In-browser antivirus programs aren’t exactly well-loved; many are little more than malware masquerading as legitimate ways to clean systems and proactively detect threats. According to SecurityWeek, however, even well-known antivirus companies sometimes play fast and loose with Chrome extension rules. That’s the problem with AVG’s Web TuneUp, which could expose users’ browsing history to the Internet at large.

Big Business

Chrome extensions are an easy way to get more functionality out of Google’s popular browser. As noted by The Next Web, some are harmless and even hilarious, but the ubiquity of these extensions also makes them popular targets for malware-makers. If packaged with legitimate-seeming software, it’s possible to gain almost unlimited access to victim computers.

Consider the tactic used by some operators of the Fiesta exploit kit earlier this year. As Softpedia explained, when it became clear their payload delivery wasn’t paying off, actors developed the bogus antivirus software “Antivirus Pro 2015,” which always detected threats and then prompted users to purchase the full version.

The solution used to be that users should only download and install legitimate, above-board antivirus software and extensions. But now, a Google security researcher has turned up evidence that antivirus-maker AVG may be putting user privacy at risk with its default Chrome extension.

Bad Practice?

According to Computing, when users install AVG’s security suite, they’re also getting the AVG Web TuneUp extension — whether they like it or not. The problem? It installs a host of JavaScript APIs, some intended to hijack search settings along with the New Tab page. The APIs make it past Chrome’s malware check owing to an extremely complicated installation process, effectively making them a kind of malicious code, one that Google security researcher Tavis Ormandy described as “trash” in an email to AVG.

But installation without permission isn’t the whole story. Ormandy discovered that while most of the APIs were broken, a few created significant flaws in Chrome. For example, a cross-site scripting bug in the “navigate” API made it possible for websites to execute scripts on other domains, such as reading email from mail.google.com or performing other actions at a distance, while AVG’s “recently” API extension exposes user browsing history to the public Internet.

What’s more, Ormandy said that with enough time and effort, attackers could leverage these APIs for use in remote code execution, giving them the ideal hidden malware vehicle: Malicious code hidden inside the offering of a legitimate antivirus company.

Fixing the Chrome Extension

AVG has been working with Google to find a fix, and version 4.2.5.169 should remedy any issues with AVG Web TuneUp — although the Chrome Web Store team has disabled in-line installations of the extension in the meantime. Problematic code aside, there’s a larger issue here: the assumed permission of extension install combined with a clear attempt at API obfuscation.

While Google is investigating possible policy violations, this kind of enforcement is no match for user action. With supposedly above-board companies now willing to play fast and loose with privacy and property, Internet citizens must take a more active role in cataloging, monitoring and removing extensions that reach too far.