The way you tilt your mobile while you're using it could allow hackers to work out your pin numbers and passwords, according to new research.
Experts at Newcastle University analysed the movement of a smartphone as the screen was used.
The team of cyber-experts claim tech companies know about the problem but can't figure out what to do about it.
Dr Maryam Mehrnezhad, from the university's school of computing science, said: "Most smartphones, tablets, and other wearables are now equipped with a multitude of sensors (gyroscope, rotation sensors, accelerometer, etc).
"But because mobile apps and websites don't need to ask permission to access most of them, malicious programmes can covertly 'listen in' on your sensor data."
The research suggests there's a problem in the tech industry because of the number of different sensors used by competing companies.
Dr Mehrnezhad said: "On some browsers we found that if you open a page on your phone or tablet which hosts one of these malicious codes and then open [another one], then they can spy on every personal detail you enter.
"And worse still, in some cases, unless you close them down completely, they can even spy on you when your phone is locked.
"People were far more concerned about the camera and GPS than they were about the silent sensors."
The team said it was able to identify 25 different sensors which come as standard on most devices.
The researchers found that everything you do - from clicking, scrolling and holding to tapping - led to people holding their phone in a unique way.
So on a known webpage, the team was able to work out which part of the page the user was clicking on, and what they were typing, by the way it was tilted.
They said they'd told all the major tech companies, like Google and Apple, about the risks but no-one has been able to come up with an answer so far.
The team is now looking at the risks around personal fitness trackers linked to online profiles.