Is the web browser on your phone slower than usual? It could be mining bitcoin for criminals.

As the popularity of virtual currencies has grown, hackers are focusing on a new type of heist: putting malicious software on peoples' handsets, TVs and smart fridges that makes them mine for digital money.

So-called "cryptojacking" attacks have become a growing problem in the cybersecurity industry, affecting both consumers and organizations. Depending on the severity of the attack, victims may notice only a slight drop in processing power, often not enough for them to think it's a hacking attack. But that can add up to a lot of processing power over a period of months or if it impacts a business's entire network of computers.

"We saw organizations whose monthly electricity bill was increased by hundreds of thousands of dollars," said Maya Horowitz, of the cybersecurity company Checkpoint.

Hackers try to use victims' processing power because that is what's needed to create — or "mine" — virtual currencies. In virtual currency mining, computers used to make the complex calculations verify a running ledger of all the transactions in virtual currencies around the world.

Cryptojacking is not done only by installing malicious software. It can also be done through a web browser. The victim visits a site, which latches onto the victim's computer processing power to mine digital currencies as long as they are on the site. When the victim switches, the mining ends. Some websites, including Salon.com, have tried to do it legitimately and been transparent about it. For three months this year, Salon.com removed ads from its sites in exchange for users allowing them to mine virtual currencies.

Rise in currency

Industry experts first noted cryptojacking as a threat in 2017, when virtual currency prices were skyrocketing to record highs.

The price of bitcoin, the most widely known virtual currency, jumped six-fold from September to almost $20,000 US in December before falling back down to under $10,000 US.

The number of cryptojacking cases soared from 146,704 worldwide in September to 22.4 million in December, according to antivirus developer Avast. It has only continued to increase, to 93 million in May, it says.

Hackers put malicious software on peoples' handsets, TVs and smart fridges to pull processing power to mine virtual currencies, like bitcoin. (Jaap Arriens/NurPhoto via Getty Images)

The first big case emerged in September and centred on Coinhive, a legitimate business that let website owners make money by allowing customers to mine virtual currency instead of relying on advertising revenue. Hackers quickly began to use the service to infect vulnerable sites with miners, most notably YouTube and nearly 50,000 Wordpress websites, according to research conducted by Troy Mursch, a researcher on cryptojacking.

Mursch says Monero is the most popular virtual currency among cyber-criminals. A report by cyber security company Palo Alto Networks estimates that over five per cent of Monero was mined through cryptojacking. That is worth almost $150 million US and doesn't count mining that occurs through browsers.

Spray and pray

In the majority of attacks, hackers infect as many devices as possible, a method experts calls "spray and pray."

"Basically, everyone with a [computer processing unit] can be targeted by cryptojacking," said Ismail Belkacim, a developer of an application that prevents websites from mining virtual currencies.

As a result, some hackers target organizations with large computing power. In what they believe might be the biggest cryptojacking attack so far, Checkpoint discovered in February that a hacker had exploited a vulnerability in a server that over several months generated over $3 million US in Monero.

Cryptojackers have also recently targeted organizations that use cloud-based services, where a network of servers is used to process and store data, providing more computing power to companies who haven't invested in extra hardware.

Abusing this service, cryptojackers use as much power as the cloud will allow them to, maximizing their gains. For businesses, this results in slower performance and higher energy bills.

Increased cryptojacking

Martin Hron, a security researcher at Avast, says that besides the rise in interest in virtual currencies, there are two main reasons for the rise in attacks.

First, cryptojacking scripts require little skill to implement. Ready-made computer code that automates cryptomining is easy to find with a Google search, along with tips on the vulnerabilities of devices.

Second, cryptojacking is harder to detect and is more anonymous than other hacks. Unlike ransomware, where victims have to transfer money to regain access to their computers blocked by hackers, a victim of cryptojacking might never know their computer is being used to mine currency. And as currency generated by cryptojacking goes straight into a hacker's encrypted wallet, the cyber-criminal leaves less of a trail.

Researchers says Monero is the most popular virtual currency among cybercriminals. (Kacper Pempel/Reuters)

Both Apple and Google have started to ban applications that mine virtual currencies on their devices. But Hron, the Avast researcher, warns that the risk is growing as more everyday devices connect to the internet — from ovens to home lighting systems — and that these are often the least secure. Hron said that cheaply made Chinese devices were particularly easy to hack.

Some experts say new techniques like artificial intelligence can help get a faster response to suspicious software.

That's what Texthelp, an education technology company, used when it was infected with a crypto-jacker, said Martin McKay, the company's chief technology officer. "The risk was mitigated for all customers within a period of four hours."

But security researcher Mursch says that these precautions won't be enough.

"They might reduce the impact," he says, "But I don't think we're going to stop it."