It is thought that tens of thousands of people could have caught been out by malicious ads on popular websites recently.

Cyber criminals racked up the victims by compromising the computers used by ad firm Unanimis, displaying ads to popular websites.

These ads appeared on the websites of the London Stock Exchange, Autotrader, the Vue cinema chain and six other websites.

Unanimis said it is now investigating how the criminals were able to inject their malicious ads into its feed.

The ads exploited vulnerabilities in software used on Windows PCs to make it appear like a machine had been attacked by a virus.  Users were redirected to a bogus screen claiming that their PC had been infected.  They were then asked for a payment to remove the “infection”.

Because Unanimis serves ads to thousands of websites, cyber criminals are able to spread these malicious attacks across a large number of legitimate websites without having to go directly through each of the websites, but rather by using a malicious ad.