XML_Badbun.A is malicious script that arrives as attachment to email messages spammed by another malware or a malicious user. It may be dropped by other malware. It may be downloaded unknowingly by a user when visiting malicious Web sites.

It accesses Web sites to download files. It also executes its downloaded files. As a result, routines of the downloaded files may also be exhibited on the affected system.

Technical details can be found at this Trend Micro page.