A Trojan designed to get past antivirus software is being spread by spam posing as an urgent Microsoft security update.
WebSense, the web monitoring company has issued a warning about the infected spam mails, which claim to protect against worm attacks.
The email header is as follows: 'Microsoft Security Bulletin MS05-039: New patch against W32/Sober, W32/Zafi, W32/Mytob'.
If a user clicks on the link at the bottom of the email, a new variant of the SDBot Trojan, which has not yet been identified by antivirus companies, is downloaded onto the PC.
Graham Cluley, senior technology consultant at Sophos, said, the lab tells me that the URL quoted in the emails is down, so there's no way at the moment for people to be infected by whatever is at the end of the web link. Because we cannot download any malware at the end of the link we cannot currently confirm whether we detect this variant or not, he added.