For seven days, hackers used Yahoo’s ad network to send malicious bits of code to computers that visit Yahoo’s collection of heavily trafficked websites, the company said on Monday.
The attack, which started on July 28, was the latest in a string that have exploited Internet advertising networks, which are designed to reach millions of people online. It also highlighted growing anxiety over a much-used graphics program called Adobe Flash, which has a history of security issues that have irked developers at Silicon Valley companies.
“Right now, the bad guys are really enjoying this,” said Jérôme Segura, a security researcher at Malwarebytes, the security company that uncovered the attack. “Flash for them was a godsend.”
The scheme, which Yahoo shut down on Monday, worked like this: A group of hackers bought ads across the Internet giant’s sports, news and finance sites. When a computer — in this case, one running Windows — visited a Yahoo site, it downloaded malware code.
From there, the malware hunted for an out-of-date version of Adobe Flash, which it could use to commandeer the computer — either holding it for ransom until the hackers were paid off or discreetly directing its browser to websites that paid the hackers for traffic.