With recent attack disclosures like Browser Syncjacking and extension infostealers, browser extensions have become a primary security concern at many organizations. SquareX’s research team discovers a new class of malicious extensions that can impersonate any extension installed on the victim’s browser, including password managers and crypto wallets. These malicious extensions can morph themselves to have the exact same user interface, icons and text as the legitimate extension, making it an extremely convincing case for victims to enter their credentials and other sensitive information. This attack impacts most major browsers, including Chrome and Edge.

Polymorphic extensions work by exploiting the fact that most users interact with extensions via the pinned in the browser toolbar. The attack begins with the user installing the malicious extension, which disguises itself, for example, as an unassuming AI tool. To make the attack even more convincing, the extension performs the AI functionality as advertised and remains benign for a predetermined period of time.

However, while all this is happening, the malicious extension starts figuring out what other extensions are installed in the victim’s browser. Once identified, the polymorphic extension completely changes its own appearance to look like the target, including the icon shown on the pinned toolbar. It can even disable the target extension temporarily, removing it from the pinned bar. Given that most users use these icons as a visual confirmation to inform which extension they are interacting with, changing the icon itself is likely sufficient to convince the average user that they are clicking on the legitimate extension. Even if the victim navigates to the extension dashboard, there is no obvious way to correlate the tools displayed there to the pinned icons. To avoid suspicion, the malicious extension can even temporarily disable the target extension such that they are the only ones with the target’s icon on the pinned tab.

Critically, the polymorphic extension can impersonate any browser extension. For example, it can mimic popular password managers to trick victims into entering their master password. This password can then be used by the attacker to log on to the real password manager and access all credentials stored in the password vault. Similarly, the polymorphic extension can also mimic popular crypto wallets, allowing them to use the stolen credentials to authorize transactions to send cryptocurrency to the attacker. Other potential targets include developer tools and banking extensions that may provide the attacker unauthorized access to apps where sensitive data or financial assets are stored.

Furthermore, the attack only requires medium-risk permissions based on Chrome Store’s classification. Ironically, many of these permissions are used by password managers themselves, as well as other popular tools like ad blockers and page stylers, making it especially difficult for Chrome Store and security teams to identify malicious intent just by looking at the extension’s code.

The founder of SquareX, Vivek Ramachandran cautions that “Browser extensions present a major risk to enterprises and users today. Unfortunately, most organizations have no way of auditing their current extension footprint and to check whether they are malicious. This further underscores the need for a browser native security solution like Browser Detection and Response, similar to what an EDR is to the operating system.”

These polymorphic extensions exploit existing features within Chrome to conduct the attack. As such, there is no software bug involved, and it cannot be patched. SquareX has written to Chrome for responsible disclosure, recommending banning or implementation of user alerts for any extension icon changes or abrupt changes in HTML, as these techniques can easily be leveraged by attackers to impersonate other extensions in a polymorphic attack. For enterprises, static extension analysis and permissions-based policies are no longer sufficient – it is critical to have a browser-native security tool that can dynamically analyze extension behaviour at runtime, including polymorphic tendencies of malicious extensions.

For more information about polymorphic extensions, additional findings from this research are available at https://sqrx.com/polymorphic-extensions.

About SquareX

SquareX helps organizations detect, mitigate, and threat-hunt client-side web attacks happening against their users in real time, including defending against malicious extensions. In addition to the polymorphic attack, SquareX was also the first to discover and disclose multiple extension-based attacks, including Browser Syncjacking, the Chrome Store consent phishing attack leading to Cyberhaven’s breach and numerous other MV3-compliant malicious extensions revealed at DEF CON 32.

SquareX’s industry-first Browser Detection and Response (BDR) solution, takes an attack-focused approach to browser security, ensuring enterprise users are protected against advanced threats like malicious QR Codes, Browser-in-the-Browser phishing, macro-based malware and other web attacks encompassing malicious files, websites, scripts, and compromised networks.

Additionally, with SquareX, enterprises can provide contractors and remote workers with secure access to internal applications, enterprise SaaS, and convert the browsers on BYOD / unmanaged devices into trusted browsing sessions.

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