Yet again, variants of the Klez worm are by far the most common viruses circulating on the Internet.
That's according to MessageLabs which stopped more than 475,000 copies of the virus in July, down from in excess of 788,000 infected emails it blocked in June. It's the fourth month in succession that Klez has topped monthly virus charts.
MessageLabs reports that virus infection rates are currently running at around one per 256 emails, which compares to one in 30 infected emails at the heights of the Goner and Love Bug epidemics.
In the last four weeks MessageLabs blocked more than 462,333 emails infected with Klez-H alone, which became the worse virus ever in May, according to the company.
By comparison, the next most common virus, Yaha-E, was blocked only 111,562 times by MessageLabs during the last four weeks, with the infamous SirCam virus trailing in a distant third with 17,427 sightings.
A chart of calls logged by AV vendor Sophos reveals a similar picture with Klez-H responsible for almost one in five (19.4 per cent) of calls to the company in May. The next most common pathogen was Frethem-Fam (17.0 per cent), closely followed by Yaha-E (16.8 per cent).
Klez is a mass-mailing worm, which searches the Windows address book for email addresses and sends messages to all recipients that it finds. The worm uses its own SMTP engine to send the messages. It can also spoof the 'From' in messages, a factor which has resulted in widespread confusion about the bug.
The subject and attachment name of incoming emails is randomly chosen, making it harder for users to spot. The attachment will have one of the following extensions: .bat, .exe, .pif or .scr. Klez is capable of infecting files.
The worm exploits a vulnerability in Microsoft Outlook and Outlook Express in an attempt to execute itself when you open or even preview the message. Information and a patch for the vulnerability can be found here. ®
Top ten viruses blocked by MessageLabs in July