This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in Microsoft SharePoint.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when handling crafted HTTP Post request. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the application, via a crafted HTTP request.
Microsoft SharePoint Server 2010 Service Pack 1
Microsoft SharePoint Server 2010 Service Pack 2
Microsoft SharePoint Server 2013
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected site.
Apply the most recent upgrade or patch from the vendor.