Hacking ISP Error Pages
This is a big deal:
At issue is a growing trend in which ISPs subvert the Domain Name System, or DNS, which translates website names into numeric addresses.
When users visit a website like Wired.com, the DNS system maps the domain name into an IP address such as 126.96.36.199. But if a particular site does not exist, the DNS server tells the browser that there’s no such listing and a simple error message should be displayed.
But starting 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 (unlike, say, mail in mail.google.com). In this case, the Earthlink/Barefruit ads appear in the browser, while the title bar suggests that it’s the official Google site.
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.
Kaminsky demonstrated the vulnerability by finding a way to insert a YouTube video from 80s pop star Rick Astley into Facebook and PayPal domains. But a black hat hacker could instead embed a password-stealing Trojan. The attack might also allow hackers to pretend to be a logged-in user, or to send e-mails and add friends to a Facebook account.
Earthlink isn’t alone in substituting ad pages for error messages, according to Kaminsky, who has seen similar behavior from other major ISPs including Verizon, Time Warner, Comcast and Qwest.