Bypassing GSuite CAPTCHA for Username Enumeration and Password Spraying

While performing a recent assessment of an organization using GSuite, I discovered that the implementation of CAPTCHA to stop automated activity is wholly inadequate, and I was able to accomplish both username enumeration and password spraying with relative ease.

Classic Username Enumeration

When you enter an invalid email address in the Google login form, you are returned an error which states -- "Sorry, Google doesn't recognize that email." But when you enter a legitimate email address, you are prompted for a password. This varied response is the basis for any username enumeration attack. Automate the process of iterating through a list of emails, then measure the responses to determine which are legitimate.

But of course, Google throws another wrench in the mix, which might seem at first to be a show-stopper. After a couple of invalid usernames are supplied, Google throws you a CAPTCHA to solve.

I quickly discovered while interacting with this, that the best way to bypass the CAPTCHA is to just ignore it altogether.

So How to Beat the CAPTCHAs?

No complex machine learning to solve CAPTCHA puzzles. No rotating IP addresses through proxies. It was as easy as just programmatically closing the browser between each task and opening a new one. In each case, you would be able to supply at least an initial username without being prompted to solve a puzzle. So to do this ad infinitum, you simply have to automate the process of opening and closing the browser between each attempt. Fortunately, with Selenium, this is extremely easy to do.

For each email in the emails list, it will open a new browser instance and test whether the username is valid. And after running, you will have a list of which emails are valid, and which ones are not. Obviously, this is just a POC and doesn't scale well if you are going to be attempting to test thousands of email addresses. Of course, this could be resolved through multi-threading and then configuring the browsers to be headless.

And Password Spraying Too!!!

Better yet, this even works for password spraying too. Just cycle to a new browser for each attempt and CAPTCHA is no longer an issue. As long as your robot knows how to open and close browsers, it no longer has to try and prove that it's human. PROBLEM SOLVED!!!

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