This indicates an attack attempt against a Cross-Site Scripting vulnerability in modules of Microsoft ASP.NET MVC.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted web page. A remote attacker may be able to exploit this to inject arbitrary web script or HTML via a crafted web page.
Microsoft ASP.NET MVC 2.0
Microsoft ASP.NET MVC 3.0
Microsoft ASP.NET MVC 4.0
Microsoft ASP.NET MVC 5.0
Microsoft ASP.NET MVC 5.1
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.