Experts at Newcastle University analysed the movement of a smartphone as the keyboard was used.
They say they cracked four-digit pins with 70% accuracy on the first guess and 100% by the fifth guess.
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.
