The AVG Web TuneUp Chrome extension, forcibly added to Google Chrome browsers when users were installing the AVG antivirus, had a serious flaw that allowed attackers to get the user's browsing history, cookies, and more.

The vulnerability was discovered by Google Project Zero researcher Tavis Ormandy, who worked with AVG for the past two weeks to fix the issue.

AVG Web TuneUp vulnerable to a universal XSS

As Mr. Ormandy explains in his bug report, the AVG Web TuneUp extension, which lists over nine million users on its Chrome Web Store page, was vulnerable to trivial XSS (cross-site scripting) attacks.

Attackers aware of this problem would have been able to access a user's cookies, browsing history, and various other details exposed via Chrome.

"This extension adds numerous JavaScript APIs to Chrome, apparently so that they can hijack search settings and the new tab page," explains Mr. Ormandy. "The installation process is quite complicated so that they [AVG] can bypass the Chrome [Store] malware checks, which specifically tries to stop abuse of the [Chrome] Extension API."

A half-baked Chrome extension

During his research, Mr. Ormandy discovered that many of the custom JavaScript APIs added to Chrome by this extension are responsible for the security issue, being broken or poorly written, allowing attackers access to personal details.

AVG's developers ignored or failed to protect their users against simple cross-domain requests, allowing code hosted on one domain to be executed in the context of another URL.

Theoretically, this would give attackers access to data stored on other websites, such as Gmail, Yahoo, banking websites, and more of the bunch. All that attackers had to do was to convince a user to access a malicious URL.

The extension rendered HTTPS connections useless

Websites hosted on HTTPS were also susceptible, Mr. Ormandy stating that users of this extension "have SSL disabled."

Version 4.2.5.169 of AVG Web TuneUp fixed this issue. In the meantime, Google blocked AVG's ability to carry out inline installations of this extension. This means that users who want to install the extension have to go to the Chrome Web Store and trigger the download with a click.

Additionally, the Chrome Web Store team is also investigating AVG for possible Web Store policy violations.