Web sites that naively call for user input, then fail to put strict checks on what that input may be, are susceptible to SQL injection attacks. That vulnerability appears to be the cause of up to 70,000 Web pages getting hacked by malicious code between Dec. 28 and Jan. 5.

The intrusions represent a whole new level of threat to users on the Internet. Instead of the attack seeking to launch a virus or worm at individual computers, it invaded Web databases and used them to host its malicious code and distribute it every time site visitors sought information beyond a home page or product page from the database. But for the fact it used an old and already guarded against Windows exploit, it might still be spreading across the Internet, security experts said.

"I don't think we've seen this scale of database intrusion before. SQL injection attacks are usually on a one-at-a-time basis," said Phil Neray, VP of marketing at Guardium, a Waltham, Mass., firm that makes database protection software.

The URL and server used to launch the attack was based in China and a Chinese site first posted information on the attack. The domain name now only shows the text, "OK" with the ^_^ emoticon.

"This was a pretty good mass-hack, and it wasn't just that they got into a server farm, as the victims were quite diverse, with presumably the only common point being whatever vulnerability they all shared," said Roger Thompson, chief research officer of Grisoft, producer of anti-virus software. Thompson is the founder of Exploit Prevention Labs, producer of LinkScan, which scans Web applications for malware. The firm was acquired by Grisoft in December.

The attacker penetrated the sites by discovering applications where the site builder expected a user name, address or other innocuous information to be typed in by the site visitor. The automated attack used such forms to inject a SQL statement instead of text, and thousands of Web applications apparently passed the statement on to the site database. The attack was launched Dec. 28. Security experts said the number of sites affected now appears to be declining, but it's not clear whether the attack has been completely stifled or might be relaunched from a new URL.

As of Jan. 5, it appeared to have peaked and was in reversal as sites closed SQL injection exposures in their databases. Neray estimated

Sites that show up in a Google search as containing the attack's original site URL include Rust-Oleum's Premiumgaragefloors.com; certain pages of CA, the system software vendor with a security product line; and the Haworth Press's seed catalogue, among many others. Thompson said in a Jan. 5 blog on the Grisoft Web site that it was Microsoft SQL Server databases that ended up as the target of the attack because the tables targeted are specific to SQL Server. But neither Neray nor Thompson could specify the purpose of the attack or what damage it might be doing beyond attaching JavaScript to text links inside the database. The intrusion of each database is massive, with a JavaScript string being attached to all text items in the database. A site user's request for an information item then leads to the attacker's JavaScript response attempting to plant code on the user's computer.