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 individuals users on the Internet, experts warned.
Blaster, which is also known as the MSBlast, the Lovsan Worm and the DCOM Worm, targets a Windows component for handling Remote Procedure Call (RPC) protocol traffic called the Distributed Component Object Model (DCOM) interface.
However, security experts familiar with the new worm said that close inspection of its code reveals shoddy workmanship.
“It’s a pretty bad worm. I keep calling it the ‘half a worm’,” said 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 which was available on the Internet, Maiffret said.
Whereas the vulnerability affects almost every computer running Microsoft Windows, the DCOM exploit used by Blaster worked only on Windows XP and Windows 2000 systems, greatly reducing the number of machines affected, he said.
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 of Helsinki.
Sophisticated worm writers would create their own exploit code that worked for more flavours of Windows and added features to detect the operating system and prevent telltale crashes, Maiffret said.
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 required the vulnerable machine to establish a separate connection to the infected machine to copy over the worm code, making the worm easier to notice on a network and providing multiple avenues to block the worm’s spread, Maiffret said.
That was the experience of IT administrators at the University of Florida (UF), a network security engineer, Jordan Wiens, said.
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 said.
“They were clueless,” Maiffret said. “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 agreed, 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 said, referring to the devastating worm of 2001.
Maiffret, Ullrich and others agree that future versions of the Blaster worm were likely, as were new worms that exploit the RPC vulnerability.
Despite its many faults, Blaster did do one thing right, according to Maiffret and others: target 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 said.
In the end, however, the emergence of a serious — but not devastating — worm like Blaster might help inoculate the Internet community against future variants that are more virulent, spurring users to patch vulnerable systems and install other protective measures like firewalls, Maiffret said.