Symantec is warning users of the RealPlayer media software to be extra careful while surfing the Web.

On Wednesday, the company's Symantec Security Response team uncovered new attack code that affects the RealPlayer 11 beta and RealPlayer 10.5 software on the Windows platform, according to Ben Greenbaum, a senior manager with the group. By late Wednesday, Symantec had tested the attack and confirmed that it worked on the English version of Windows XP Service Pack 1 running Internet Explorer 6. Tests for the more-recent XP service pack 2 and IE 7 browser were ongoing.

For the attack to work, the criminal would have to trick the victim into playing a maliciously encoded Web page. The flaw lies in a browser helper object, software that RealPlayer uses to help users who are experiencing technical difficulties.

Once the exploit is run on the victim's machine, the attacker can download and install whatever software he wants, Greenbaum said.

So far, Symantec has just seen the one sample of the attack code, submitted by a customer in the U.S., but Greenbaum expects it to soon become more widespread. Symantec has not yet seen the code posted to public hacker forums, he added.

Symantec's products are now protecting its customers from the attack, but other users can protect themselves by disabling JavaScript and ActiveX in their browsers and by being careful about the sites they visit, Greenbaum said.

Greenbaum did not know whether the flaw also exists on Linux and Mac OS versions of RealPlayer, but he said that even if they were vulnerable, the attack code that Symantec has seen would probably need to be rewritten for those platforms.

RealPlayer's maker, RealNetworks, did not immediately respond to requests for comment on this issue.