Cryptojacking – the act of hijacking a user’s computer to mine cryptocurrencies – is likely to “become a regular source of income for website owners,” a British government agency warned Tuesday.

The U.K.’s National Cyber Security Centre, the technology wing of the Government Communications Headquarters, highlighted cryptojacking as a “significant” concern in its latest “cyber threat to UK business” report.

In particular, the report singled out websites which mined cryptocurrencies without users’ permission, noting that 55 percent of businesses worldwide suffered from cryptomining attacks last December. The report specifically referred to monero, which has frequently been mined through browser application Coinhive, as one example.

Nor have the attacks waned in 2018. The report noted that more than 4,000 websites secretly mined a cryptocurrency using a plugin for visually-impaired users. The report noted that “the only way users may notice their devices are being cryptojacked is a slight slowdown in performance.”

The document continued:

“The technique of delivering cryptocurrency miners through malware has been used for several years, but it is likely in 2018-19 that one of the main threats will be a newer technique of mining cryptocurrency which exploits visitors to a website.”

The agency added that “we assume the majority of cryptojacking is carried out by cyber criminals, but website owners have also targeted visitors to their website and used the processing power of visitors’ CPUs, without their knowledge or consent, to mine cryptocurrency for their own financial gain.”

That being said, some sites may mine cryptocurrencies with their users’ permission, the report noted, citing Salon as an example. The publication announced in February that it would let its readers choose between advertisements and cryptocurrency mining in order to access its content, as previously reported. However, the report noted that this was only a trial.

The report advised users to use ad blockers and anti-virus programs which include browser mining blocks to prevent their computers from being hijacked.

Monero image via Sharaf Maksumov / Shutterstock