Ahead of next month's unveiling of iOS 10, Apple quietly issued "an important security update" to its operating system, pushing out iOS 9.3.4, addressing a memory corruption issue that blocks IOMobileFrameBuffer, a bug that could enable an app “to execute arbitrary code with kernel privileges.” 

This bug was injected by Team Pangu, a Chinese hacker gang, whose jailbreak tools enable users, willing to forgo Apple warranties, to install unauthorized apps.

The upgrade affects iPhone 4s and later, iPad 2 and later, and iPod touch (fifth generation) and later.

As Apple's previous upgrade, 9.3.3, followed five beta versions and two months of user feedback, it was widely expected to be the last release before the launch in September of iOS 10. 

Apple recommended in its release notes that all users download iOS 9.3.4. The patch also fixes issues with the dictionary and battery widgets.