Online ads are more than just annoying—according to Edward Snowden, they’re also dangerous.

“Everybody should be running adblock software, if only from a safety perspective,” Snowden, a former NSA contractor who in 2013 leaked a trove of secret documents about the National Security Agency, told The Intercept in a wide-ranging interview.

“As long as service providers are serving ads with active content that require the use of Javascript to display,” he added, “that have some kind of active content like Flash embedded in it, anything that can be a vector for attack in your Web browser—you should be actively trying to block these.”

Snowden is right. Online advertising is sometimes used to serve malware to unsuspecting users. In fact, advertising malware rates have tripled in the past year, according to cybersecurity firm Cyphort.

It’s not just sketchy sites selling off brand medication or discount electronics that have ads serving malware. Even popular sites like Forbes, the Economist, and Yahoo have recently suffered instances of ads serving malware to readers.

Not only can these online ads be hosts for viruses, but they also often attach so-called ‘cookies’ to readers, allowing the advertising company to track users across the Web. If you’re not using ad-blocking software, you are late to the party. According to a report published earlier this year by Adobe and PageFair, almost 200 million Web users are actively using ad-blocking software.

Online is all about identifying and eliminating potential weaknesses. Blocking ads is a good way to start.

H/T The Intercept | Illustration by Max Fleishman