation

Posted by Isabella Chen, Software Engineer

We launched a fully revamped Sign-In API with Google Play services 8.3 providing a much more streamlined user experience and enabling easy server authentication and authorization. We’ve heard from many developers that they’ve found these APIs simple and less error prone to use. But when we look at applications in the Play Store, we see many that are still using legacy Plus.API / GoogleAuthUtil.getToken and do not follow best practices for authentication and authorization. Not following best practices can make your apps easily vulnerable to attack.
It’s also worth noting that developers who don’t want to worry about managing the security implications of different API flows or keeping up to date with the latest  APIs can use Firebase Authentication to manage the entire authentication lifecycle.

Ensuring your apps are secure

Developers should ensure that their apps are not open to the following vulnerabilities:

• Email or user ID substitution attack After signing in with Google on Android, some apps directly send email or user ID in plain text to their backend server as the identity credential. These server endpoints enable a malicious attacker to easily forge a request and gain access to any user’s account by guessing their email / user ID.
  
  Figure 1. Email / User ID Substitution Attack
  We see a number of developers implement this anti-pattern by using getAccountName or getId from the Plus.API and sending it to their backend.
  
  Problematic implementations, DO NOT COPY
  

• Access token substitution attack After signing in with Google on Android, many apps send an access token obtained via GoogleAuthUtil.getToken to their backend server as the identity assertion. Access tokens are bearer tokens and backend servers cannot easily check if the token is issued to them. A malicious attacker can phish the user to sign-in on another application and use that different access token to forge a request to your backend.
  
  Figure 2. Access Token Substitution Attack
  Many developers implement this anti-pattern by using GoogleAuthUtil to retrieve an access token and sending it to their server to authenticate like in the following example:
  
  Problematic implementation, DO NOT COPY

Solution

1. Many developers who have built the above anti-patterns into their apps simply call our tokenInfo (www.googleapis.com/oauth2/v3/tokeninfo) which is debug-only or unnecessarily call the G+ (www.googleapis.com/plus/v1/people/me) endpoint for user’s profile information. These apps should instead implement our recommended ID token flow explained in this blog post. Check out migration guide to move to a both secure and efficient pattern.
2. If your server needs access to other Google services, e.g. Drive, you should use server auth code flow. You can also check out this blogpost. Worth mentioning, you can also get an ID token using server auth code flow, from which you can retrieve user id / email / name / profile url without additional network call. Check out the migration guide.

Improving the user experience and performance of your apps

There are still many apps using GoogleAuthUtil for server auth and their users are losing out the improved user experience while the developers of those apps need to maintain a significantly more complicated implementation.
Here are some of the common problems that we see:

Requesting unnecessary permissions and displaying redundant user experience

Many apps request GET_ACCOUNTS permission and draw their own customized picker with all email addresses. After getting the email address, the app calls either GoogleAuthUtil or Plus.API to do OAuth consent for basic sign in. For those apps, users will see redundant user experience like:
Figure 3. GET_ACCOUNTS runtime permission and redundant user experience

The worst thing is the GET_ACCOUNTS permission. On Marshmallow and above, this permission is displayed to the user as ‘Contacts’. Many users are unwilling to grant access to this runtime permission.

Solution

Switch to our new Auth.GOOGLE_SIGN_IN_API for a streamlined user consent experience by providing an intuitive one-tap interface to provide your app with the user’s name, email address and profile picture. Your app receives an OAuth grant when the user selects an account, making it easier to sign the user in on other devices. Learn more

Figure 4. New streamlined, one-tap sign-in experience

Getting ID Token from GoogleAuthUtil for your backend

Before we released revamped Sign-In API, GoogleAuthUtil.getToken was the previously recommended way to retrieve an ID token via a “magic string.”

Wrong pattern, DO NOT COPY

GoogleAuthUtil.getToken needs to take an email address, which usually leads to the undesirable user experience in Figure 3. Also, user’s profile information like name, profile picture url is valuable information to store in your server. The ID token obtained via Auth.GOOGLE_SIGN_IN_API will contain profile information and your server won’t need additional network calls to retrieve them.
Solution Switch to the ID token flow using the new Auth.GOOGLE_SIGN_IN_API and get the one-tap experience. You can also check out this blogpost and the migration guide for more details.

Getting auth code from GoogleAuthUtil for your backend

We once recommended using GoogleAuthUtil.getToken to retrieve a server auth code via another “magic string.”

Wrong pattern, DO NOT COPY

In addition to the possible redundant user experience in Figure 3, another problem with this implementation was that if a user had signed in to your application in the past and then switched to a new device, they would likely see this confusing dialog:

Figure 5. Confusing consent dialog for returned user if using GoogleAuthUtil.getToken for auth code
Solution

To easily avoid this “Have offline access” consent dialog, you should switch to server auth code flow using the new Auth.GOOGLE_SIGN_IN_API . We will issue you an auth code silently for a previously signed-in user. You can also check out this blogpost and migration guide for more info.

Should I ever use GoogleAuthUtil.getToken?

In general, you should NOT use GoogleAuthUtil.getToken, unless you are making REST API call on Android client. Use Auth.GOOGLE_SIGN_IN_API instead. Whenever possible, use native Android API in Google Play services SDK. You can check out those APIs at Google APIs for Android.
And starting from Google Play services SDK 9.0, you will need to include -auth SDK split to use GoogleAuthUtil.getToken and related classes
AccountChangeEvent/AccountChangeEventsRequest/AccountChangeEventsResponse.
dependencies { compile 'com.google.android.gms:play-services-auth:9.0.0' }