Researchers at Cato Networks have discovered two dozen malicious Google Chrome browser extensions and 40 associated malicious domains that are being used to introduce adware on victim systems, steal credentials, or quietly redirect victims to malware distribution sites.
The security vendor discovered the extensions on networks belonging to hundreds of its customers and found that they were not being flagged as malicious by endpoint protection tools and threat intelligence systems.
Etay Maor, senior director of security strategy at Cato Networks, says such extensions can pose risks for enterprise organizations. "Security researchers have found extensions performing malicious activity that ranged from stealing usernames and passwords to stealing financial data," he says. The theft of personal and corporate data is a real threat for organizations, and there have already been multiple instances of extensions doing so, he notes.
While malicious extensions are an issue with all browsers, it's especially significant with Chrome because of how widely used the browser is, Maor says. It's hard to say what proportion of the overall Chrome extensions currently available are malicious. It's important to note that just a relatively small number of malicious extensions are needed to infect millions of Internet users, he says.
One case in point was Awake Security's discovery last June of over 100 malicious Google Chrome extensions that were being used as part of a massive global campaign to steal credentials, take screenshots, and carry out other malicious activity. Awake Security estimated that there were at least 32 million downloads of the malicious extensions. In February 2020, Google removed some 500 problematic Chrome extensions from its official Chrome Web Store after being tipped off to the problem by security researchers. Some 1.7 million users were believed affected in that incident.
In a soon-to-be-released report, Cato says it analyzed five days of network data collected from customer networks to see if it could identify evidence of extensions communicating with command-and-control servers. The company basically correlated Chrome browser extension behavior with network traffic to preliminarily classify extensions as benign or malicious. The exercise resulted in Cato identifying 97 out of 551 unique extensions on customer networks as being potentially problematic. Researchers from the company then manually inspected each extension to see if they could definitively classify them as malicious or benign. That process in turn ended up identifying 87 extensions as being definitely malicious. Out of that number, 24 had not been previously identified as being malicious.
Google, like other browser makers, has implemented multiple measures to vet the security of extensions uploaded to its Chrome store. According to Cato, the process of uploading an extension to Google's official store can take weeks and involves both automated and manual reviews of the extension code and activity. Chrome's standard security settings also block installations of extensions sourced from outside of Chrome Web Store. Even so, Cato's research showed threat actors employing at least four different approaches to introduce malicious extensions into users' browsers.
One common way is to sneak it in via extension installation files from unofficial stores. "Some developers prefer not to go through the Google’s set of installation restrictions and offer their extensions for download from unofficial stores," Maor says. While not all extensions on unofficial sites are malicious, it's still a risk to get Chrome extensions from anywhere but Google's official Chrome Web Store. Attackers have found ways to bypass Chrome's blocking of unofficial extensions by using iframes, a mechanism for embedding documents and other content inside a webpage, he says.
In other instances, an attacker may sneak malicious code into a Chrome browser extension update. Maor points to several ways this can happen. A developer, for instance, might sell code to a third party that later injects malicious code into it. Or a developer might initially release a benign browser that performs as advertised but then gets updated with malicious properties once it gets popular. Developers could also get scammed into giving up control of their account to an attacker. "In almost every instance, the app initially is not harmful but rather updated later with malicious code, as it is easier to bypass security checks that occur at the Google store that way," Maor says.
Adversaries have also been known to purchase rights to a legitimate Chrome extension and then modify it later with malicious code or to use a malicious extension to download additional malicious extensions.