Browser

Browser-based worms, which primarily work against Internet Explorer, make use of vulnerabilities that exist in web-browsers. What generally happens is that when a users visits a malicious website, an exploit will make Internet Explorer download and execute code. As there are well known vulnerabilities in Internet Explorer at all times that are not yet fixed, the bad guys usually have a couple of days or weeks to spread their code. Of course, the infection rate heavily depends on the number of visitors on the website hosting the exploit. One approach that has been used in the past to gain access to a wider 'audience' involved sending mail to thousands of users in an attempt to get the users to visit a malicious website. Another approach involved hacking advertisement companies and changing their content in order to make them serve exploits and malware on high profile sites.