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.
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
getIdfrom 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
- 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.
- 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.
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
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
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.