Researchers warn about a kind of malware that can deliver profits without being obvious to users.
The attack is the first major incident made public in which a new breed of hackers took over a large numbers of websites to effectively create currencies like bitcoin which are generated by using computing power.
The attacks made public over the weekend by British security researcher Scott Helme showed more than 4,000 website were infected in this manner, including those of the British data protection and privacy watchdog and the US federal courts system.
