The Kargo.F worm opens a hole to Microsoft systems through TCP ports 3067 and 113. Users who operate unpatched Windows 2000 and Windows XP systems are open to attack, according to anti-virus firm Symantec.
The worm is commonly used in phishing scams. When opened the wom has the ability to steal personal information and banking details.
Symantec raised the threat level from two to three, after receiving multiple user warnings about the worm.
The firm has known about Korgo.F since April.