Hackers have discovered a new vulnerability in Internet Explorer, and they've released code that could be used to attack users of Microsoft's popular browser.
The vulnerability is similar to a bug that Microsoft patched last month in a multimedia component of Internet Explorer, according to Vincent Hwang, a group product manager with Symantec's Security Response team.
Though a sample exploit of the vulnerability was posted earlier this week by hackers on the xsec.org Web site, Symantec has yet to see the code used in any attacks, according to Hwang.
To take advantage of the exploit code, attackers would first need to trick users into viewing a maliciously encoded Web page, but they could then run unauthorized code on a victim's computer.
It is unclear right now which versions of Windows and Internet Explorer are affected by the vulnerability. Researchers at Secunia said they were able to create a "fully working" exploit for the latest version of Windows XP running Internet Explorer 6. Windows 2000 users are also vulnerable, Secunia said.
Microsoft security researchers were unavailable to comment on the issue, but a spokesman for the company's public relations agency said that the matter was under investigation.
Symantec calls the bug "critical," and Secunia rates the issue as "highly critical," its most severe rating. The Secunia alert can be found online.
The xsec.org hackers referred to their code as a 0day, meaning an exploit for a previously undisclosed vulnerability. But one well-known hacker said the flaw was not difficult to find using publicly available security tools, such as the AxMan ActiveX fuzzing software.
"Calling it 0day is a stretch," said HD Moore, the head of the Metasploit project, via e-mail.
Moore wrote an automated ActiveX testing tool called AxMan that uncovered a handful of IE bugs, including the one exploited by on xsec.org. Although Moore recently launched a project called the Month of Browser Bugs, in which he disclosed a new browser vulnerability every day for the month of July, he said he had refrained from disclosing this particular vulnerability.
"This is one of the many exploitable bugs that can be discovered using AxMan and one of the few that I didn't include in Month of Browser bugs due to the ease of exploitation," he said. "I still have three or four left in IE that have similar impact."
This is the second unpatched flaw that Microsoft is looking at patching right now. Earlier this month, attackers began exploiting a vulnerability in the company's Word software.