By Jose Pagliery
October 15, 2014: 3:37 PM ET
NEW YORK (CNNMoney)
An especially sneaky type of hack is on the rise. Hackers can infect your computer by piggybacking on Web ads--even on trusted websites.
Hackers are slipping malware into legitimate-looking online advertisements. When you visit sites that serve those ads, you're automatically and unknowingly downloading computer viruses.
"Malvertising" has hit Amazon, Answers.com, Dictionary.com, Examiner.com, The Jerusalem Post, Last.fm, The Pirate Bay, The Times of Israel, Yahoo, and YouTube this year.
And it's blowing up. The number of malicious ads has nearly doubled every year since 2011, according to data from security firm RiskIQ. Its researchers have discovered 432,374 of them so far this year.
"The ad tech industry recognizes this is a serious problem," said Geir Magnusson, CTO of online ad platform AppNexus.
Malvertising makes up a microscopic fraction of the 5 trillion online ads displayed each year in the US alone, according to trackers at comScore. But that's still half a million times our computers could get infected.
Hackers have used malvertising to steal bank account information and lock up files to hold them for ransom.