All virtual currency transactions are recorded on public ledgers called blockchain, which are managed jointly by PC users around the world. People can obtain the digital coins through mining — a process where remuneration is earned by adding new transaction records to the ledgers.

In many cases, high-performance PCs are used for this mining operation because it involves a huge volume of data calculation. There is also a method called pool mining, in which multiple PCs share the same task.

The illicit websites in question abuse this pool mining method. PC or smartphone users are directed to the illicit websites without their knowledge if they browse certain fake websites or ads. If their device’s operating software or apps have flaws, the device then automatically downloads a malicious program, which starts the mining operation. Any digital coins that are obtained are sent to the operators of the illicit websites.

PCs that run this mining program see a drop in their performance speed in some cases, but it is difficult for users to tell if their PCs are being used for mining.

Information security firm Trend Micro Inc. in Tokyo confirmed that there were a total of 1,749 such illicit websites from July to September, about 12 times the figure in the April-June period.One bitcoin, which had a value of about ¥100,000 at domestic exchanges at the beginning of this year, topped ¥1 million for the first time on Nov. 26 and ¥2 million earlier this month.

“If your PCs are used without your permission, that means you are forced to join in a plot by an illicit website operator to gain revenue,” a Trend Micro official said. The official called for keeping software up-to-date at all times, saying, “There is also a danger of viruses entering your PC.”