IT security and control firm Sophos is warning computer users to be on their guard following the discovery of a malicious Trojan horse spam campaign disguised as Microsoft's monthly security bulletin.

The messages were first discovered yesterday and continue to cause problems today, coinciding with Microsoft's monthly 'Patch Tuesday' cycle – when the software giant issues an update of genuine critical patches.
Samples intercepted at SophosLabs have the subject line 'Security Update for OS Microsoft Windows' and claim to come from Steve Lipnser at securityassurance@microsoft.com. The emails attempt to fool unsuspecting computer users that the attached file is a high priority update that should be installed by users of various flavours of Microsoft Windows.
However, running the attached file infects Windows computer users with the Mal/EncPk-CZ Trojan horse, and could give hackers control over your PC.
"Computer users need to learn that Microsoft never sends out security updates as email attachments, and that they should always visit the genuine Microsoft website, or use automatic updating processes, to keep their systems current," says Brett Myroff, CEO of regional Sophos distributor, Sophos South Africa.
"By timing their attack to coincide with Microsoft's genuine monthly patch cycle, the spammers are hoping to trick more unwary computer users who might be awaiting the update, keen to defend themselves against future cyber attacks.  However, falling for this scam will do precisely the opposite and could result in identity theft or financial losses," he says.