This indicates an attack attempt against an Information Disclosure vulnerability in Microsoft Internet Explorer.
The vulnerability is due to improper validation of user supplied data when handling crafted HTTP Requests. A remote attacker can exploit this to gain unauthorized access to sensitive information on the affected machines via crafted packets.
Internet Explorer 9
Internet Explorer 10
Internet Explorer 11
Internet Explorer 11 on Windows 10
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems
Apply the most recent upgrade or patch from the vendor.