ASP.NET in Microsoft .NET Framework 1.1 SP1, 2.0 SP2, 3.5, 3.5.1, 4, 4.5, 4.5.1, and 4.5.2, when the customErrors configuration is disabled, allows remote attackers to obtain sensitive configuration-file information via a crafted request, aka "ASP.NET Information Disclosure Vulnerability."
https://docs.microsoft.com/en-us/security-updates/securitybulletins/2015/ms15-041