The next time you open up your laptop, you may want to cover up the built-in webcam with a piece of duct tape.

A computer science professor at John Hopkins University in Maryland says he’s found a way to turn on the cameras on some Apple computers without activating the green LED light that lets the user know he or she is being recorded.

“We were able to turn on the built-in camera on certain older model laptops and desktops while keeping that associated green indicator light turned off,” Stephen Checkoway told CTV News Channel Thursday.

Checkoway said he and one of his graduate students were able to covertly turn on the cameras in 2007-08 MacBooks and iMac computers.

“There’s no reason to assume that a similar technique couldn’t work for other computers, but we just don’t know,” he said.

Although Apple “went to great lengths” to ensure that the camera system would keep the green light on when the webcam is in use, Checkoway was still able to bypass the security system.

Checkoway said he doesn’t think it’s a matter of bad design on Apple’s part, but a “small mistake” that allowed him to hack into the cameras.

The good news is, worried computer users can simply place a sticker or tape over the tiny camera lens to protect their privacy, Checkoway said.

Keeping your computer software up to date will also help keep hackers and malware at bay, he said.