Microsoft Corp. Tuesday released its monthly security update with a patch to repair a newly discovered "important" vulnerability in Windows that can allow remote code execution in Windows Explorer. Both Windows 2000 Service Pack 3 and Windows 2000 Service Pack 4 are affected by the security bulletin. The vulnerability is not found in Windows XP or 64-bit XP, or in Windows Server 2003 and Server 2003 64-bit operating systems, according to the company.

The vulnerability is in Windows 98/98SE and Windows Millennium Edition, but the company no longer provides security updates for those older operating systems unless they are rated "critical."

Microsoft Security Bulletin MS05-024 said the patch fixes a remote code-execution vulnerability found in Windows Explorer"s file management utility. The vulnerability involves the way that Web View in Windows Explorer handles certain HTML characters in preview fields, according to the company.

Microsoft rates the vulnerability as "important," the third-highest level of its four-level Maximum Severity Rating system. The highest level of update importance is "critical."

Stephen Toulouse, a security program manager for Microsoft"s Security Response Center, said the vulnerability could allow an attacker to run or install malicious software on a user"s computer, or it could allow an attacker to view or delete files remotely.

Such an attack, however, would require user intervention, he said, because a user would have to click to execute and open a file sent by an attacker. "It"s not an automated attack," Toulouse noted.

The vulnerability was identified about four weeks ago on a security mailing list, Toulouse said, before Microsoft had an opportunity to create a patch to repair it. Usually, vendors are given notice of such vulnerabilities before they are made public so that fixes can be made ahead of attacks, he said. "We believe it puts people at risk," he said of the public announcement before the patches were made available.