The MITRE CVE dictionary describes this issue as:
An issue was discovered in certain Apple products. iOS before 11.3 is affected. Safari before 11.1 is affected. iCloud before 7.4 on Windows is affected. iTunes before 12.7.4 on Windows is affected. watchOS before 4.3 is affected. The issue involves the fetch API in the "WebKit" component. It allows remote attackers to bypass the Same Origin Policy and obtain sensitive information via a crafted web site.
Find out more about CVE-2018-4117 from the
MITRE CVE dictionary dictionary and