UPDATE: A patch has been issued in iOS 13.4.5 beta, with an expected final release soon. No word on patches for earlier iOS versions.
A zero-day exploit has been discovered in the iOS Mail app. The security hole has existed as far back as iOS 6 (September 2012), and extends to the current iOS (13.x).
As of today (4/22/2020) this has NOT been patched. It is recommended that you DISABLE iOS mail at this time.
We advise that you update as soon as an iOS patch is available.