The report said that Fox IT revealed the threat. The researches of the company said that the users of the portal were redirected to websites with malware, exploiting vulnerability of Java, when they clicked advertisements.

Ashkan Soltani, a security researcher said that such attacks are “the result of hacking an existing ad network”. He added that the culprits may have simply submitted the malicious software as ordinary ads, sneaking past Yahoo’s system for filtering out malicious submissions.

Fox IT said that malicious payloads were being delivered to around 300,000 users per hour. The company guesses that around 9 percent of those, or 27,000 users per hour, were being infected.

Largest number of infected computers fell to share of Romania, Great Britain and France, the company underlined.

Fox IT said that Yahoo users have been getting infected since at least from 30 December 2013. More Recently, the firm said, the volume of infections has tapered off, perhaps due to efforts by Yahoo’s security team.

“At Yahoo, we take the safety and privacy of our users seriously,” a Yahoo spokeswoman said in a Saturday email to the Washington Post. “We recently identified an ad designed to spread malware to some of our users. We immediately removed it and will continue to monitor and block any ads being used for this activity.”