According to MITRE, Browser-in-the-Middle (BitM) is an attack where “an adversary exploits the inherent functionalities of a web browser, in order to establish an unnoticed remote desktop connection in the victim’s browser to the adversary’s system.” This attack has been used by many attackers to trick victims into unknowingly entering credentials and providing sensitive information on an attacker controlled window. The attack was first disclosed in a paper by researchers from the University of Salento in 2021, and we have seen many cases of BitM being used in the wild since then.

However, one key flaw of the BitM attack is that it still requires the victim to land on a malicious site and perform an action to open up the noVNC pop-up window. As the parent window still has a malicious URL in its address bar, this will likely raise suspicion among more security aware users at the point of credential entry.

SquareX’s research team has observed multiple instances of the browser’s FullScreen API being exploited to address this flaw by displaying a fullscreen BitM window that covers the parent window’s address bar, as well as a limitation specific to Safari browsers that makes fullscreen BitM attacks especially convincing. The article below will recap how BitM attacks work, explore the Fullscreen API requirements and why Safari browsers are particularly vulnerable to fullscreen BitM attacks.

Traditional Browser-in-the-Middle (BitM) Attacks
To illustrate how a typical BitM attack works, we will use a real attack that targeted Counter-Strike 2 gamers. Incentivized by cryptocurrency and skin giveaways, victims were tricked into entering their Steam credentials. These compromised accounts were then sold on the black market for up to $300,000. Here is how it works:

Note: The case study below actually used the Browser-in-the-Browser (BitB) technique, where instead of using remote desktop, the attackers uses HTML, CSS and JavaScript most commonly to mimic login pop-ups of popular SaaS or Single Sign-On (SSO) services. We chose this example as it is a well documented attack and because the social engineering and principles behind this attack can also be used in BitM attacks.