A critical vulnerability in Windows Vista has been found and fixed, a Microsoft security manager admitted on Wednesday. However, he argues that the flaw in the company's malware scanning engine isn't in the operating system's core code.

Microsoft has repeatedly touted Vista as its most secure version of Windows ever, so watchers are eagerly awaiting the first vulnerability to be spotted and fixed.

On Tuesday, Microsoft released 12 security updates to fix 20 vulnerabilities, 11 of them pegged as "critical" in the company's four-step scoring system. One of the bulletins, MS07-010, patched a critical bug in the malware scanning engine used by Microsoft's security products, including Windows OneCare, Windows Defender, Forefront Security and Antigen.

Windows OneCare and Windows Defender run on Vista client systems; Defender is in fact built into Vista - so technically, this is a Vista bug.

"The underlying component [of Windows Defender] does ship with Vista," acknowledged Mark Griesi, security program manager for the Microsoft Security Response Center (MSRC). "That's the correlation people are drawing. The vulnerability is in the malware engine, which is part of Defender, which is part of Vista. So, yes, you can say that Vista [itself] is vulnerable.

"But is it a direct vulnerability in Vista? I wouldn't characterise it like that," he added.

One point of confusion with the scanning engine bug is how it's patched. In Vista, for instance, it would never appear in the Windows Update window. Although Windows Defender relies on Vista's own update mechanisms to retrieve new signature files and security updates such as the MS07-010 patch, it does that behind the scenes. Windows OneCare, on the other hand, has its own internal update feature that circumvents Windows Update. Forefront and Antigen, meanwhile, use the Forefront Server security update service.

"By default, all have auto update turned on," said Griesi. "Enterprises can set [Forefront and Antigen] to not auto update, of course, but users should see an update on whatever cycle the software's set to." While Microsoft throttles down updates - a process that means it may take several days for all users to receive critical updates - Griesi said that the scanning engine fix has probably already been downloaded and installed on most affected users' machines.

Some security researchers took the Vista's-not-flawed side of the argument. "Technically, it's not a Vista vulnerability," said Amol Sarwate, manager of Qualys' vulnerability lab. "But nothing is impervious. We're not seeing [a vulnerability in Vista] at this point, but we anticipate that there will be one."

Whather or not it’s a Vista flaw, other experts were critical of Microsoft for letting it slip through the cracks. "This is exactly the kind of thing that you would have expected Microsoft to catch," Minoo Hamilton, a senior security researcher at patch management vendor nCircle Network Security said Tuesday.

In fact, Alex Wheeler, one of the researchers with IBM's Internet Security Systems X-Force group credited with discovering the scanning engine bug, is noted for digging up vulnerabilities in antivirus and other security software. Among the vendors whose products Wheeler has pinned with bugs are Kaspersky, McAfee, Sophos and Symantec.

Griesi rejected critics who said Microsoft's emphasis on security during development should have ensured a bug-free scanning engine, especially since similar flaws had been found in other antivirus software more than two years ago.

"Yes, [the scanning engine] was developed under Security Development Lifecycle," he said, referring to the process has Microsoft instituted in several steps since 2002 to make its software more secure. Windows Vista was the first operating system completely written under those guidelines. "But there's no software that's 100 percent [secure].

"I wouldn't call this an 'embarrassment,'" said Griesi when asked about how nCircle's Hamilton characterised the bug in Vista. "We beta-tested Vista more thoroughly than any other version of Windows, [but] there are always new threats."

And when the company finds bugs, as in this case, it will patch them, he said. "We're always learning."