The security woes continued for Microsoft after Chinese security researchers mistakenly released the code needed to hack a PC by exploiting an unpatched vulnerability in Internet Explorer 7.

At one point, the code was traded for as much as $15,000 (£10,000) on the underground criminal markets, according to iDefense, the computer security branch of VeriSign, citing a blog post from the Chinese team.

The problem in Internet Explorer 7 means a computer could be infected with malicious software merely by visiting a website, one of the most dangerous computer security scenarios. It affects computers running IE7 on Windows XP, regardless of the service pack version, Windows Server 2003 running Service Pack 1 or 2, Windows Vista and Windows Vista with Service Pack 1 as well as Windows Server 2008.

Microsoft has acknowledged the issue but not indicated when it will release a patch.

The vulnerability was first revealed earlier this week by the Chinese security team "knownsec." Knownsec said on Tuesday they mistakenly released exploit code thinking that the problem was already patched, iDefense said.

"This is our mistake," knownsec said in a Chinese-language research note.

That mistake could mean that more hackers will try to build websites in order to compromise users PCs since the exploit code is more freely floating around on the Internet. However, other information indicates that hackers already knew how it worked before the release. According to knownsec, a rumour surfaced earlier in the year about a bug in Internet Explorer, iDefense wrote.

Information on the vulnerability was allegedly sold in November on the underground back market for $15,000. Earlier this month, the exploit was sold second or third hand for $650, said iDefense, citing knownsec.

Eventually, someone developed a Trojan horse program - one that appears harmless but is actually malicious - that is designed to steal information related to Chinese-language PC games, a popular target for hackers.

Now, other websites are being built that incorporate the exploit. Hackers then usually try to get people to visit those sites through spam or unsolicited instant messages. The Shadowserver Foundation has published a list of domains that are hosting the exploit and subsequent Trojan, although users are highly advised not to visit the websites. Most are ".cn" domains, the top-level domain for China.

iDefense said in a note that the vulnerability is "really nasty" and that computer security professionals could be in for a rough ride.

Microsoft issued its biggest group of patches in five years on Tuesday, and is not due for a regular patch release until 13 January, although it could opt to do an emergency release.

"Chances are this will be unpatched for around about a month, and that leaves plenty of time for attackers to take advantage," said Toralv Dirro, a security strategist based in Germany for McAfee's Avert Labs. "This should be taken pretty seriously."

iDefense said there aren't many options for users to defend themselves, but there is an easy one. The SANS Institute, which runs computer security training courses, recommended that people use a browser other than Internet Explorer.

Toralv said it could be tough to get the Internet Service Providers hosting the dodgy websites to take them offline, since the process is time intensive and service providers can be slow to respond.

The IE vulnerability compounds what looks to be a tough month for Microsoft, with the publication of another 0-day vulnerability in Microsoft's WordPad application.

That problem is somewhat less severe since a user would have be tricked into opening a maliciously-crafted document attached to an email. It also does not affect computers running Windows XP Service Pack 3 and Vista.

It does, however, affect Windows 2000 Service Pack 4, Windows XP Service Pack 2, Windows Server 2003 Service Pack 1 and Windows Server 2003 Service Pack 2, according to Microsoft.

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