Written by Feike Hacquebord and Chenghuai Lu
Recently we discussed in some detail a collection of rogue DNS servers, which are related to Zlob Trojans . Here we present additional evidence that these rogue DNS servers are used for fraud with pay-per-click services and for unauthorized personal and confidential information disclosure.
Domain Name System servers resolve human readable domain names to IP addresses that are assigned to computer servers on the Internet. Normally, when an Internet user types a Web address in the address bar of his Internet browser, “www.google.com” for example, a DNS server resolves that domain name to an IP address that is hosting the Google Web page. In this way, his computer knows where to fetch www.google.com. If a user mistypes the domain, e.g. “wwe.google.com”, the DNS server fails to resolve the domain and the user gets an error message.
Most Internet users automatically use the DNS servers of their ISP. DNS-changing Trojans silently modify computer settings to use foreign DNS servers. These DNS servers are set up by malicious third parties and they translate certain domains to fallacious IP addresses. As a result, victims are redirected to possibly malicious Web sites without them noticing it.
Rogue DNS servers can silently monitor the browsing habits of infected users for a long time because modified DNS server settings on victims’ computers may remain unnoticed when the rogue DNS servers work properly and do not drop requests. Apart from click fraud and personal information theft, controllers of rogue DNS servers can therefore launch very specific targeted attacks by giving different DNS replies to different infected users at different times.
Network of 600+ rogue DNS servers
Here we focus on a network of more than 600 (apparently) identical rogue DNS servers, which IP addresses are hardcoded in DNS-changing malware. As far as we know, all of these DNS servers get their Internet connectivity from US-based hosting companies Intercage and Pilosoft. It has been reported that the spread of the corresponding DNS-changing Trojans shows remarkable advanced technical and social engineering tricks [4,2].
The rogue DNS servers exhibit interesting behavior. We found that the DNS servers resolve most domains correctly at the times we queried them. However, they show deviating behavior as well, such as:
- Domains with typos (non-existent domain names) are resolved to IP addresses by the rogue DNS servers, where a normal DNS server gives back an error message.
- Some domain names known for hosting malware and C&C servers are resolved differently.
- A number of parked domain names are resolved differently.
- Some sub domain names used by advertising companies for registering clicks are resolved to foreign servers. This makes click fraud possible.
- Sub domains of some popular dating sites are resolved differently. This may lead to leaking of confidential information.
For non-existent domain names, the 600+ rogue DNS servers do not return the usual error message but they instead resolve the domain names to a malicious IP address. Whenever an infected user mistypes a domain name in his browser, he is shown adult Web sites. See  for details.
Clicks generated by other malware
Another interesting thing we found is that the 600 rogue DNS servers hijack some known bad domain names that host malware or C&C servers. Resolving bad domain names differently has the result that other malware, which might be present on the victim’s computer, may work in another way than they were originally designed. In particular, a built-in update function of a Trojan that polls a Web site for updates may now generate automated clicks on adult Web pages. See  for more details.
Parked domain names
Parked domain names are inactive Web sites with no real content except for advertisements. Specialized companies have thousands of these parked domain names. Because of the large volume, parked domains may get substantial traffic from Internet users who attempt to visit an old and no longer existing Web page or who mistype domain names.
The network of 600+ rogue DNS servers appear to resolve a number of parked domains names differently so that infected users can not load advertisements of the companies who own the parked domains. Instead the infected users are shown advertisements from a foreign Web site.
We have seen that parked domains owned by Sedoparking, Hitfarm, Domainsupport and Fastpark get resolved to foreign IP addresses by the 600+ rogue DNS servers.
Click fraud through a sub domain vulnerability
Some advertising companies use several sub domains to register clicks that get generated by Web sites, which show advertisements to their visitors. Whenever a user loads an advertisement, the click gets registered by e.g.
In both cases, the owner of Web site A gets paid for the click by the advertising company.
The controller of a rogue DNS server can take advantage of the use of multiple sub domains by resolving one of the domains to a foreign IP address. The foreign server changes the affiliate tag and redirects the victim to another sub domain, which gets resolved normally. The end result is that the click is registered as if it was generated by another Web site.
We have seen this kind of click fraud targeting advertising companies like Ccbill Inc, ValueClick Media (Fastclick.net), WP Associates (Webpower.com), Alexa, Penthouse Media Group Inc. and a number of pornography distributors. See the table presented below.
Table 1: Whenever a user infected with a Zlob-related DNS changer Trojan loads an advertising link pointing to, for example, refer.ccbill.com, he gets directed to foreign IP address 18.104.22.168. The foreign server changes tracking tags and then lets the user load an advertising link at ref.ccbill.com with the tracking tags changed
More dangerous applications of rogue DNS servers
So far we gave examples where rogue DNS servers are used for click fraud. Rogue DNS servers can be used for more harmful attacks like stealing personal information.
We found that Internet users of a number of popular dating sites are vulnerable for leaking personal information to third parties when they are infected with DNS-changing malware. The popular dating site Friendfinder accepts login information on both friendfinder.com and www.friendfinder.com. The 600+ rogue DNS servers appear to make use of this by resolving friendfinder.com to a foreign IP address and www.friendfinder.com normally. An Internet user who wants to log in on the dating site usually sends his login information to http://friendfinder.com/p/login.cgi. When he is infected with a DNS-changing Trojan, his login information is sent to a foreign copy of friendfinder.com. The foreign server accepts the login information and redirects the user to www.friendfinder.com. This has the effect that personal login information of victims gets leaked to an unknown third party without the victims noticing it.
The affected dating sites claim to have tens of millions of registered users. Login information of these registered users is probably valuable for social engineering and targeted attacks.
Figure 1. An Internet user infected with a DNS changer Trojan visits a Web site. Advertisements on the web site are fetched via a malicious foreign server instead of directly from an ad server. The foreign server changes ad tags so that the wrong party gets paid for showing the advertisement.
Figure 2. An Internet user infected with a DNS changer Trojan logs in on friendfinder.com. The login information is sent to a foreign server. This foreign server sends the login info to www.friendfinder.com, so that the infected user does not notice he has leaked his personal information to a third party.
Rogue DNS-changing Trojans and their corresponding rogue DNS servers are serious threats for Internet users. In this article, we gave concrete examples of how a collection of 600 rogue DNS servers is being used for click fraud and personal information theft. The fact that there is a large cluster of well-connected identical rogue DNS servers and the advanced methods to spread the DNS-changing malware strongly suggest that bad guys are making a lot of profit by deploying their rogue DNS servers.