A muffled voice hidden in a YouTube video can issue commands to a nearby smartphone without you even knowing it, researchers reveal.

According to Micah Sherr, professor at Georgetown University, voice recognition has taken off quickly on phones thanks to services like Google Now and Apple's Siri but voice software can also make it easier to hack devices. 

“It might not work every time but it's a number's game. If a million people watch a kitten video with a secret message embedded, 10,000 of them might have their phone nearby. If 5,000 of those load a URL with malware on it, you have 5,000 smartphones under an attacker's control,” pcworld.com quoted Sherr as saying. 

If the hackers know the ins and outs of the voice-recognition software itself and know its internal workings, they can create voice commands that are even harder to decipher by humans. 

To guard against the threat, developers of voice recognition software could incorporate filters to differentiate between human and computer-generated sounds, the report added.