The latest variant of the Netsky series of worms is spreading without the use of an attachment, because it launches simply by being viewed in an e-mail program.
Rather than attaching the worm's executable code to an e-mail message, Netsky.V uses two separate vulnerabilities in Microsoft software to download the code from an infected PC. Many e-mail gateways now block all e-mail attachments, so the worm is a way of getting around that precaution.
While the worm has not yet spread widely since its discovery last Wednesday, anti-virus companies 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 antivirus 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 firms 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.
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 e-mail messages without banning all HTML messages, Sophos' Cluley said, speculating that future worms might be able to use Netsky.V's method of propagation more successfully by using standard ports and exploiting more recent vulnerabilities.
The worm modifies the registry so that it runs automatically each time the affected machine boots up. Like previous Netskys, the worm will attempt 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.
Matthew Broersma writes for Techworld.com