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Malicious sites abuse 11-year-old Firefox bug that Mozilla failed to fix

Malware authors, ad farmers, and scammers are abusing a Firefox bug to trap users on malicious sites.

This wouldn’t be a big deal, as the web is fraught with this kind of malicious sites, but these websites aren’t abusing some new never-before-seen trick, but a Firefox bug that Mozilla engineers appear to have failed to fix in the 11 years ever since it was first reported back in April 2007.

The bug narrows down to a malicious website embedding an iframe inside their source code. The iframe makes an HTTP authentication request on another domain. This results in the iframe showing an authentication modal on the malicious site, like the one below.

firefox-authentication-demo.png

For the past few years, malware authors, ad farmers, and scammers have been abusing this bug to lure users on sites where they show all sorts of nasties, such as tech support scams, ad farms that reload the page with new ads in a loop, pages that push users to buy fake gift cards, or sites that offer malware-laced software updates.

Whenever users try to leave, the owners of these shady sites trigger the authentification modal in a loop. Every time the user dismisses it, another request is made, and a new modal appears, effectively keeping the user captive on the malicious sites until they close the browser altogether, and are forced to start a new browsing session.

But despite being reported over and over again for seven other times [1, 2, 3, 4, 5, 6, 7], this issue has gone unfixed, for unknown reasons, and crooks have gladly abused it all this time.

The latest example of abuse comes from a user who reported the issue once again today, after landing on one of these shady sites that tried to force him into installing a suspicious Firefox extension.

“At first, it is opened full screen mode. With some fake Windows dialog (I am using Linux so I know it is fake),” the user said. “It tried to [force] me install their extensions.”

“Then I press ESC to exit full screen. I click the close button of tab or window, but it doesn’t work because it has this login dialog. I click close button of the login dialog or cancel button. Then the dialog will appear again. I click the ‘Don’t allow’ button of extension installation pop over, but it seems not clickable. I killed the Firefox process, which is the only solution for me.”

firefox-bug-abused.png
Image: Guo Yunhe

Sure, Mozilla is an open source project, and it doesn’t have unlimited resources to handle all the reported issues, but you’d think that after more than 11 years a Firefox engineer would find the time to fix an actively exploited issue.

Based on the feedback left by other users on the reported issue, the Firefox team’s best bet is to follow how Edge and Chrome have dealt with this same issue.

Edge: The delay between authentication modals in Edge is large enough to allow the user to close the tab or the browser.

Chrome: The authentication dialog window has been moved from the browser window level to each tab’s level. This means the aggressive authentication dialogs only blocks the tab, and not the entire browsers, allowing the user to easily close the abusive tab.

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