The latest variant of the hugely effective Netsky series of worms is causing trouble by spreading without the use of an attachment. Slipping past many email gateways, it can launch simply by being viewed in an email program.

Rather than attaching the worm's executable code to an email message, Netsky.V uses two separate vulnerabilities in Microsoft software to download the code from an infected PC. Many email gateways now block all email attachments, so the worm is a way of getting around that precaution, experts said.

While the worm has not yet spread widely since its discovery on Wednesday, anti-virus vendors said any Netsky variant is potentially dangerous. "Out of the top 10 viruses at the moment, eight are Netskys," said Graham Cluley, senior technology consultant with anivirus firm Sophos. "This is a pretty unusual method, and it is particularly interesting in this case because a Netsky author is doing it. The people behind it have written some of the most successful viruses of all time."

Since the vulnerabilities used are both more than a year old, any PC with up-to-date patches will not be affected. Anti-virus vendors have also updated their virus definitions to detect the variant's executable file, as well as the exploit code used to download it. Cluley noted that a properly configured firewall would also probably block the worm from opening ports 5557 and 5556, which it uses to transfer the executable file.

However, history has shown that large numbers of PCs - mostly belonging to home users and small businesses - are left unpatched, and lack a firewall or up-to-date anti-virus software, allowing worms such as last month's Netsky.Q to flourish. Worms such as Blaster are expected to continue to spread via unpatched machines for years to come, creating an increasingly hostile environment for corporate networks.

Like previous variants, Netsky.V appears as a "Mail delivery failed" message, and spreads using e-mail addresses in the victim machine's address book.

Instead of an attachment, the message body contains code exploiting the Internet Explorer XML Page Object Type Validation vulnerability (detailed in Microsoft Security Bulletin MS03-040), which allows malicious code to be trusted, installed and executed on a target PC.

Using port 5557, the target machine then uses an HTTP connection to download an html page from an infected computer. This page exploits the IE5 ActiveX "Object for constructing type libraries for scriptlets" vulnerability, described in Microsoft's Bulletin MS99-032, to open an FTP connection on port 5556 and download the worm executable file from an infected machine.

It would be difficult for system administrators to block exploit scripts built into email messages without banning all HTML messages, Sophos' Cluley said. Future worms might be able to use Netsky.V's method of propagation more successfully by utilising standard ports and exploiting more recent vulnerabilities, he speculated.

The worm modifies the registry so that it runs automatically each time the affected machine boots up. Like previous Netskys, the worm will attemtp to launch denial-of-service attacks on the sites of hackers and peer-to-peer networks; between 22 April and 28 April it will target www.keygen.us, www.freemule.net, www.kazaa.com, www.emule.de and www.cracks.am.

Anti-virus companies issuing detection and cleaning tools for Netsky.V include Symantec, McAfee and Sophos.