Browser-makers are finally starting to fight back against the bitcoin mining threat. According to ZDNet, the beta version of Opera 50 is the first major browser to offer a built-in mechanism that blocks cryptojacking, a new form of malware that leverages the victim’s computer to mine cryptocurrency without permission.

The new antimining tool, called NoCoin, is part of the ad blocker function in Opera. It is disabled by default but can be enabled through the Settings or Preferences page. According to an Opera blog post, the tool blocks cryptocurrency mining scripts just as an ad blocker would disable banner ads and pop-ups.

The Danger of Cryptojacking

Without a tool such as NoCoin, a cryptocurrency miner might go undetected until the central processing unit (CPU) usage dramatically increased for no obvious reason, preventing the victim from executing his or her own programs.

Coinhive was the first such malware to hit the web in September, Bleeping Computer reported. The JavaScript code mines for Monero using the victim’s CPU resources. It was initially presented as a way for site owners to bypass the need to display ads for revenue. Pirate Bay was one of the first to use Coinhive for this purpose, but the torrent site removed it in response to user outcry.

Researchers have also observed attempts to hide JavaScript cryptominers using covert pop-under windows, ZDNet reported. This enables fraudsters to continue using the victim’s CPU even after the user has navigated away from the website. Some crafty criminals even integrated JavaScript miner scripts right into their fake security warning browser lock screens.

Dusting for Fingerprints

Another Bleeping Computer article reported that Google has already looked at the service for its Chrome browser, but has rejected the idea of blacklisting or fingerprinting this kind of JavaScript action. The technology giant believes that a malware-laden site could easily mutate the script to bypass this mechanism.

While specific extensions can be added to browsers to help users avoid cryptomining, Opera’s built-in functionality makes this protection readily available.