With Microsoft's monthly patch release expected on Tuesday, scammers are sending out fake security bulletins that attempt to install malicious software on victim's computers.

The email messages claim to describe a "Cumulative Security Update for Internet Explorer" that fixes a critical security flaw in the browser. It comes with a link entitled "Download this update".

When users click on this link, they are taken to a server that attempts to install malicious software known as Trojan-Downloader.Win32.Agent.avk.

This Trojan software then attempts to reach out to other computers on the internet in order to install more programmes on the victim's computer.

The SANS Internet Storm Centre received its first and only report of the scam last Thursday night but a second sample has also been posted to the Chinese Internet Security Response Team blog.

SANS volunteer Lenny Zeltser believes that the criminals behind this scam may be gearing up for more activity. The trojan looks for three different servers and two of them have domains that haven't yet been registered. He suspects the authors of the scam may be planning to register those domains before embarking on a more widespread campaign.

The two email samples contained obvious errors that would be caught by technically savvy users. For example, although the patch Zeltser examined claimed to have been issued in June 2007, it was entitled MS06-4 instead of the more-plausible MS07-004.

Still, these scams need to fool only a small percentage of victims in order to be successful, said Zeltser, information security practice leader at US-based Gemini Systems. "You wonder, does it really matter that there are these strange discrepancies in the way the fake security alert is written," he said. "People who would notice probably would be the kind of people who wouldn't click on the link."

Another tip-off: Microsoft does send out notification email when it publishes security bulletins, but the links in these alerts take users to the bulletins themselves, not to executable downloads.