Football fans who access free streams of top matches are putting their devices, and personal privacy, at great risk, according to a study.

It says the most popular sites are attracting upwards of eight million visits per month.

Like many free services, the pirate sites rely on advertising.

But with few reputable brands willing to attach their name to illegal distribution, the sites turn to malicious ads to pull in profits.

Of the thousands of streams studied, the researchers said that as many as half planted malicious software on the users’ machine through forced ads and other deceptive techniques.

As well as pop-up and overlay advertising, they observed an increase in sites demanding users install browser plug-ins in order to watch a free stream.

They said that meant not only were malicious ads appearing on the football sites, but the software was also hijacking normally safe websites.

“[To watch the stream] you have to install the extension, and once the user installs the extensions, it can infect any website the user is visiting,” lead researcher Zubair Rafique told the BBC.

“So, if a person installs an extension to watch a stream, and then visits a site like BBC.com, this extension can actually change the contents of BBC.com and include malicious links.

“This is extremely dangerous.”

“Additionally, we found that more than 64% of parties providing these streams have been reported at least once for violating the copyrights of content owners.”

In the UK, several internet service providers have been told by the courts that they must block certain websites. It makes it much more difficult for web users to reach those specific sites, but does little to stop others appearing elsewhere.

More on this story here : http://www.bbc.com/news/technology-35434765