Security experts are warning Internet users to be careful where they click, thanks to a nasty unpatched bug in the way Microsoft's Internet Explorer browser handles the JavaScript computer language. The bug is of particular concern because security researchers in the U.K. have now published "proof of concept" code showing how hackers could exploit the problem and possibly take over a Windows system.

The proof of concept code was published Monday by Computer Terrorism, a London security research firm. It exploits a problem in the way Internet Explorer processes the "Window()" function in JavaScript, a popular scripting language used by Web developers to make their sites more dynamic.

Though security experts had known about this JavaScript problem for months, they did not know that it could be used to do anything more than crash a user's computer, said Russ Cooper, editor of the NTBugtraq newslist and a scientist with security vendor Cybertrust. "The vulnerability has been around since May. It's only now that somebody has figured out how to turn it into something that runs exploitable code," he said.

Users would need to be tricked into clicking on a Web link in order to launch the malicious code, Cooper said. But once that was done, it could set up a chain of events that could ultimately let a hacker gain control of the user's system, he said.

All users of Internet Explorer version 5.5 and 6.x are affected by the vulnerability, Computer Terrorism said.

The problem is serious enough that Cooper believes that Microsoft will patch Internet Explorer in advance of its next monthly security update, which is scheduled to occur Dec. 13. "I would expect Microsoft to go into emergency patch mode and push something out very quickly," he said.

No one at Microsoft was available for immediate comment on the issue.

To avoid the problem, the SANS Internet Storm Center is advising users to turn off JavaScript, which can be done by disabling "Active scripting" in Internet Explorer's Internet Options menu, or to use an alternative browser like Firefox or Opera.