ttack of the Worm could be the name of a "B" horror movie, but many businesses around the world experienced the real thing during the weekend of Jan. 25-26, 2003. The latest Internet worm, called "SQL Slammer," struck global Web networks carrying data, Web pages and e-mails. The ambush clogged Internet traffic and hindered hundreds of thousands of computers and ATM machines. Many were left wondering how this could have happened and what might be next.

Disruptions caused by the computer virus-like program were more troublesome than damaging, but the assault revealed a vulnerability in the interdependence of the Internet and services such as banking, emergency dispatchers and health care - believed by many to be impervious from such attacks.

The worm struck at 12:30 a.m. Jan. 25, burrowing aggressively into networks and making 200,000 to 300,000 attacks per hour on computers in North America, Asia and Europe (a worm is a computer program that propagates itself over a network, reproducing along the way). Businesses running Microsoft SQL Server 2000, a popular database software that has sold more than 1 million copies, were targets although more proved to be victims.

In July 2002, a British researcher reported a flaw in the SQL Server 2000 software, making it vulnerable to hackers. In response, Microsoft Corp. released a "patch" in July 2002 that would enable companies to protect their servers; however, many were not prepared when the worm struck. Users attempting to download the patch over the weekend were blocked from doing so by Internet congestion caused by the worm.

Many financial institutions suffered from the cyber assault. Service was disrupted at a majority of Bank of America Corp.'s 13,000 ATMs. Customers could not use the machines, and customer-service representatives could not access customer information. Bank of America said that no damage was done to customers' funds or personal information.

Customers of the Canadian Imperial Bank of Commerce in Toronto also were unable to withdraw money from the bank's cash-dispensing machines. American Express Co. said customers couldn't reach its Web site to check credit statements and account balances during parts of the weekend.

The worm interrupted service at several thousand of Seattle-based Washington Mutual Inc.'s ATMs across the United States. In addition, customers were unable to pay certain bills online or transfer funds over the telephone, the Seattle Post-Intelligencer reported.

The White House and Canadian defense officials are investigating how the worm affected ATM banking networks that should remain immune from traditional Internet outages. One of President Bush's cyber-security advisors, Howard Schmidt, said early reports suggested private ATM networks overlapped with parts of the public Internet.

"This is one of the things we've been talking about for a long time, getting a handle on interdependencies and cascading effects," Schmidt was quoted as saying.

Non-financial institutions also were hit, such as The Boeing Co., and police and fire officials in Seattle lost access to their computerized 911 emergency dispatcher services. Even Microsoft Corp. was attacked when the worm dug into its own servers that had not yet received the patch.

Most businesses were able to isolate and fix the problem within a few hours, slowly decreasing Internet congestion from the attack over the weekend. Much of it was cleared up by Monday. Microsoft reported that the number of users downloading its patch reached 6,800 per hour on Monday.

So far, the origins of "Slammer" remain a mystery.