source: https://www.securityfocus.com/bid/149/info

Microsoft IIS and other NT webservers contain a vulnerability that allows remote users to obtain the source code for an ASP file. When one appends ::$DATA to an asp being requested, the ASP source will be returned, instead of executing the ASP. For example: http://xyz/myasp.asp::$DATA will return the source of myasp.asp, instead of executing it.

The following proof of concept was provided:

http://xyz/myasp.asp::$DATA

This will cause IIS to disclose the contents of the page, myasp.asp, to an attacker.