Security experts say the flaw lies in the way Adobe's software executes JavaScript code, and Adobe offered a few workarounds in its advisory to help users avoid being hacked by the attack.
The simplest way is to turn off JavaScript in Reader and Acrobat. Security experts have long recommended this option, because a number of Adobe attacks already depend on the use of JavaScript. To disable JavaScript, select Edit > Preferences and then pick the JavaScript category. There, users can uncheck the "Enable Acrobat JavaScript" choice.
Criminals have been sending out malicious PDF files since Friday that include this new attack code, but these attacks have not been widespread. However, security experts worry that as information on the bug spreads, these attacks will become a bigger problem.
Several hacker sites on Tuesday claimed to have published samples of the attack, which means that the code could soon be picked up by even more criminals.
"This is legit and is very bad," the anti-malware volunteer group Shadowserver said in a post to its Web site late Monday.
The flaw lies in the current version of Adobe Reader on Windows, Macintosh and Unix systems. Macintosh and Unix computers will crash when they try to open the malicious files, but Adobe and outside security experts say that, so far, the attack code only works on some versions of Windows. Older versions of Reader and Adobe Acrobat are also affected by the issue, Adobe said.
Windows Vista and Windows 7 use a Data Execution Prevention technology that prevents the attack from doing anything more than crashing Reader, Adobe noted.
Adobe is not saying when it will patch the issue, but its next set of Reader and Acrobat patches is due Jan. 12.
