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Despite infecting tens of thousands of computers worldwide, the recent W32.Blaster worm is poorly written and inefficient, blunting its impact, according to security experts.

However, future versions of the worm could correct Blaster's flaws and spread much more quickly, resulting in service outages on infected networks and causing far greater harm to businesses and individual users on the Internet, experts warn.

Blaster, which is also known as the MSBlast, the Lovsan Worm, and the DCOM Worm, surfaced on Monday and quickly spread to computers worldwide by exploiting a known security vulnerability in Microsoft Windows.

The worm targets a Windows component for handling Remote Procedure Call (RPC) protocol traffic called the Distributed Component Object Model (DCOM) interface. By Tuesday morning, it had spread to more than 30,000 systems, according to Johannes Ullrich, chief technology officer of the SANS Internet Storm Center.

Sloppy Worm

However, security experts familiar with the new worm say that close inspection of its code reveals shoddy workmanship.

"It's a pretty bad worm. I keep calling it the 'half a worm,'" says Marc Maiffret, chief hacking officer at security company eEye Digital Security.

Rather than write new code, Blaster's author or authors copied and pasted a well-known exploit for the vulnerability that was available on the Internet, Maiffret says.

Whereas the vulnerability affects almost every computer running Microsoft Windows, the DCOM exploit used by Blaster works only on Windows XP and Windows 2000 systems, greatly reducing the number of machines affected, according to Maiffret.

Unlike other successful worms, the Blaster code is unable to detect what kind of operating system is installed on the machine it is attacking, and chooses randomly between the exploit for Windows XP systems and Windows 2000 systems, according to security company F-Secure.

In doing so, Blaster frequently uses the wrong exploit for the installed operating system, which causes Windows XP machines to reboot with an error message that mentions RPC, F-Secure representatives say.

Sophisticated worm writers would create their own exploit code that works for more flavors of Windows and add features to detect the operating system and prevent telltale crashes, Maiffret says.

Easily Identifiable

Similarly, Blaster's authors created a noisy and an inefficient method for spreading the worm code from an infected machine to a vulnerable, but uninfected machine. The worm requires the vulnerable machine to establish a separate connection to the infected machine to copy the worm code, making the worm easier to notice on a network and providing multiple avenues to block the worm's spread, Maiffret says.

That was the experience of technology administrators at the University of Florida, according to Jordan Wiens, a network security engineer.

While Blaster uses port 135 to spread from computer to computer, it also opens a back door to the computer on port 4444, which is used to issue commands that download the worm code.

UF administrators were quickly able to stop the worm from spreading without affecting other applications in use on campus by blocking traffic to port 4444, Wiens says.

"They were clueless," Maiffret says. "A real worm writer with any type of skill wouldn't have needed to connect back (to an infected machine) in order to get infected."

Ullrich agrees, calling Blaster's infection method "a bit primitive" and pointing to the worm's habit of stopping after it scans only 20 or so machines to check for infections.

"Code Red scanned 100 or 200 machines at a time," he says, referring to the devastating worm of 2001.

Mutations Likely

Maiffret, Ullrich, and other security experts agree that future versions of the Blaster worm are likely, as are new worms that exploit the RPC vulnerability.

Those variants might patch the holes in Blaster's code or modify it. For example, one might redirect the worm's programmed denial of service (DoS) attack against Microsoft's windowsupdate.com site to a different Internet domain or IP address, Maiffret says.

The Internet Storm Center had not received reports of any Blaster variants Tuesday, Ullrich says.

Despite its many faults, Blaster does one thing right, the experts say: It targets an easily exploitable and ubiquitous security flaw that affects home users more than just closely monitored servers.

"Even as poorly written as [Blaster] is, it's still having an effect and we're seeing a lot of impact from the worm right now. That's really the scariest part," Maiffret says.

And the worm's programmed DoS attacks against Microsoft could still cripple the networks used to launch the attack, even if they don't crash the Windows update servers, Maiffret says.

Good Lesson

Like any worm, Blaster will also be hard to eradicate, according to Mikko Hypponen, antivirus research director at F-Secure.

"We're still fighting Code Red from 2001, so [Blaster] will keep spreading for a very long time. I expect it will still be scanning networks in 2005," Hypponen says.

In the end, however, Blaster could do users a favor. A serious--but not devastating--worm of its type might help inoculate the Internet community against future variants that are more virulent, the experts say.

Every report of a widespread, if only pesky, worm can help spur users to patch vulnerable systems and install other protective measures like firewalls, Maiffret says.