Anyone using Apple Inc.’s iPhones and iPads should immediately update to the latest version of iOS 9 to ward off a security flaw that apparently can turn the device into a surveillance tool.

Apple updated its mobile operating system to version 9.3.5 after security researchers said they discovered previously unknown iOS bugs were being used to help foreign governments spy on their citizens. The software was able to get past Apple’s rule that only approved software may run on the device by exploiting iOS flaws to “jailbreak” the device.

The spyware can track movements, log messages and download personal data. In a statement Thursday, Apple said it had been “made aware of this vulnerability and immediately fixed it.”

The discovery of sophisticated spyware called “Pegasus” running on iPhones pierces the perception that Apple’s flagship device is immune from the troubles plaguing personal computers, Android phones and other consumer electronics.

It’s another reminder that keeping software up-to-date isn’t just good hygiene for hardware performance, but for keeping devices safe from unsavory tools. Even for Apple users. (That’s not to say you should run “beta” software, as Joanna Stern points out, just the latest version released to the public.) And even people who have armored up with best practices such as two-factor authentication still need to be mindful of the links and images they click and tap.

Apple devices run more-current versions of their OS by far when compared with Android counterparts. While that is due to Apple’s closer control of the devices running iOS, the Android camp—hardware makers, carriers and Google—needs to work harder to get the latest versions of Android out sooner. Version 7.0, called Nougat, is rolling out now, but only to Nexus- and Pixel-branded Google Android devices.

wsj.