These attacks use spoofed packets against amplifiers in order to overload the victim's connection. This is done by sending a single packet to a broadcast address with the victim as the source address. All the machines within the broadcast domain then respond back to the victim, overloading the victim's Internet connection. Since smurfing accounts for more than half the traffic on some backbones, ISPs are starting to take spoofing seriously and have started implementing measures within their routers that verify valid source addresses before passing the packets. Spyware products wrap other commercial products, and are introduced to machines when those commercial products are installed.
Trojans are one of the leading causes of breaking into machines.
If you pull down a program from a chat room, new group, or even from unsolicited e-mail, then the program is likely trojaned with some subversive purpose. For example, a trojaned login program might be programmed to accept a certain password for any user's account that the hacker can use to log back into the system at any time. Rootkits often contain a suite of such trojaned programs. A program designed to create Trojans. Some of these tools merely wrap existing Trojans, to make them harder to detect.