Phel program attempts to use an HTML file to infect computers running Windows XP Service Pack 2.
The program--dubbed "Phel," an anagram of "Help"--infects visitors to a maliciously-created Web site through Internet Explorer's Help controls, Symantec warned in an advisory this week. A bug in the malicious program may prevent it from infecting some computers, the security company said.
The Trojan horse exploits a vulnerability, found in October, in how Internet Explorer and Windows XP Service Pack 2 handle help files called from Web pages.
The flaw is unrelated to the recent help-file flaws outed by a Chinese security company last week. In that instance, Microsoft took the Chinese security group to task for disclosing the vulnerability without giving the company a chance to develop a way to fix the problem.
"Microsoft is working to forensically analyze the malicious code in Phel and will work with law enforcement to identify and bring to justice those responsible for this malicious activity," a company spokesperson said.
A patch is not yet available from Microsoft for the October flaw, nor the most recent flaws, but the software giant said its programmers are working on the issue.
"Microsoft is taking this vulnerability very seriously, and an update to correct the vulnerability is currently in development," the spokesperson said. "We will release the security update when the development and testing process is complete, and the update is found to effectively correct the vulnerability."
Microsoft has had its Web browser in 2004. As a result, the freely available open-source browser Firefox has gained market share. Security experts have recommended that computer users consider other browsers and some schools have told their students to use a non-Microsoft browser.
The Symantec advisory can be found on the company's Web site.