One of the sneakiest viruses to date began spreading rapidly across the Internet this weekend.

Mimail, which poses as an email from a potential victim's own sysadmin or ISP, suggests that a user's email account is about to expire.

Potential victims are urged to open an attachment message.zip, containing a copy of the virus.

Users who unzip the file find another innocent-looking HTML file inside, named 'message.html'.

This file contains an embedded EXE file, when opened in vulnerable versions of Internet Explorer, will drop an executable named foo.exe and run it. More information on the IE MHTML vulnerability used here can be found in an April 2003 advisory by Microsoft.

On infected machines, the virus searches for email addresses on a user's hard drive. Mimail uses its own SMTP server to spread sending copies of the malicious code to email addresses harvested from an infected PCs.

Mimail also has a backdoor component. The virus attempts to send data from a victim's machine to certain email addresses, coded into Mimail.

More detailed descriptions of Mimail can be found in advisories from F-Secure, Symantec and McAfee.

Vendors generally rate the virus as a medium level threat.

Windows users are advised to update their AV signature files and to apply patches from Microsoft if they haven't already done so.

Apple and GNU/Linux users need have no fear. As is so often the case, they're immune from the latest Windows nasty. ®