The light sensor is supposed to adjust the brightness of your screen, but boffins have found that it can capture images of your hands as you use your phone or tablet. They did this by using special videos on the screen that matched the data from the light sensor.

This means hackers could see what TV shows you are watching, what websites you visit, or even what passwords you are typing. The images are not very clear and take a long time to get, but they could still be used for targeted attacks.

Security expert Lukasz Olejnik, who warned about the light sensor before, said: "The light sensor is a privacy nightmare. It could be used for spying on chosen targets. But a new standard limits access to the light sensor, which is good news."

Yang Liu, a PhD student at MIT who did the research, said: "The light sensor is still a problem for Android apps. Some devices log the data from the light sensor in a file that anyone can see. We also found that we could get faster and better images by lowering the resolution. But this is not a big threat for smartphones, as the screens are too small. We just wanted to show how mobile devices can have hidden security risks."