After reading some blog posts about Mozilla’s Addons websites, I was fascinated from this python-based platform and decided to focus on it.
The XSS vector led basically to nowhere. The folks at Mozilla did excellent job curing and properly sanitizing every user input.
This led me to change my direction and search for the most fun vulnerabilities – logic flaws.
The logic flaws logic
Most people don’t know, but the fastest way to track logic issues is to see things logically. That’s it. Look at a JS function – would you write the same code? What would you have changed? Why?
Mozilla’s Addons site has a collections feature, where users can create a custom collection of their favorite addons. That’s pretty cool, since users can invite other users to a role on their collection. How, do you ask? By email address of course!
A user types in the email address of another user, an AJAX request is being made to an ‘address resolver’ and the ID of the user who owns this email address returns.
When the user press ‘Save Changes’, the just-arrived ID is being passed to the server and the being translated again to the email address, next to the user’s username. Pretty weird.
So, If the logic, for some reason, is to translate an email to an ID and then the ID to the email, we can simply interrupt this process in the middle of it, and replace the generated ID with the ID of another user.
The following video presents a proof of concept of this vulnerability, that exposed the email address of any of addons.mozilla.org users.
It is a bad practice to do the same operation twice. If you need something to be fetched from the server, fetch it one time and store it locally (HTML5 localStorage, cookie, etc.). This simple logic flaw jeopardized hundreds of thousands of users until it was patched by Mozilla.
The patch, as you guessed, was to send the email address to the server, instead of sending the ID.