Microsoft has unveiled the second stage of its April security updates by releasing five security bulletins that patched eight vulnerabilities – including one that was missed during the company's Windows Vista beta testing and ended up in the final version of the new operating system.

Of the four updates that addressed bugs in Windows, the MS07-021 update was clearly the one to patch pronto, said researchers. "This is my first [to patch] choice," said Amol Sarwate, manager of Qualys' research lab.

"It affects everyone," agreed Minoo Hamilton, senior security researcher with patch management vendor nCircle Network Security.

The update, which fixes three different bugs, includes one marked critical that affects all supported editions of Windows, from 2000 through XP and Server 2003 to Vista.

The vulnerability in the error message processing of the Windows Client/Server Run-time Subsystem (CSRSS) can be exploited remotely, said Microsoft, and could result in a complete compromise of the PC. And the most likely way to deliver an attack is to dupe users into visiting a malicious website.

Ironically, this MsgBox flaw was acknowledged by Microsoft more than three months ago, and was reported to the company's security team about the same time as the animated cursor (.ani) bug patched by an emergency fix last week.

"Number one, it affects earlier operating systems as well as Vista, so it's similar to the .ani vulnerability in that it's likely Microsoft reused [older] code in Vista," said Sarwate. "Two, it's a zero-day; it was out there [publicly] before today. Three, it has a web-based attack scenario. And fourth – last but not least – it affects both clients and servers and is a core component of the operating system."

Hamilton concurred, especially on the reused code and the similarity with .ani. "It confirms a lot that we discussed about the ANI bug and the type of vulnerabilities Vista will likely have," he said. "The file handing- and process- and local privilege escalation-type vulnerabilities will be what attackers focus on with Vista."

But Hamilton also cast back to 2006, when Vista was still in beta testing. "This vulnerability was in the betas, so it escaped from the beta all the way to now."

The US federal government's National Vulnerability Database confirmed Hamilton's charge. In a vulnerability summary first posted Dec. 21, 2006, a day before Microsoft acknowledged the bug, it listed Vista Beta 1 and Vista Beta 2 as affected.

Of the remaining four updates, Sarwate and Hamilton were split on the next, most dangerous bugs. Sarwate highlighted MS07-018 and MS07-019, both ranked critical by Microsoft; Hamilton, meanwhile, picked MS07-020 as the second most-important update he recommended users deploy.

"MS07-019 is potentially wormable," said Sarwate, because a hacker can actively send packets to two listening ports and there's no user interaction required." Hamilton's choice, MS07-020, is rated critical on both Windows 2000 and Windows XP SP2, which account for the bulk of Windows clients.

"The one bright spot," said Hamilton, "is that MS07-020 shows us a trend. Take a look at the 'Mitigating factors' in the bulletin. It shows a progressive locking down of the operating system from Windows XP SP2 to Windows Server 2003 to Windows Vista.

"But then you turn around, and there's MS07-021, and you see Vista breaking out of those protections."