These are not so big but very powerful programs, which provide to users full IRC service. Besides it these clients have a lot of additional features like scripts and automatic react to a variety of events that occur on IRC when people join and leave the channel they are on, enter some words and so on. It is possible also to reject a user from the channel, to send a private message to any user, to send a message only to users that join or leave channel, e.t.c. The first report about IRC worm was registered at the end of 1997 from mIRC users. Script worms are very simple and in some days there were several new worms created, based on original one.
Every worm has the same principle of operation.
This infected script files substitutes original one in the target remote system, and after reloading the affected IRC client software will carry the worm code and send infected script to other victims. Because of these features all kinds of received files are saved in client's home directory. These scripts are automatically loaded by client and used as main script during client is running. There are also special commands to start DOS and Windows executable files and internal command in client's script languages. This feature makes able to appearance new generation of IRC worms, which infects scripts and also drops executable viruses and trojans.