A new and possibly more virulent version of the "CodeRed" computer worm was detected circulating the Internet over the weekend, attacking machines and leaving them vulnerable to other intruders, a leading Internet security site reported.

The Systems Administration, Networking and Security Institute (SANS) said in an advisory on its Web site that the latest variant of the computer virus seems to leave a "back door" in infected systems that makes them easy for an intruder to infiltrate.

CodeRed surreptitiously infects computers running Microsoft Corp.'s (Nasdaq:MSFT - news) Windows NT or 2000 operating systems and its IIS Web server software and then makes infected machines scan the Internet for more victims.

If the new worm spreads as quickly as last week's CodeRed outbreak, hundreds of thousands of Web sites could be left open to computer hackers. Machines that had already been "patched" with Microsoft software aimed at thwarting the virus were not vulnerable to the new CodeRed, computer experts said.

The SANS Institute said several sources reported that the number of probes to their home networks had increased and that a new worm, similar to CodeRed, started circulating on Saturday.

The Internet security Web site said the most obvious difference between previous variants of CodeRed and the latest one was that Web server logs will record a GET request containing "XXXXXX" instead of the familiar "NNNNNN" of CodeRed.

CodeRed first became a threat in mid-July, when the worm hit some 350,000 machines, including the official White House Web site.

White House technicians had to change the IP address, the series of numbers and dots that identifies the physical address of each machine connected to the Internet, to avoid being shut down by the worm.

Last week, another version of the worm infected an estimated 300,000 computers worldwide, but it did not cause any measurable impact on Web performance.

Some undisclosed Web sites, however, had to be taken off-line because the worm halted or overloaded routers and systems. The worm also knocked out Web servers at companies of various sizes as it commandeered them to scan for new victims.

Last week's onslaught also disturbed Defense Department systems, Pentagon officials said.

The worm spreads by latching onto computer servers and then randomly sending itself to 100 other IP addresses, which in turn start scanning the Internet for more computers to hit. Since the Internet has no national boundaries, the worm has quite likely spread globally, and hits have been reported in South Korea, France and Britain.