Microsoft has patched 34 vulnerabilities in Windows, Internet Explorer (IE), Office and other software, 15 of them labeled "critical" by the company.
The large number of updates -- as well as the fact that Microsoft issued them two hours later than usual -- will put pressure on enterprise administrators, one expert said.
"No doubt IT administrators will have to pick and choose where to act first," said Wolfgang Kandek, chief technology officer for Qualys.
Of the 16 updates, which Microsoft calls bulletins, nine were pegged critical, the most-serious rating in the company's four-step scoring system, while the remaining seven were tagged "important," the next-most-dangerous category.
While the number of bugs patched today was significantly less than the record 64 Microsoft fixed in April , it was the second-highest total for the year. The 16 bulletins were just one off the record, also set last April.
Fifteen of the 34 total vulnerabilities were rated critical, 17 were ranked important, and two were marked as "moderate."
Microsoft picked four of the 16 updates to highlight, and urged customers to roll out the quartet as soon as possible.
"Our top priorities are MS11-050, MS11-052, MS11-043 and MS11-042," Jerry Bryant, group manager with the Microsoft Security Response Center (MSRC), said in an interview earlier today. Bryant listed the four in the order of priority.
Among the deploy-immediately bulletins, MS11-050 offered 11 patches for IE that Microsoft and independent experts pinned to the top of their lists.
"This one is at the top of the list, as it always is when Microsoft patches IE," said Andrew Storms, director of security operations for nCircle Security. "But it's also the first IE9 update, and certainly does look to be true that Microsoft had this bug at the time it launched IE9, or a few days later."
Storms was referring to Microsoft's testing process, which usually lasts two months or more. That timeline would have precluded an IE9 patch in April, the first update scheduled after the browser shipped.
Microsoft habitually patches IE on even-numbered months; the last time it issued a security update for its browser was in April, when it fixed five flaws. Today's, however, was the first critical update for IE9, the browser that Microsoft shipped in mid-March. Four of the 11 patches in MS11-050 affected IE9, said Microsoft.
Nine of the 11 bugs in IE that Microsoft patched today could be exploited by attackers with a "drive-by" attack that requires users to simply visit a malicious Web site.
MS11-052 also affected IE, although Microsoft labeled it as a Windows update.
"The vulnerability is in Windows, but the attack vector is through Internet Explorer," said Bryant. "But IE9 is not affected by this update. [The issue] was addressed before IE9 released..., so that's part of the 'newer is better' message we're getting out to customers," Bryant added.
Only IE6, IE7 and IE8 can be used to exploit the vulnerability patched in MS11-052, not rival browsers, Bryant confirmed.
MS11-043 and MS11-042 were also called out by Bryant today. The former, which patches a single vulnerability in how Windows handles the SMB (server message block) protocol, could be used in what Bryant called a "browse-and-own" attack.
On the bright side, said both Bryant and Storms, many companies have blocked outbound SMB traffic at the firewall, which would prevent exploits of the flaw patched in MS11-043 .
"I think this may be difficult to exploit in the real world," said Storms.
MS11-042 updates DFS (distributed file service), which is used by administrators to group shared folders located on different servers, to patch a pair of bugs -- one critical the other important, in Windows. Microsoft rated the flaw as critical only on Windows XP and Windows Server 2003.
"[MS11-042 and MS11-043] are interesting, but I think they're technically more challenging to attackers," said Kandek.
In fact, Kandek rated MS11-045 , an eight-patch update for Excel, the spreadsheet included with Microsoft Office on Windows and Mac, as the second-most-serious of today's collection, immediately after the IE-oriented combo of MS11-050/MS11-052.
"Microsoft ranks it only as 'important' because the user is required to open an attacker-provided file, but we believe that attackers have shown often enough that they have the skills to make opening the file enticing to users," said Kandek.
"If I was the attacker, this would certainly be one I would use, if only because users tend to trust Excel files," Kandek added.
Of the eight vulnerabilities patched in the Excel update, only one affected the newer versions, Excel 2007 and Excel 2010 on Windows; two impacted Excel 2011 on the Mac.
"It's blatantly clear that the newer Office software is much better and more secure," said Storms.
Microsoft also issued updates today for SQL Server, its Forefront 2010 security product, the .Net Framework and Silverlight development platforms, and the virtualization hypervisor included with Windows Server 2008 and Server 2008 R2.
June's security updates can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services (WSUS).