The booming price of Bitcoin and other crypto-coins has kicked off a gold rush among malicious hackers keen to cash in, warn security firms.
Many sites are now harbouring code that secretly uses a visitors' computer to mine the valuable e-cash coins.
The code is inserted by hackers who exploit poor site security or web software bugs.
The problem has led Google engineers to consider putting protections against mining in the Chrome browser.
Freshly minted
Earlier this month, hundreds of websites were found to be running code created by the Coin-Hive project. This let the web domains generate coins for the Monero crypto-currency by using the processing power of visitors' computers.
The change would let users see when their browser was being used for mining and let them choose if it continued. Blocking the activity any other way would be difficult, he said.