"It's possible that this particular campaign is going after low quality traffic—but not necessarily bots —and rather than serving typical ads that might be wasted, they chose to make a profit using a browser-based Monero miner," they noted.
Five identical domains have been identified using the same CAPTCHA code but with different Coinhive site keys. At least two of them had over 30 million visits per month while the traffic combined from all five domains amounted to about 800,000 visits per day with an average four minutes spent on the mining page.
"We believe there are several more domains than just the few that we caught, but even this small subset is enough to give us an idea of the scope behind this campaign," Segura said. "It is difficult to determine how much Monero currency this operation is currently yielding without knowing how many other domains (and therefore total traffic) are out there. Because of the low hash rate and the limited time spent mining, we estimate this scheme is probably only netting a few thousand dollars each month.
"However, as cryptocurrencies continue to gain value, this amount could easily be multiplied a few times over."
Over the past few months, security experts have observed a steady rise in cryptojacking attacks, malware-based miners and browser-based cryptominers to ensnare the processing power of millions of devices to generate digital currencies without the knowledge or consent of users.
"Forced cryptomining is now also affecting mobile phones and tablets en masse—not only via Trojanized apps, but also via redirects and pop-unders," Segura noted. "While these platforms are less powerful than their Desktop counterparts, there is also a greater number of them out there.
Over the weekend, more than 4,000 websites in the US, UK, Australia and other nations were hijacked with hackers tweaking the code of a plugin named BrowseAloud to secretly mine cryptocurrency.
Some of the compromised websites included thousands of government ones including those of the Information's Commissioner's Office, the United States Courts, the Queensland Government's legislation, Student Loans Company, NHS services and more.
"Similar to what we see with IoT devices, it's not always the individual specifications, but rather the power of the collective group altogether that matters," Segura said.