[S]tarting in August 2006, Earthlink instead intercepts that Non-Existent Domain (NXDOMAIN) response and sends the IP address of ad-partner Barefruit's server as the answer. When the browser visits that page, the user sees a list of suggestions for what site the user might have actually wanted, along with a search box and Yahoo ads. The rub comes when a user is asking for a nonexistent subdomain of a real website, such as http://webmale.google.com, where the subdomain webmale doesn't exist.... In this case, the Earthlink/Barefruit ads appear in the browser....

[S]ubdomains are only as secure as Barefruit's servers, which turned out to be not very secure at all. Barefruit neglected basic web programming techniques, making its servers vulnerable to a malicious JavaScript attack. That meant hackers could have crafted special links to unused subdomains of legitimate websites that, when visited, would serve any content the attacker wanted. The hacker could, for example, send spam e-mails to Earthlink subscribers with a link to a webpage on money.paypal.com. Visiting that link would take the victim to the hacker's site, and it would look as though they were on a real PayPal page.