According to security researchers this vulnerability is being actively exploited since January 2018.
Vulnerable component: Apple iOS
CVSSv3 score: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H/E:H/RL:U/RC:C
CWE-ID: CWE-787 - Out-of-bounds Write
The vulnerability allows a remote attacker to compromise vulnerable system.
The vulnerability exists due to a boundary error when processing email in the iOS MobileMail. A remote attacker can send a specially crafted email message, trigger an out-of-bounds write and execute arbitrary code on the target system. No user interaction is required to execute arbitrary code.
Note, this vulnerability is being actively exploited in the wild.