Disable Windows authentication for specific file in ASP.Net

Hi sds,

You need to set the location tag for allow access to anonymous users for error file.

<configuration>
    <system.web>
        <authentication mode="Windows" >
        </authentication>
        <authorization>
            <deny users="?" />
        </authorization>
    </system.web>
    <location path="error.htm">
        <system.web>
            <authorization>
                <allow users ="*" />
            </authorization>
        </system.web>
    </location>
</configuration>

For more details refer below link.

Authorization Permissions