The cross-site scripting vulnerability could have allowed trivial account takeover. Facebook has awarded a security researcher $20,000 for discovering a cross-site scripting (XSS) vulnerability in the Facebook Login SDK, which is used by developers to add a “Continue with Facebook” button to a page as an authentication method. Exploitation could allow threat actors to hijack accounts. Security researcher Vinoth Kumar identified a Document Object Model-based (DOM) XSS flaw in the window.postMessage() method of the platform’s code. This method is supposed to enable secure cross-origin communication between Windows objects. Kumar said he discovered the flaw when he went digging for client-side vulnerabilities—more specifically, XSSI, JSONP and postMessage issues, according to a recent blog post.