Wednesday, March 16, 2011

Best Practices for User Authentication

By now, many of you have seen our recent announcement regarding 2-step verification for Google Accounts. It’s an optional way of protecting your Google Account from unauthorized access, providing a level of security beyond that of a password alone. The initial announcement did not detail the impact enabling 2-step verification has on programmatic account access from code written against one of Google’s official APIs. We want to go into some more detail regarding the implications of 2-step verification on various authentication (and authorization) techniques, and offer best practices that you as a developer should follow.

There are three forms of authentication supported by almost all of Google’s APIs. AuthSub and OAuth (either version 1 or the newer OAuth 2) are similar web-based authentication mechanisms in which the user logs in on a web page hosted by Google. The other approach to authentication, ClientLogin, relies on your application soliciting the user’s account address and password, and then sending that information to Google.

If your code uses AuthSub or OAuth, then you don’t have to do anything special to accommodate users who have opted-in to 2-step verification. The web-based login flow currently allows users to enter both their normal passwords as well as the additional verification code, and this extra step is transparent to you as the developer.

ClientLogin, however, does not fare as well for accounts that have 2-step verification enabled. There is no concept of an additional verification code in the ClientLogin process, and a user’s account address and password are no longer sufficient for authenticating them once 2-step verification is turned on. If you make a ClientLogin authentication request for such an account, you’ll get back an HTTP 403 error response from our servers with the following in error included in the response body:


There are two solutions to these failed ClientLogin attempts. The first solution, which does not require changing any existing code, is to ask your users to generate an application-specific password and to provide that, instead of their Google Account passwords, when making your ClientLogin request. You can point your users to this article for a full explanation of how application-specific passwords work.

The second, and recommended, solution requires some work on your part as a developer: moving away from ClientLogin completely, in favor of OAuth 2. If your code runs as part of a web application, then OAuth 2’s web-based login flow is trivial to integrate. Even applications that are installed on a user’s computer or other device can leverage OAuth 2, though. This guide explains how to launch a web browser to handle the login process, and then redirect control back to your application.

While it may take some effort to migrate your code away from ClientLogin, your users will be grateful that you did. Even those who haven’t enabled 2-step verification will benefit from entering their credentials on a web page accessed via HTTPS and hosted by Google, as opposed to sharing their password information directly with your third party code.


  1. Hey Google team,

    I want to use the OAuth 2.0 workflow for Authentication, not for Authorization. Is this possible? I can get the users email adress via which is enough user information for me, but a userID would be better. In this OAuth 2.0 flow, the user is always promt to give the App permission. How can I skip that part, and redirect the user straight to the app?

  2. So, does that mean that my thunderbird client can not login automatically anymore (without app specific password)?
    That could be a bit annoying. I mean to set up app specific password for every mail client I have.