Sirefef.ac is a Trojan horse that manipulates web traffic and redirects users to sponsored websites full of ads or infected sites that may install malware. Most common variant is Sirefef. All of the time, this Trojan horse redirects Google search results, but it may target any other website or even inject different content, usually ads. Sometimes scammers run their own ad network but it's rather difficult to find new customers so they try to steal money from already established companies that have weak anti-fraud mechanisms. Sirefef is one of the most widely spread malware family nowadays. Search redirect is a very common problem. You can easily find hundreds if not thousands different forum threads and blog posts complaining or describing this problem in one way or another. Usually, users complain that their web browsers are being redirected to spammy sites. At some point, Sirefef.ac and other variants of this Trojan is responsible for at least 30% of all redirects.
Sirefef.ac can be updated to even more sophisticated malware if needed. There are numerous other modules including spyware module that can be used to steal sensitive information or disable antivirus software. But scammers usually try to avoid antivirus detection rather than disable it. In such way they can be sure that malware stays undetected as long as possible or needed. This Trojan horse can be also updated manually targeting only selected computers. For example, computers from UK. As you can see, it's a very sophisticated and without any doubts profitable malware. What is more, there are no other visible symptoms of this infection other than web browser redirects. It doesn't even slow down the system. If your web browser is sometimes or very often redirect to shady websites full of ads than your computer can be infected with Sirefef.ac Trojan. To remove this Trojan from your computer, use an automatic spyware removal tool given below.