Microsoft patched six vulnerabilities, most marked 'critical', in Windows, Word, Publisher and its antivirus software during May's Patch Tuesday series of updates.

The most important patch to apply, said analysts, is MS08-028, a critical fix that updates the Jet Database Engine in Windows 2000, Windows XP SP2 and Windows Server SP1. "We have to address this first," said Andrew Storms, director of security operations at nCircle. "There are public exploits out for this."

"Jet Database should be done first," agreed Amol Sarwate, the manager of Qualys' vulnerability research lab. "This is a zero-day that Microsoft themselves acknowledged as having seen not only proof-of-concept code, but also public exploits."

Two months ago, Microsoft confirmed critical vulnerabilities in Jet Database Engine, a Windows component that provides data access to applications such as Microsoft Access and Visual Basic, and posted a security advisory that acknowledged "limited, targeted attacks" using Word documents to trigger the Jet Database bug.

Microsoft knew of the Jet Database bugs for more than two years, but had not patched the problems because it thought it had blocked the obvious attack vectors, a manager in the Microsoft Security Response Center (MSRC) said several days later. Mike Reavey, the group's operations manager, said Microsoft might replace the version of Jet in Windows 2000, XP and Server 2003 SP1 to fix the flaws. According to MS08-028, Microsoft is doing just that.

The company also reiterated that attacks have been spotted in the wild exploiting the vulnerability. "Microsoft had received information that this vulnerability was being exploited," the company said in the security bulletin issued on Tuesday.

The Jet Database Engine included in Windows Vista, Windows Server 2003 SP2 and the just-released Windows XP SP3 is not vulnerable to the attacks, and doesn't require replacement.
What is less clear, however, is how Microsoft patched Word and Outlook to shut down the attack vectors that the public exploits have used to leverage the Jet Database problem.

Storms wasn't sure exactly what Microsoft fixed in MS08-026, the security bulletin it released on Tuesday for Word. "We're all kind of asking 'huh?' about that," Storms said. "My guess is that we're not the only ones asking what's been fixed."

In late March, when Reavey admitted that Microsoft had not conceived of Jet Database exploits that used Word to trick users into opening malformed .mdb files, he also said that the MSRC was considering a patch to "prevent Word documents from loading MDB files without prompting".

On Tuesday, it sounded like Microsoft had taken that route. "In addition to the changes that are listed in the 'Vulnerability Details' section of this bulletin, this update includes logic enhancements to security warnings that mitigate Word as an attack vector used to exploit vulnerabilities in Microsoft Jet Database Engine," the MS08-026 bulletin said. "Word was vulnerable to attacks when opening a specially crafted Word document containing a malicious Jet database file. After applying this update, Word will prompt a user for confirmation before running SQL commands or queries when opening Word documents."

However, Microsoft did not spell out the changes to Word in a separate CVE (Common Vulnerabilities and Exposures) listing, as is its usual habit. At times, the company has been criticised for bundling multiple fixes in a single bulletin without detailing each.

Although Sarwate of Qualys echoed Storms - "it is a little confusing", he said, referring to the information about Jet Database in the Word bulletin - he said fixing it on Windows' end was not only the most important part of the exploit chain to patch, but the only patch really necessary.

MS08-026 patches a pair of critical bugs in how Word and Outlook render RTF (rich text format) files and documents with Cascading Style Sheets (CSS). Microsoft pegged the flaws as critical in Word 2000 and Outlook 2007, and 'important' - the second-highest threat rating in the company's four-step system - in Word 2002, 2003 and 2007, as well as in the versions of Word included with Office 2004 for Mac and Office 2008 for Mac.

Another bulletin, MS08-027, patches a vulnerability in several versions of Microsoft Publisher, the company's consumer- and small business-grade desktop publishing program. Although Microsoft designated the fix as critical, Sarwate downplayed the patch as nothing new. "This is just another of the kind we've seen in the past year where client-side applications are being targeted," he said.

The fourth security update, however, caught both Sarwate's and Storms' attention. MS08-029 quashes two bugs in Microsoft's anti-malware scanning engine, which is used by a whole host of Microsoft security apps, including Windows Live OneCare, Microsoft Antigen, Windows Defender and Microsoft's Forefont line.

Sarwate disagreed with Microsoft's 'moderate' ranking of the two bugs. "That may work for the immediate consequences, but if you think of the impact of closing down your anti-virus, you're basically exposing yourself to viruses and all sorts of bad stuff," he said. "I think it should have been at least one level up, to 'important'," said Sarwate.

Storms also noted that it was possible that an attack aimed at Microsoft's vulnerable security products could make a system, a server, for example, unbootable.

"One of the two vulnerabilities talks about 'disk space exhaustion'," said Storms. "When an anti-virus scanner scans a file, it usually creates a temp file in a safe zone and scans it there. This bug could cause the scanner to open so many temp files that it would exhaust the available disk space. That makes this a little more interesting than Microsoft says, because if the system doesn't clean itself up as it crashes, [the attack] could render the machine unbootable."

Sarwate also reminded users that Microsoft didn't deliver on one expected patch. "Last month, on April 17, Microsoft issued an advisory that talked about a zero-day it called an elevation of privilege bug," said Sarwate.

Almost a month ago, Microsoft warned of a bug that could give attackers significantly greater access to Windows XP Professional SP2, and all versions of Windows Server 2003, Windows Vista and Windows Server 2008. The flaw could be exploited through custom Web applications running in Microsoft's Web server, Internet Information Services (IIS), or via SQL Server, added Microsoft.

May's security updates can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services.