An Estonian man has been sentenced to seven years and three months in prison in the U.S. for his role in a cybercriminal operation that infected over 4 million computers with DNS hijacking malware.Vladimir Tsastsin, 35, from Tartu, Estonia, was one of the key players in a US$14 million click fraud scheme. He is the sixth individual to be sentenced in the case and has received the longest prison sentence. The sentence was handed down Tuesday in U.S. District Court for the Southern District of New York.According to the Department of Justice, between 2007 and 2011, Tsastsin and his co-conspirators set up companies that masqueraded as publisher networks and entered into agreements with advertising brokers to display ads on their properties.In order to earn more money, the group artificially increased the number of user clicks and views for the ads they displayed by installing a malware program called DNS Changer on unsuspecting users' computers.It's estimated that DNSChanger infected over 4 million computers worldwide, including 500,000 in the U.S., before its operation was shut down by the FBI.As its name implies, the DNSChanger malware altered the computer's DNS (Domain Name System) settings, forcing them to use DNS resolvers run by the attackers.The DNS is the Internet's phonebook, translating domain names into numerical Internet Protocol (IP) addresses that computers use to communicate.\u00a0By controlling the DNS servers used by infected computers, the attackers could point users to other websites when they wanted to reach specific domain names. This capability was used to perform click fraud.Tsastsin and his co-conspirators used DNSChanger to redirect users who clicked on links in search results to websites that they didn't intend to visit. They also replaced the advertisements that users should have normally seen on various websites with ads for which they were being paid.While DNS hijacking was used for click fraud in this case, the technique is very powerful and can be used for more serious attacks against users, such as phishing.For example, when asked for an online banking website's IP address, a rogue DNS server could respond with the address of a Web server hosting a rogue copy of the website. Despite this, the domain name showing up in the user's browser would be the correct one, making it hard to tell that a phishing attack was in progress.This kind of attack happened in 2014 in Poland, but instead of using malware to change the DNS settings of individual computers, the attackers compromised home and small business routers and changed the DNS settings for entire networks.Most computers and mobile devices are configured to automatically obtain their Internet connection settings from the router serving the network they're connected to. This makes\u00a0routers an attractive target for DNS hijacking attacks.Over the past several years security researchers have observed many large-scale attacks designed to change the DNS settings on routers, either by exploiting vulnerabilities in their firmware or by hijacking users' browsers when they visited compromised websites.Router owners should periodically check their device manufacturer's website for firmware updates and install them, especially if they contain security fixes. They should also change the default administrator password on their routers and make sure to always log out from the router's management interfaces after accessing it through a browser.