PornHub users may be looking over their shoulders for another reason, after news emerged that cybercriminals have been targeting the website.

Millions of visitors to the site may have been exposed to the Kotver malware, which generates revenue by clicking on ads in the background, with users left oblivious.

Know as a 'malvertising' attack, it could have easily delivered more malicious ransomware or information gathering software instead, according to Daily Mail.

Computer security experts from Sunnyvale-based Proofpoint first raised the alarm about the hack attack.

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Countries most heavily hit over the more than year-long campaign include the US, UK, Canada, and Australia.

Users install what they believe is an update to popular browser related software like Chrome, Firefox and Adobe's Flash player.

Instead, their systems are infected by the virus.

Following notification from Proofpoint, PornHub and the Traffic Junky advertising network worked to remove the infected content and keep visitors safe.

Kevin Epstein, vice president of threat operations at Proofpoint: "This campaign uses clever social engineering to trick users into installing fake updates that appear as soon as they visited a page containing a malicious ad.

"Once users clicked on what they thought was an update file, they may not have even noticed a change in their systems as the malware opened an invisible web browser process, clicked on ads, and generated potential revenue for cybercriminals.

"We encourage consumers to run anti-malware security solutions to ensure systems are clear and organisations to update web gateways to detect related traffic."

In a written statement to MailOnline Corey Price, Pornhub vice president, added: "Pornhub's commitment to providing their viewers with an optimal online experience has made security a top priority, allowing us to respond quickly to cybercrime and safeguard our customers.

"Over the course of the past year, we've taken several measures to further ensure the safety of our users.

"We announced a bug bounty program through HackerOne to reward researchers that find security bugs on our platform with bounties as high as $25,000.

"This program has been extremely successful thus far, providing some of our savvy fans with a chance to earn some extra cash.

"More importantly, it ensures the safety of our 80 million daily visitors.

"Additionally, we went all-in on encryption and switched to HTTPS by default across the entirety of our site to help ensure our users' privacy and offer heightened security against hackers and malware."

This is not the first time that visitors to porn sites have been warned about the potential dangers of their online activities.

In September, researchers discovered that watching mobile porn on your smartphone puts you at much higher risk of having your data leaked than watching it on your PC.

Experts from Wandera, a London-based mobile phone consultancy, looked at the websites that are most likely to contain malware, and found that the adult apps were also the most likely to have malicious bugs.

They examined content viewed on 10,000 mobile devices across the US and UK.

They discovered that 34 out of every 10,000 devices are accessing inappropriate content on a daily basis.

A further analysis of the results showed that inappropriate mobile activity was highest on Fridays, followed by Thursdays, while Monday was the least popular day for inappropriate mobile activity.

In terms of time of day, inappropriate usage was found to increase from 8pm, peaking at around 2-3am, and remaining low throughout the working day.

Gambling, cam, adult and ad networks were found to be by far the biggest risks for mobile users.