Cryptocurrency Miners hidden in websites now run even after users close the browser

Some websites use a simple trick to keep their cryptocurrency miners scripts running in the background even when the user has closed the browser window.

Website administrators and crooks are looking with an increasing interest at JavaScript-based cryptocurrency miners due to rapid increase in cryptocurrency prices.

These scripts exploit the CPU power of their visitor’s PC to mine Bitcoin or other cryptocurrencies. Some websites use a simple technique to keep their cryptocurrency mining JavaScript under the radar and secretly running in the background even when the users close his web browser.

In many cases the scripts are used as an alternative monetization model to banner ads. Recently, the Pirate Bay was spotted using the Coinhive browser-based cryptocurrency miner service.

The scripts can mine cryptocurrencies as long as the visitors are on their site, they lost access to the computer processor and associated resources when the Window is closed.

Experts from security firm Malwarebytes have discovered that some websites use a simple trick to keep their cryptocurrency mining scripts running in the background even when the user has closed the browser window.

The technique leverages a hidden pop-under browser window that is opened by the mining window and that fits behind the taskbar and hides behind the clock on Microsoft’s Windows computer.

This hidden window is used to run the crypto-miner code consumes CPU cycles and power from visitor’s computer until he will not spot the window and close it.

“The trick is that although the visible browser windows are closed, there is a hidden one that remains opened. This is due to a pop-under which is sized to fit right under the taskbar and hides behind the clock.” reads the blog post published by MalwareBytes.

“The hidden window’s coordinates will vary based on each user’s screen resolution, but follow this rule:

• Horizontal position = ( current screen x resolution ) – 100
• Vertical position = ( current screen y resolution ) – 40

If your Windows theme allows for taskbar transparency, you can catch a glimpse of the rogue window. Otherwise, to expose it you can simply resize the taskbar and it will magically pop it back up:”