The flaw, first revealed at a hacker conference in Germany over the holidays, exists in a plug-in that enables Acrobat users to view PDF files within Web browsers. By manipulating the Web links to those documents, hackers and online thieves are able to commandeer the Acrobat software and run malicious code when users attempt to open the files, according to Ken Dunham, director of the rapid response team at VeriSign's iDefense Intelligence.

Dunham gave this hypothetical scenario: an attacker finds a PDF file on a banking Web site. The attacker creates a hostile Web site that links to the bank's PDF file. Included is malicious JavaScript code that will run on the unsuspecting user's computer once the link is clicked. "PDF is trusted and tried and true — everyone uses it," Dunham said. "But instead of just viewing the file, you've initiated script that shouldn't be executed. All you have to do is click on the PDF and the ball starts rolling."

The flaw appears to target Microsoft Corp.'s Internet Explorer 6.0 Web browser and earlier versions, and Mozilla's Firefox browser, the researchers said. They recommended that users protect themselves by upgrading Internet Explorer or changing Firefox's user options so the browser does not use the Acrobat plug-in.

Researchers said it's unclear how pervasive or harmful any future attacks might be."Given that it is easy to exploit, I would expect that we will see this method used considerably in the coming days and weeks, until it is resolved," a Symantec researcher said in a posting on a company Web log.