SWF_Dloader.YVN is a malicious Shockwave Flash (.SWF) object that may be downloaded by SWF_DLOADER.YVM. It may also be downloaded from certain remote sites, or hosted on a Web site and run when a user accesses the said Web site.

It is a specially crafted .SWF file that exploits a zero-day vulnerability found in Adobe Flash Player.

When executed using Flash player, it then executes arbitrary codes which leads to the accessing, downloading, and execution of malicious files from certain Web sites. Trend Micro detects the downloaded files as TROJ_WIESSY.J and WORM_OTWYCAL.BO. As a result, routines of the downloaded Trojan and worm are also exhibited on the affected system.

Technical details can be found at this Trend Micro page.