The short version: Some computer scientists have found a way to get web servers to do calculations based on the error detection features of the TCP/IP protocol. There’s no need to hack the web server or take advantage of some kind of bug or misconfiguration in the server. Simply because of the way the TCP/IP protocol works, the web server’s error detection calculations will give a client information about a problem the client is trying to solve.
You can read the full article in the 17 Nov 2001 issue of Science News. Below is an edited/condensed version of the article. The full article also has as short description of how the TCP protocol does error detection.