Fitness trackers are a leaky haven of data for hackers, according to a new study that found if you want a device with a high security rating, you better buy an Apple Watch.

A study of seven Android fitness trackers by security firm AV-Test revealed varying levels of security vulnerabilities, with many of the devices lacking basic secure connections and tamper protection.

Apple Watch was evaluated with different criteria because of its operating system, but still scored a high security rating. Researchers found the device is “almost impossible to track” because of it mostly uses encrypted connections. But it does have some potential vulnerabilities.

Some of the Android fitness trackers’ security is so weak hackers were able to access or tamper with user data, a problem that was discovered by the same research firm during a study last year. Apple copy-cat Xiaomi scored the worst marks because it stores its entire data unencrypted on the smartphone.

The Pebble Time, Microsoft Band 2 and Basis Peak scored the highest marks among fitness trackers, with devices made by Runtastic, Striiv and Xiaomi having the weakest security.