This is the Backdoor:W32/Zapchast Family Description.
Zapchast variants use an mIRC client to create a backdoor that gives an attacker access to, and control over, the infected system. The client itself is detected as Riskware:W32/mIRC (also detected as Client-irc.win32.mirc). By itself, the mIRC client is not malicious. Zapchast and its variants can however use malicious configuration files (detected as Backdoor.IRC.Zapchast) to turn the mIRC-client into a backdoor.
The mIRC client and the necessary files (not all of them may be malicious) are usually dropped somewhere on the hard drive. A registry entry is then created, which starts the mIRC client every time the computer is started.
The mIRC client, which now functions as a backdoor, then joins a predefined IRC channel. Using this channel, the attacker can then issue commands to the backdoor and effectively control the infected system. Sometimes, Zapchast variants will use additional batch files which provide added functionality, such as performing registry changes to create a launchpoint for the backdoor. These auxiliary batch files are detected as Trojan.BAT.Zapchast.
The mIRC scripting language used to create the malicious configuration files enables the backdoor to do numerous tasks, such as downloading files, acting as a proxy or stealing information by logging keystrokes. The actual functionality of the backdoor depends on the variant in question.