Malware
Malware authors target bugs, or loopholes, to exploit. A common method is exploitation of a buffer overrun vulnerability, where software designed to store data in a specified region of memory does not prevent more data than the buffer can accommodate being supplied. Malware may provide data that overflows the buffer, with malicious executable code or data after the end; when this payload is accessed it does what the attacker, not the legitimate software, determines.
Shnakule
Shnakule is the biggest malware network in existence. It averages over 2000 hosts and has had as many as 4357. It targets users while visiting trusted sites and routes them to malware by using relays, exploits, and payload servers. These are continually shifted to new domains.