Yahoo ads were infected with malware for nearly a week before anyone noticed
Yahoo's banner adverts were infected with malware for a week before it was noticed, when hackers launched an attack on 28th July. According to Malwarebytes, the attack involved hackers buying ads on Yahoo's more popular websites, like news and sports, and targeted Windows users using older versions of Adobe Flash, downloading a malicious malware code onto their desktops. Users simply had to visit the page to become infected, and it was not dependent on a user interacting with an ad.
For nearly a week, nobody from Yahoo noticed that its banner ads were inflicting people’s computers with malware. The New York Times eeports that hackers infiltrated banner ads on Yahoo’s ad network using an Adobe Flash vulnerability beginning on July 28 until yesterday.

Researchers from security firm Malwarebytes said that the hackers bought ads on Yahoo’s family of highly trafficked websites, like news and sports, and targeted Windows users using older versions of Adobe Flash. Then, a malicious malware code was automatically downloaded to their desktops that was used to remotely control the computer, like locking people out until they paid the hackers.