A widespread attack that targeted major E-commerce sites and secretly planted hacker tools on the computers of Internet surfers is subsiding, security experts say.

Security experts estimate that thousands of Web sites were compromised in the past week. The attack, which may have begun as early as Sunday, didn't attract much attention until late Thursday evening when it was identified by Internet security firms.

Most of the Web sites known to have been infected have been cleaned, security analysts say. Also, Internet service providers have blocked access to, or "black-holed," the Russian server that was planting the hacker tools on user PCs.

Alfred Huger, senior director of engineering for Internet security firm Symantec Corp., says Web sites running Microsoft's Internet Information Services software version 5.0 were attacked and infected with a malicious JavaScript application. When Web surfers visited affected sites, their computers were subsequently infected through multiple vulnerabilities in Internet Explorer.

Once a Web surfer's system was attacked by the malicious JavaScript application, the surfer's computer was connected to a server located in Russia and infected with hacker tools such as backdoors and keystroke loggers, which could be used to take control of the user's system or steal confidential information.

While patches are available for most of the Internet Explorer vulnerabilities used in the attack, no patch is available for one of the flaws, commonly known as the ADODB vulnerability.

It's still unclear how the attackers managed to successfully compromise Web servers running Microsoft's IIS software, security experts say. "It's something we're looking into," Huger says.

It's possible, but unlikely, that systems running Microsoft IIS 5.0 software could have been attacked by a "zero-day" vulnerability, which is a new software flaw that's unknown and unpatched by software vendors, says Marcus Sachs, director, of The SANS Institute's Internet Storm Center. "That's the worst-case scenario," he says.

Other possibilities include Web servers that administrators believed to have been patched but were not, or Web servers that could have been attacked through vulnerabilities unrelated to IIS 5.0.

Security experts warn that future attacks are possible. "Others may attempt copycat attacks, especially if there is a zero-day attack in IIS," Sachs says.

Major antivirus companies have updated their software to spot the malicious code downloaded to end-user systems in this attack.

Microsoft is urging Web-site operators running Windows 2000 Server and IIS to apply a patch found in Microsoft Security Bulletin MS04-011.

Microsoft has published a Web site with more information about this attack and the IIS and Internet Explorer vulnerabilities; it's located at www.microsoft.com/security/incident/download_ject.mspx.