The vulnerability, which has now been patched, was the result of a window message event handler's failure to accurately verify the message's origin, providing attackers access to users’ sensitive data.

PostMessage API 

The PostMessage API (also known as the HTML5 Web Messaging API) is a communication mechanism that permits safe cross-origin communication between several windows or iframes inside a web application. The API enables scripts from different origins to exchange messages, overcoming the restrictions the Same-Origin Policy imposes, that normally restricts data sharing between distinct sources on the web.

The API includes methods named window.postMessage() and an event message. The postMessage() method is used to send a message from the source window to the target window or iframe, while the message event is triggered on the receiving end when a new message is received. The team discovered a script in TikTok's web application during the code analysis that seemed to be involved in user tracking. 

The Imperva report states that “the first step in discovering the vulnerability was to identify all the message event handlers in TikTok's web application. This involved a comprehensive analysis of the source code in locating instances where the PostMessage API was being used[…]Once all the message event handlers were identified, we proceeded to carefully read and understand the code for each handler. This allowed us to determine the purpose of each handler and evaluate the security implications of processing untrusted messages.” 

Exploiting the Vulnerability 

Attackers could send harmful messages to the TikTok web application through the PostMessage API by taking advantage of this vulnerability and getting around the security precautions. The malicious message would then be processed by the message event handler as if it were from a reliable source, giving the attacker access to private user data.

The vulnerability was promptly addressed after being reported to TikTok by the Imperva Red Team, and Imperva appreciated TikTok for its swift action and cooperation. This disclosure should serve as a reminder of the value of adequate message origin validation and the risks of enabling interdomain communication without the necessary security precautions.