This indicates an attack attempt to exploit a Cross Site Scripting vulnerability in Microsoft IIS Server.
The vulnerability is due to insufficient validation of user supplied data in the application. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the application, via a crafted request.
Windows Vista Service Pack 2
Windows Vista x64 Edition Service Pack 2
Windows 7 for 32-bit Systems Service Pack 1
Windows 7 for x64-based Systems Service Pack 1
Windows Server 2008 for 32-bit Systems Service Pack 2
Windows Server 2008 for x64-based Systems Service Pack 2
Windows Server 2008 for Itanium-based Systems Service Pack 2
Windows 8.1 for 32-bit Systems
Windows 8.1 for x64-based Systems
Windows Server 2012
Windows Server 2012 R2
Windows 10 for 32-bit Systems
Windows 10 for x64-based Systems
Windows Server 2016 for x64-based Systems
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user's browser
Apply the most recent upgrade or patch from the vendor.