This indicates a padding oracle attack against DotNetNuke, an open source platform for building web sites based on ASP.net.
The vulnerability is caused by a design error when the vulnerable software handles multiple HTTP requests for a web resource. It allows a remote attacker to bypass security checking of vlunerable systems.
DotNetNuke based on Microsoft .NET Framework 4.0 and prior versions
System Compromise: Remote attackers can gain control of vulnerable systems.
Information Disclosure: Remote attackers can gain sensitive information from vulnerable systems.
Refer to the following web site for the suggested workaround: