Microsoft today said it will deliver six security updates on Tuesday, including one that will patch a vulnerability in Internet Explorer (IE) the company admitted only last week.

The updates will patch a total of 12 flaws in Windows, IE and Microsoft Office, the company said in a follow-up entry to its security response center's blog .

At the top of the patch list, even Microsoft's own, will be an update for IE 5.01, IE6, IE7 and IE8 that has been pegged as "critical," the firm's highest severity rating in its four-step scoring system.

The update will address an IE zero-day vulnerability that Microsoft confirmed Nov. 23 in a security advisory. "I want to point out that Internet Explorer 8 is not affected on any platform and that running Protected Mode in Internet Explorer 7 on Windows Vista mitigates this issue," said Jerry Bryant, a spokesman for the Microsoft Security Response Center (MSRC), in a blog post announcing the advisory last week.

Microsoft's advisory was its reaction to proof-of-concept attack code that had gone public several days before, when it was posted to the popular Bugtraq security mailing list. The sample code exploited a flaw in IE's layout parser, and could be used to hijack fully-patched Windows machines.

Next Tuesday's update, however, will quash bugs in all still-supported versions of IE, not just IE6 and IE7, a fact Microsoft confirmed today . "We want to make customers aware that we will be addressing the vulnerability discussed in Security Advisory 977981 in the IE bulletin on Tuesday," Bryant said in another blog post.

The advisory Bryant called out was the one Microsoft issued last week. "We know that customers are concerned about this issue and we are also aware that Proof of Concept (PoC) code is available publicly."

Microsoft's advance notification spelled out the significance of the problem with IE: All versions of its browser contain one or more flaws when run on Windows 2000, Windows XP, Vista, Server 2003 and even Windows 7. Only the company's newest server products -- Server 2008 and Server 2008 R2 -- are somewhat safer.

However, that doesn't mean IE 5.01 and IE8 are suddenly vulnerable to last week's zero-day exploit, said Andrew Storms, director of security operations at nCircle Network Security. Microsoft confirmed to Storms that the IE update contains fixes for multiple vulnerabilities; it's not unusual for Microsoft to quash several bugs in a single update.

"Last week's zero-day is still not applicable to IE8," Storms said after he consulted with MSRC. "Some other bug is also being patched in the same bulletin."

The attack code that went public Nov. 20 was not only unreliable, according to security experts who dived into the exploit, but was touted as only affecting IE6 and IE7 on Windows XP. Later, Microsoft said that Vista users would be protected to a degree not enjoyed by XP customers because the former's "sandbox" would limit the ability of the exploit to compromise the PC.

Most outside security researchers, including Storms, were pessimistic last week when asked whether Microsoft could scramble fast enough to fix the flaw in IE6 and IE7. Today, he was impressed. "I would have wavered last week [on] whether they would fix it," he admitted. "But given the impact [of the vulnerability] and the fact that there's code out there, I'm not surprised that they managed it."

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