Security analysts are warning IT managers and users about an email hoax that is playing off people's concerns about the Blaster worm.

A new backdoor Trojan, named the Graybird-A, is being disguised as a patch for the Microsoft Windows vulnerability that the Blaster worm has been exploiting for the past week. The bogus patch is coming attached to an email.

Microsoft has issued a statement warning users that the company never delivers software directly via email. Patches can be downloaded from the official Microsoft Web site or from CD-Roms or floppy disks.


''Packaging Graybird as a Microsoft patch is a very devious trick,'' says Chris Belthoff, a senior security analyst at Sophos, Inc., an anti-virus and security company. ''Blaster is believed to have infected hundreds of thousands of computers around the world, and this is a deliberate attempt to exploit users' panic. Never trust unsolicited executable code that arrives via email. Businesses should consider blocking all executable code at the email gateway so it cannot reach their users.''

Users will be getting its Windows patches from a different Microsoft Web address than usual.

Because of the Blaster worm, Microsoft also has killed off its windowsupdate.com Web site. The Blaster worm was launching a distributed denial-of-service attack (DDoS) through infected computers against the windowsupdate.com Web site starting on Saturday, Aug. 16.

Instead of suffering through the attack or trying to ward it off, Microsoft simply shut down the address.

Users can access Windows patches via the www.Microsoft.com site or the new http://windowsupdate.microsoft.com site.

''Users should not think this step means they no longer have to do anything about the Blaster worm,'' says Graham Cluley, senior technology consultant for Sophos. ''All computer users still have a responsibility to ensure this worm has no hiding place on their PCs. So, install the patch from Microsoft, ensure your firewall is properly configured, and confirm your anti-virus is up-to-date.''

The Blaster worm was first detected on Aug. 11. It quickly spread from machine to machine across the globe through a flaw in the Windows operating system. But the worm doesn't carry a destructive payload, only causing a small percentage of infected computers to reboot because of a flaw in its own coding.

Instead, Blaster, otherwise known as LovSan and Poza, is specifically aimed at causing trouble for Microsoft. The worm is geared to harvest as many vulnerable systems as possible and launch a DDoS attack on the windowsupdate.com Web site. By focusing all the net congestion on that Web site, the author of the worm was deliberately trying to make it difficult for IT managers and individual users to download the patch they need to secure their systems against the worm.

Blaster exploits a flaw in the Remote Procedure Call (RPC) process, which controls activities such as file sharing. The flaw enables the attacker to gain full access to the system. The vulnerability itself, which affects Windows NT, Windows 2000, Windows 2003 and Windows XP machines, affects both servers and desktops, expanding the reach of any exploit that takes advantage of it.

Where the vulnerability affects servers and desktops in such popular operating systems, there are potentially millions of vulnerable computers out there right now. The security industry sent out a widespread warning about two weeks ago, spurring many companies to install the necessary patch, which was available from Microsoft a month ago.