Attackers Leverage Locally-Loaded Chrome Extension for Data Exfiltration

A recently investigated malicious attack was abusing a locally loaded Chrome extension to exfiltrate data and establish communication with the command and control (C&C) server.

While the use of malicious Chrome extensions in attacks is not something new, this attack stands out from the crowd due to the use of ‘Developer mode’ in the browser to enable loading of a malicious extension locally.

The extension was dropped in a folder on the compromised workstation, while the ‘Developer mode’ was enabled directly from the browser (it is available in More Tools -> Extensions). Any user can leverage this legitimate function by clicking ‘Load unpacked.’

The malicious add-on used in this attack, SANS Internet Storm Center (ISC) handler Bojan Zdrnja explains, claimed to be Forcepoint Endpoint Chrome Extension for Windows, although it had nothing to do with the cyber-security firm, aside from the stolen name and logo.

The threat actor behind this attack, Zdrnja says, was focused on the manipulation of data in an internal web application their victim had access to.

“While they also wanted to extend their access, they actually limited activities on this workstation to those related to web applications, which explains why they dropped only the malicious Chrome extension, and not any other binaries,” the researcher says.

Analysis of the code revealed that the attackers were using a legitimate method to set up a listener and enable communication between extensions.

Furthermore, specific keys the code was found to set were being synced to the logged-in victim’s Google cloud, allowing the attackers to log into their own Chrome browser with the same account, and then abuse Google’s infrastructure to communicate with the browser on the victim’s network.

“While there are some limitations on size of data and amount of requests, this is actually perfect for C&C commands (which are generally small), or for stealing small, but sensitive data – such as authentication tokens,” Zdrnja points out.

After testing and verifying the method, the researcher confirmed that both C&C communication and data exfiltration can be performed this way. Detecting requests abused in this attack is rather difficult, due to the use of legitimate infrastructure.

The researcher recommends control over the Chrome extensions in the local environment, especially since Google does allow administrators to use group policies to allow/approve specific extensions and block all others.

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