VBS_Badbun.A is Visual Basic script (VBS) that may arrive bundled with malware packages as a malware component.

It drops files. Trend Micro detects the dropped files as IRC_BADBUN.A, JS_BADBUN.A, and WORM_BADBUN.A. As a result, routines of the dropped malware are also exhibited on the affected systems.

This script also launches a flood attack against Web sites that are mostly related to antivirus companies. This action starts a distributed denial of service (DDoS) attack on the target sites.

Technical details can be found at this Trend Micro page.