By Brian Krebs
washingtonpost.com Staff Writer
Sunday, March 21, 2004; 2:17 PM 

A quickly spreading Internet worm destroyed or damaged tens of thousands of personal computers worldwide Saturday morning by exploiting a security flaw in a firewall program designed to protect PCs from online threats, computer experts said.
The "Witty" worm writes random data onto the hard drives of computers equipped with the Black Ice and Real Secure Internet firewall products, causing the drives to fail and making it impossible to restart the PCs. Unlike many recent worms that arrive as e-mail attachments, it spreads automatically to vulnerable computers without any action on the part of the user.
At least 50,000 computers have been infected so far, according to Reston, Va.-based computer security firm iDefense and the Bethesda, Md.-based SANS Institute.
The firewalls were developed by Atlanta-based Internet Security Systems. Chris Rouland, vice president of the company's X-Force research and development division, said that as many as 32,000 corporate computers could be infected. The company does not know how many home computers are infected. ISS released a patch on Wednesday, and Saturday published a detailed writeup of the affected products.
Most infected computers will have to be rebuilt from scratch unless their owners instead decide to buy new ones, said Ken Dunham, a computer security expert at iDefense.
"The thing looks like it will corrupt or crash most drives enough so that reinstallation is going to be required," he said. "This is a very destructive worm."
Officials at the Department of Homeland Security, which is in charge of the government's cybersecurity efforts, were unavailable for comment.
Internet worms, viruses and other malignant software often install software or open "back doors" that allow hackers to control infected computers. That often gives them access to private data that people keep on their computers, and allows them to use those computers to send out e-mail spam that cannot be traced back to its real owner. The Witty worm is different and in some respects more destructive because it renders the computer useless.
Johannes Ullrich, chief technology officer for the SANS Internet Storm Center, said that the worm does not create files on infected computers so most antivirus software will not detect it.
Security vulnerability research firm eEye Digital Security identified the flaw on March 8. The Aliso Viejo, Calif.-based company discovered that it could trick some versions of Black Ice and Real Secure into processing Internet traffic that would allow attackers to transfer dangerous data to vulnerable computers.
The Witty worm gets its moniker from a message buried within its code that says: "insert witty message here." That comes just before the code that overwrites the infected hard drives.
Joe Stewart, a senior security researcher at Chicago-based security services company Lurhq, said he expects the worm to die out over the next few hours as vulnerable computers quickly become useless hosts.
"With all these hard drive problems, the infection rates are going to shrink pretty quickly as all these affected machines grind themselves to a halt," Stewart said.
In an analysis of the worm, Stewart wrote that the worm corrupts the computer hard drive in a similar way that a pathogen feeds on its host in order to spread.
Like a pathogen that feeds on its host in order to spread, the Witty worm corrupts the hard drive as it tries to take over other vulnerable machines, Stewart wrote in his analysis of the worm. "This worm has been found to be highly malicious, slowly destroying the systems it infects. Because of this activity, at some point this worm will cease to exist -- unfortunately it will take all the affected systems with it."