From: Apple Product Security
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APPLE-SA-2016-07-18-2 iOS 9.3.3

iOS 9.3.3 is now available and addresses the following:

Calendar
Available for:  iPhone 4s and later,
iPod touch (5th generation) and later, iPad 2 and later
Impact:  A maliciously crafted calendar invite may cause a device to
unexpectedly restart [...]