A researcher has documented almost 2,500 sites that are actively running cryptocurrency mining code in the browsers of unsuspecting visitors, a finding that suggests the unethical and possibly illegal practice has only picked up steam since it came to light a few weeks ago.

Willem de Groot, an independent security researcher who reported the findings Tuesday, told Ars that he believes all of the 2,496 sites he tracked are running out-of-date software with known security vulnerabilities that have been exploited to give attackers control. Attackers, he said, then used their access to add code that surreptitiously harnesses the CPUs and electricity of visitors to generate the digital currency known as Monero. About 80 percent of those sites, he added, also contain other types of malware that can steal visitors' payment card details.

"Apparently, cyberthieves are squeezing every penny out of their confiscated assets," he said.

One of the affected sites is shop.subaru.com.au. When I visited the site on Tuesday, the fan on my MacBook Pro, which I hadn't heard in months, soon started whirring. The activity monitor showed that about 95 percent of the CPU load was being consumed. As soon as I closed the site, the load dropped to about 9 percent. Besides putting a noticeable strain on my computer, the site also draws additional electricity from my office. The arrangement allows the attackers to reap the benefit of my hardware and electricity without providing anything to me in return. A recent report from security firm Trustwave's SpiderLabs estimated that the electricity cost for a single computer could range from about $2.90 to $5 per month, presumably if the cryptomining page was left open and running continuously over that time. The figure doesn't include the wear and tear on hardware as it performs complex mathematical problems required to generate the digital coins.

Thanks, Coinhive

The site that makes all of this possible is Coinhive.com, which Ars covered last week . It offers an easy-to-use programming interface that any website can use to turn visitors' computers into vehicles for generating—or in the parlance of cryptocurrency people, mining—Monero. Coinhive gives participating sites a tiny cut of the proceeds and pockets the rest. Coinhive doesn't require that sites provide any notice to users.

de Groot said that about 85 percent of the 2,496 sites he tracked are generating currency on behalf of just two Coinhive accounts. Depending on the total number of visitors, the amount of time they stay on an affected site, and the power of their computers, the revenue collected by those accounts could be considerable, as would be the total amount of additional charges those accounts made to visitors' electric bills. The remaining 15 percent were spread over additional Coinhive accounts, but de Groot has evidence suggesting those accounts are controlled by a single individual or group. Most of the affected sites concealed the connection to Coinhive by adding a link to the domain siteverification.online or one masquerading as a Sucuri firewall. Those disguised sites, in turn, hosted the crypto-mining JavaScript that interacted with Coinhive.

de Groot's findings suggest that drive-by cryptomining has grown more widespread in the week since Ars first covered it or at least that the phenomenon shows no signs of abating. The earlier Ars article cited research from security firm Sucuri that found 500 sites running hacked versions of the WordPress content management system that were participating in the Coinhive mining. Ars also reported that two Android apps with as many as 50,000 downloads from Google Play had recently been caught putting cryptominers inside hidden browser windows. On Wednesday, researchers from Ixia reported finding two additional such apps with as many as 15 million downloads combined. (In fairness, one of the apps informed users it would use their phone's idle time to generate coins and provided a way for that default setting to be turned off. The apps have since been modified to curtail the practice.)

There are other indications that the in-browser cryptomining racket is getting worse. In a report published Tuesday, endpoint security provider Malwarebytes said that on average it performs about 8 million blocks per day to unauthorized mining pages.

People who want to avoid these cryptojacking scams can use Malwarebytes or another antivirus program that blocks abusive pages, install this Chrome extension, or update their computer host file to block coinhive.com and other sites known to facilitate unauthorized mining. As the phenomenon continues to grow and attract copycat services, blocklists will likely have to be updated, requiring regular updates to blocklists as well.