Tl;dr: I’ve recently been playing around with Google services, poking here and there for security vulnerabilities. It’s been a quite a roller-coaster experience with some interesting results as well as some devastating rejections (I should definitely write a separate blog about those). Nevertheless, I’ve found one simple, but interesting auth-bypass on Google groups (that landed me a $500 bounty) that I think is worth sharing.





First things first: GWT

Have you ever heard of or played with Google Web Toolkit (GWT)? I have to admit that although it’s not exactly cutting-edge technology ( in fact it’s been around from as early as 2006 and became open source in 2013 ), I had never really come across it before I started looking at Google products. When you first look at GWT HTTP requests they look, well… different. At first glance, they might even be intimidating. I mean, just look at this Googley witchcraft:









Luckily, however, the age of GWT means that security research has already been done on it (I’ve even found an old OWASP ppt .), which helped me decipher some of the request elements.





Class enumeration is a nice start

Now that we have our bearings, let’s go back to Google-Groups . If you’ve never played with this service before, Google-Groups is basically a forum platform. You can create your own forums (‘groups’), invite participants to discuss topics, and manage the group’s permissions (decide who is allowed to view/update which thread). Oh, and yes, Google-Groups is built with GWT.

Generally speaking, my quick and dirty approach to manipulate GWT requests was to ‘stick to the middle,’ i.e. not to mess with the signature bit of the request (the pipe-separated numbers at the beginning and the end of the above body), but to stick to the parameters.

I quickly found a parameter worth manipulating. In the following GWT request, I discovered that the marked integer points to a specific class on the server-side:









And so, I quickly launched a Burp-intruder session to extract classes’ names:





While not a huge issue on its own (I’ve reported the issue to Google who justifiability deemed it “low risk”), it gave me the motivation to keep going and believing that I could find a gap in this GWT implementation.





Viewing other group’s data

Armed with my new confidence, I’ve looked back at the initial GWT request from before. One of the strings in the request was the name of a testing group I’d created:





The response included basic information about the groups, including the group’s email, topic and description. But could I only view info about my own groups?

I quickly spawned a new group under a 2nd Google account (setting all permissions to ‘private,’ meaning that only invited users are allowed to view anything about the group), and then queried it from my 1st account. Turns out that authorization check was lacking, and I was able to extract some of the private group’s data:





While auth-bypass is always nice, it’s not a critical issue. I wasn’t able to extract the group’s conversations, but only the group’s email and description (which might be sensitive). Still, it was my first GWT exploit (and one of my first Google bounties) and altogether quite a nice bug :)

Timeline: