This indicates an attack attempt to exploit a Cross Site Scripting vulnerability in Microsoft ASP.NET Frame work.
The vulnerability is due to an error when the vulnerable software attempts to handle a maliciously crafted webpage. An attacker can exploit this by tricking a user into visiting a malicious webpage and execute arbitrary script code within the context of the application.
ASP.NET Framework 2.0 or prior version
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