This indicates an attack attempt against an Information Disclosure vulnerability in Apple Webkit.
The vulnerability is caused by an error when the vulnerable software handles a specially crafted HTML file. A remote attacker can exploit this to gain unauthorized access to sensitive information.
Apple iOS before 8.3
Apple Safari before 6.2.5
Apple Safari 7.x before 7.1.5
Apple Safari 8.x before 8.0.5
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Refer to the vendor web site for suggested workaround.