Gribble is a trojan that has been available an a web page that uses a Java Virtual Machine vulnerability in Internet Explorer to drop and execute the worm.
Further infromation about the vulnerability, including a fix, is available from Microsoft: https://www.microsoft.com/technet/security/bulletin/MS00-075.asp
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When the trojan is executed, it creates "jb.vbs" file into the root of the current drive. This file is executed.
Afther that the trojan drops another VBS file, "C:\lipreffs.vbs". This file is executed as well when it adds itself to to the registry in a such way that it is executed when the system is restarted. It also starts to send large ICMP echo (ping) packets to the gnc.com.
Next Gribble goes trough all drives, and if it is able to locate a mIRC installation, it will create "script.ini" and modify the "mirc.ini" so, that it will be loaded when the IRC client is started.
Finally the trojan deletes "c:\jb.vbs".
The modified "script.ini" will attempt a Denial of Service attack agains grc.com when a connection is made with the mIRC client. DoS attack consists of random HTTP requests to either www.grc.com or grc.com, and garbage sent to the grc.com DNS server.
Further the trojan sends offensive messages when another user joins a channel where the infected user is.