This indicates an attack attempt to exploit an Information Disclosure Vulnerability in Apple WebKit.
The vulnerability is due to an error in the vulnerable application when handling a maliciously crafted webpage. An attacker can exploit this by tricking an unsuspecting user into visiting a malicious webpage and disclose sensitive information within the context of the application.
WebKit in iOS before 11.4
WebKit in Safari before 11.1.1
WebKit in iCloud before 7.5 on Windows
WebKit in iTunes before 12.7.5 on Windows
WebKit in tvOS before 11.4
WebKit in watchOS before 4.3.1
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Apply patches or fixes from the vendor, available from the website: