Antivirus companies are warning customers about a new email virus that is spreading on the internet.
W32.Mimail is a mass mailing email worm that arrives in email in-boxes disguised as an administrative email sent from an organisation's own administrator.
Messages use the subject "Your Account" and contain the virus in an executable attachment called "message.zip."
When released, the Mimail virus captures email addresses from a user's hard drive and sends copies of itself out to recipients using a built-in SMTP (Simple Mail Transfer Protocol) engine, according to F-Secure of Helsinki.
Companies including Symantec, Network Associates, F-Secure and others issued warnings about Mimail on Friday.
Most companies rated the new worm a "medium" level threat, indicating that the worm was infecting customer sites and spreading.
Despite the warnings it is still not clear how quickly, or even whether, Mimail is spreading, according to Vincent Gullotto, senior director at Network Associates' McAfee AVERT Labs.
"The initial numbers look like people are getting hit pretty hard -- maybe even a high alert," Gullotto says.
However, the large number of reports about Mimail may just be evidence of a spam-like initial distribution, or "seeding" of the virus, he says.
That would make Mimail similar to another recent email containing a malicious program, Downloader-DI, he says.
That virus set up a secret back door on infected machines and downloaded instructions from a hacker website.
After flooding email in-boxes in an initial spam distribution, however, Downloader-DI died out when other users failed to open the attachment that installed the Trojan program and replicated the message, Gullotto says.
However, the Mimail's spread could also be due to its ability to mask itself as an internal administrative message, tricking users into trusting the message, he says.
In addition, Mimail's malicious code is embedded in a compressed format file, making it difficult for some gateway antivirus products to detect the attack, he says.
While it appears Mimail simply steals email addresses and sends copies of itself out to unsuspecting users, McAfee AVERT is still studying the virus for other malicious activities such as installing Trojan programs that could allow malicious hackers to gain access to the machine at a later date,' Gullotto says.
Antivirus companies, including Network Associates' McAfee antivirus unit, posted updated virus identity files for Mimail Friday and encouraged users to update their antivirus software.