http://www.eweek.com/article2/0,1759,1582536,00.asp

A new worm has been detected by the virus research community which spreads through the LSASS vulnerability in various versions of Windows. Researchers and security companies are alarmed, and have assigned an elevated threat level to the worm-named Sasser.A-even
though it has not yet spread far.

Unlike the Gaobot variant found several days ago which exploited the same hole in Windows, Sasser can spread and infect automatically through the hole. Gaobot is technically a "bot," meaning that it has to be manually run by the user. The vulnerability was patched weeks
ago by Microsoft.