ELMAH (Error Logging Modules and Handlers) is an application-wide error logging facility that is completely pluggable. It can be dynamically added to a running ASP.NET web application, or even all ASP.NET web applications on a machine, without any need for re-compilation or re-deployment. If ELMAH is not properly configured, the elmah.axd handler can be accessed without authorization. This page will list all the error messages generated by the web application and may disclose sensitive information to an attacker.
Adjust web.config to deny access to this entity without proper authorization.