The DNS infrastructure of the Internet plays a critical role in resolving host and domain names into IP addresses. A great deal of effort has gone into ensuring that DNS works efficiently and is resilient in the face of server failures, incorrect data, or malicious attempts to disrupt the system. But even with these safeguards in place, the system is still subject to attack.
The potential benefit for someone involved in Internet fraud is huge. If you can change the DNS records for a major bank so that they point to your fake site, then you can potentially capture the account numbers and passwords of anyone who logs into the system. This approach sidesteps the need to send out email messages that try to get users to log in, but it does require a high level of technical sophistication. Two approaches have been used: DNS Poisoning and Pharming .
DNS servers around the Internet keep their tables updated by querying other more authoritative servers. The structure is a hierarchy with the network root servers at its origin. In a DNS poisoning attack, DNS servers are manipulated to fetch updated, incorrect DNS records from a server that has been set up by the attacker. This is a sophisticated type of attack to which modern DNS servers are largely immune. But successful attacks do still take place, usually by exploiting bugs in the server software. In March 2005, the SANS Internet Storm Center reported one such attack in which users were redirected to sites that contained spyware, which was then downloaded to users' computers. A detailed report on this attack can be found at http://isc.sans.org/presentations/dnspoisoning.php.
Pharming is somewhat of an umbrella term for several different approaches to manipulating DNS records. Rather than going after DNS servers directly, an attacker may try to con a domain registrar into changing the authoritative DNS record for a domain to point to their fake site. Examples of this form of social engineering have included someone simply calling a registrar on the phone and persuading them that they represent the owner of the target domain.
One example of this involved the New York-based Internet service provider Panix. In January 2005, an attacker was able to transfer control of its DNS records to a server in the United Kingdom, with all company email being redirected to a server in Canada. Even though the problem was spotted quickly, the impact on the company and its customers was substantial.
Another form of attack takes advantage of the fact that most operating systems have a local file of hostname-to-IP-address mappings that will be queried before making a remote DNS query. If such a file contains a match, then that address will be used without any further lookups. This has been exploited by a computer virus called the Banker Trojan. In addition to logging user keystrokes, it adds lines to the end of a host file on a Windows system that will redirect users to fake bank sites. Many variants of this trojan have been found.
DNS is fundamental to the operation of the Internet and usually works so well that people take it for granted. Attacks like these are a reminder that all components of the Internet are vulnerable.
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