Attacks on legitimate Web domains, including some belonging to the United Nations, have infected 500,000 pages since earlier this week.

The attacks have expanded dramatically, shutting down numerous governmental and commercial Web pages.

Security researchers said hundreds of thousands of pages have been hacked by SQL injection attacks, which hackers attack on a database-driven Web site, and execute unauthorized SQL commands.

An insecure code on a system connected to the Internet is the main vulnerable hole to penetrate a Web page.

The number of hacked sites was estimated in the low six figures. However, F-Secure said its infected-page count was above half a million.

One anti-virus Vendor said the sites might have been compromised through "security issue" in Microsoft's Web server software that has already been reported to Microsoft's engineers.

NAT/GM