This indicates an attack attempt to exploit a Cross Site Scripting vulnerability in Microsoft SqlServer Reporting Services.
The vulnerability is a result of the application's failure to properly sanitize user input before using it in a HTTP request. As a result, a remote attacker can send arbitrary script code to a vulnerable server.
Microsoft SQL Server Reporting Services Build 11.0.2100.60
System Compromise: Remote attackers can execute arbitrary script code within the context of the application.
Apply the most recent upgrade or patch from the vendor.