Affected site clocking visits of 7 billion a month.
A huge malvertising attack on the advertising network run by Yahoo has alarmed security researchers due to the company’s website handling some 7 billion visits per month.
Hackers were said to have leveraged the Microsoft Azure website to deliver malware into the network from July 28 onwards, with ransomware such as CryptoWall thought to be one of the viruses the attackers may have been planning to spread.
Jerome Segura, senior security researcher at Malwarebytes, which uncovered the campaign, said: "Malvertising is a silent killer because malicious ads do not require any type of user interaction in order to execute their payload.
"The mere fact of browsing to a website that has adverts (and most sites, if not all, do) is enough to start the infection chain."
Malvertising works through infection of an advertising supply chain, putting infectious adverts into a legitimate network that lead to "drive-by downloads" that take place on clicking the advert or simply by browsing the site.
Infecting such networks can lead to many sites becoming harmful due the use of multiple advertising networks by many media groups, which leads to some security experts recommending users install script blockers, or even disable Flash and similar plugins entirely.
Following Malwarebytes report Yahoo confirmed that it would continue to investigate the problem, saying: "Unfortunately, disruptive ad behavior affects the entire tech industry.
"Yahoo has a long history of engagement on this issue and is committed to working with our peers to create a secure advertising experience."
Commenting on whether Yahoo could have prevented such attacks, Mark James, security specialist at ESET, confirmed they could have, but said it would be impractical and difficult to do.
"All companies strive for the most cost effective means to deliver content to the users that want it, they must look at costs both incoming and outgoing, from a cost point of view why invent the wheel when it comes to advertising," he added.