Be careful the next time you click that link to YouTube to watch the latest popular video – you might find yourself downloading some nasty spyware.

As the popularity of online video skyrockets, hackers are targeting them for all sorts of malware, including adware, spyware, and keyloggers. Some of the malware is downloaded via Windows codecs, the software that allows compressed files to be viewed.

According to a BBC report, Security analysts at Norman Sandbox are seeing more instances of spyware and adware companies producing bundles that they claim contain many codecs, but are booby trapped with malware.

Pop-up ads are the least of the problems caused by this malware; keyloggers can also be installed and go undetected by virus protection software. One security company found a codec that hid a program that then claimed to find problems with a user’s PC and solicited payment to get rid of them.

The malware often escapes detection by security software because the initial download is benign, but then initiates a download of malware after it’s been scanned and passed.