Named Asprox, after the toolkit used in its attacks, this network gained attention in May and June when it infected an estimated tens of thousands of Web pages on more than 1,000 Web domains, typically infecting the Web sites of small businesses, schools and local governments.

"After several months of no activity, this botnet is back to its old tricks," wrote Gary Warner, director of research in computer forensics with the University of Alabama at Birmingham, in a Thursday blog post.

Security vendor SecureWorks picked up on the attack "a couple of days ago," when it noticed an uptick in so-called SQL injection attacks against the company's clients, according to Jason Milletary, a security researcher with the company. However, it's not clear whether the attacks are as virulent as before, he said.

In a SQL injection attack, the criminals take advantage of database programming errors in order to trick Web sites into posting their attack code. With Asprox, this SQL injection process is automated, so it can add malware to a lot of Web sites in a very short amount of time.

Asprox places a bit of JavaScript code on the hacked Web site that generates an invisible HTML element, called an iFrame, which in turn launches the attack code. According to Warner, at least some samples of the current Asprox code exploit a bug in Adobe's Flash Player.

Researchers with the security watchdog group Shadowserver say they've tracked more than 2,000 Web pages that have been infected by this latest Asprox attack, far fewer Web pages than were attacked with the first version of the malware.

In an e-mail interview, Shadowserver's Mike Johnson said that the Asprox gang has revised its malware, changing the configuration file structure of their code and adding new command and control computers that they haven't used in the past. "It almost looks as if they're starting over from scratch after losing control of the previous botnet," he said.

Asprox is not currently a major problem for most Web users, security experts say; it's just another sign of the ever-present dangers on the Web.

"People should be expecting malicious sites," Johnson said. "People should be expecting innocent sites being compromised in some way, shape or form that then in turn try and attack the browser."