A bug which is actively being exploited in Mozilla Firefox allows malicious webpages to lock users’ browsers, leaving them unable to exit without force-quitting through Task Manager.
Ars Technica reported that upon visiting the malicious webpage, users are shown a pop-up message which cannot be closed that states the following:
“Please stop and do not close the PC… The registry key of your computer is locked. Why did we block your computer? The Windows registry key is illegal. The Windows desktop is using pirated software. The Window desktop sends viruses over the Internet. This Windows desktop is hacked. We block this computer for your safety.”
A dialog box then appears which tells the user that they are about to log into the website using admin credentials. Clicking cancel or the “x” button on the dialog box does not work, and since the dialog box cannot be closed, the user can also not exit the tab or window.
The only way to solve this issue is to force a shutdown of the entire browser – either using Task Manager on Windows, or Force Close on macOS.
However, if the user has “restore tabs” enabled, the same website will automatically open upon relaunching Firefox – presenting the same issue once again.
In this case, there are two ways to avoid the issue:
- Quickly close the tab upon restarting the browser before the webpage can load.
- Disconnect from the internet before opening Firefox so that the page doesn’t load, allowing you to exit this tab.
The exploit was discovered by Jérôme Segura, head of threat intelligence at Malwarebytes, who said this bug is being exploited on several websites that he has come across.
Firefox has told Segura that Mozilla is actively working on a fix to the problem.