The SirCam mass mailing worm is becoming a major nuisance for Internet users as its numbers have steadily risen over the last week.

Unlike other similar viruses, such as the Love Bug or Anna Kournikova worm, the outbreak of SirCam has not contained, and although it had a slow start it has now become easily the most common virus on the Internet.

MessageLabs, a managed services firm which scans its clients email for malicious code, has intercepted 6941 copies of the bug from 2817 different email addresses.

Alex Shipp, senior anti-virus technologist at MessageLabs, said that the SirCam virus, which first appeared a week ago, was likely to rise throughout the week before peaking and gradually fading out.

As previously reported, SirCam spreads itself as an attachment to email messages, and may in certain cases delete files from a victim's hard disk.

SirCam is similar to the Magistr virus in its ability to arrive in an email with a random subject, body text and attachment name. However infected attachments contain a double extension, which gives users a clue that an email might contain a virus.

The subject line of an email is the name of a file found on the sending PC. The attachment will carry the name of this document file, with a second extension such as COM, EXE, PIF, LNK. A document file is included in the executable that the worm mails, which means there is a possibility of confidential or embarrassing material being mailed out.

The inclusion of a document means much larger emails than are currently spread by viruses will be created, which is having an impact on Internet performance even for those who aren't infected by the bug, but are still been deluged with unwanted emails.

The worm contains its own SMTP routine which is used to send email messages to email addresses found in the Windows address book and the temporary internet folder, where cached internet files are kept. Because the virus has its own email engine copies of emails sent will not show up in a user's email client sent file.

Antivirus vendors are in the process of updating their software to deal with the virus and, in most cases, the necessary protection is already in place. Users are also advised to delete any suspicious emails without opening them and to update their antiviral protection. ®

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Write up on SirCam by Symantec

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