Security companies have warned that another variant of the Mytob worm began wiggling its way into inboxes.

The variant, dubbed "Mytob.bi" by some security researchers, scans the hard drive of an infected machine and sends copies of itself to e-mail addresses it finds in the Windows Address Book, said anti-virus firm Trend Micro. The worm poses as a message from an IT administrator, warning recipients that their e-mail account is about to be suspended, Trend Micro said.

Possible subject headers for the worm include "*IMPORTANT* Please Validate Your Email Account" and "Notice: **Last Warning**."

The latest variant is the fourth iteration of the Mytob family of worms that were first detected in late February. It has backdoor capabilities and can open a random port, allowing a hacker to remotely access an infected machine.

The variant also prevents the infected machine from accessing several antivirus and security Web sites by redirecting the connection to a local machine, the security firm added.

While prevalence of the worm is still low, the damage potential is high, Trend Micro said. Anti-virus company Sophos also rated the worm as a concern, due to the severe damage it could cause.

Researchers speculated that the Mytob worm family is popular with hackers because its code base is relatively easy to manipulate to create a new variant. Another version, Mytob.ar, was detected earlier this week, containing added spyware and adware elements.

Future Mytob variants could take advantage of the .ar version to reap monetary benefits from spyware, Trend Micro warned.

Internet users are advised to update their antivirus software to protect themselves from the new threats.