Yahoo’s ad network has been attacked by so-called ‘malvertising’ hackers according to online security firm Malwarebytes.

Malwarebytes spotted the activity – which targeted ads across Yahoo.com, and the firm’s sports, finance, celebrity and gaming sites – on 28 July and described it as “one of the largest malvertising attacks we have seen recently”.

According to data from SimilarWeb, Yahoo’s main website attracts an estimated 6.9 billion visits per month.

Malwarebytes said that Yahoo had been targeted by a hacker group with a record for recent high profile malvertising campaigns, which have exploited weaknesses using something called ‘The Angler Exploit Kit’. The malware software targets users who click on ads by redirecting them to an imposter website that then affects the user’s computer with malware.

Malwarebytes said it contacted Yahoo as soon as it detected the malicious activity and the technology giant immediately took action.

Yahoo issued a statement, which read: “Yahoo is committed to ensuring that both our advertisers and users have a safe and reliable experience.

“As soon as we learned of this issue, our team took action and will continue to investigate this issue. Unfortunately, disruptive ad behaviour 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.”

In a blog post published on Monday evening, Malwarebytes wrote: “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.”