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Stored XSS on facebook and twitter!

I and my colleague Prakash were testing random stuffs to find a target that would be worth looking into. We found a new feature on Facebook which allows a user to visit the website of page-owner.The “Shop-now” feature looked interesting with different restrictions for different input fields.
The app-link field caught my eyes, because “deep-link” URL had particularly idiosyncratic example :

 

The field was sanitized for special characters like < , “, ‘ , > and didn’t allow any tags to enter. The output for such characters would be:

 

I thought of trying ()  . Surprisingly, they weren’t being sanitized, and of course : wouldn’t be filtered for sure because it would always be used in a URL. I still wasn’t sure if any javascript would be executed. I saved the details to test it out with the applink as javascript:alert(document.domain).

But then I realized I was only couple of clicks to see if the test was successful. In m.facebook.com, under the “shop now feature” was the script being stored. and then,

(Detailed exploit scenario is shown in video)

Script was straightforward inserted into “a href” tag and anyone clicking on the button link would be exploited.

Now, along with client side filtering, it doesn’t execute the script even if you managed to circumvent the client side protection in some way.

A very similar endpoint existed in twitter, too. You could define the action of a button click by yourself

 

but more on that later.

Both vulnerabilities have been patched by respective security teams.

Thanks for the read!