Sophos PLC reported evidence of new malware planted on Web sites that exploits a previously announced -- and as-yet-unpatched -- Internet Explorer security vulnerability. The Clunky-B Trojan horse allows hackers to install and run malicious software on users' machines when they visit sites containing the malware, said Graham Cluley, a senior technology consultant at Sophos.

The code is only the latest of several Trojan horses seeking to exploit an Internet Explorer vulnerability that Microsoft first disclosed in May. At that time, the flaw wasn't considered very serious because it was believed that hackers who exploited it would only be able to shut browsers down while causing little other damage, Cluley said.

The recent emergence of Trojan horses targeting the flaw, however, prompted Microsoft to issue an advisory last week. That advisory was updated on Tuesday to inform users that it was aware of the proof-of-concept code and is investigating the matter.

"Upon completion of this investigation, Microsoft will take the appropriate action to help protect our customers. This may include providing a security update through our monthly release process or providing an out-of-cycle security update, depending on customer needs," the company said in that advisory. Microsoft's next security update is due on Dec. 13.

For the moment, at least, there is little evidence that the Trojan horse code poses a major threat to most Internet users, Cluley said. The only users likely to get infected are those who visit pornographic sites. But it's possible that hackers could soon start planting the malware on other sites they may have compromised, he said.