The company also debuted a new defensive measure to help users ward off ongoing attacks that are exploiting a known bug in Internet Explorer (IE).
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The light load -- just two security updates, or "bulletins" as Microsoft calls them -- was announced last week , making for an easier beginning to the new year than the end of 2010, when in December the company shipped a record 17 updates that patched a near-record 40 bugs.
One of today's updates was classified as "critical" by Microsoft, the firm's top threat ranking, while the other was marked as "important," the second-most dangerous rating.
MS11-002 was the update that security researchers and Microsoft recommended users apply first. The update patched two vulnerabilities, one critical, the other important.
"Attackers can exploit the critical vulnerability in MS11-002 by getting users to browse to a malicious website," said Amol Sarwate, manager of Qualys' vulnerabilities research labs. The tactic, usually called a "drive-by" attack, relies on enticing users to click a link that's offered in a baited email.
"It's exploitable through a drive-by," confirmed Sarwate.
The bug is in the Microsoft Data Access Components (MDAC), a set of components that lets Windows access databases such as Microsoft's own SQL Server. The flaw is in the MDAC ActiveX control that allows users to access databases from within IE.
Only users running IE are at risk from attacks exploiting the critical bug Microsoft disclosed in MS11-002, said both Sarwate and Andrew Storms, the director of security operations at nCircle Security.
Microsoft also urged customers to apply MS11-002 first, noting that all client versions of Windows, including XP Service Pack 3 (SP3), Vista, and Windows 7 were vulnerable. The server editions of the operating system are vulnerable as well, but for them Microsoft rated the threat as important, not critical.
Hackers will probably come up with reliable attack code to exploit the bugs patched by MS11-002 in the next 30 days.
The other update, dubbed MS11-001, is less important, said Sarwate and Storms, because it applies only to Windows Vista.
The Backup Manager bug is one of several so-called "DLL load hijacking" or "binary planting" vulnerabilities in Windows.
Today's fix for Vista was the seventh update Microsoft's released to repair flaws that researchers disclosed last August. Microsoft shipped five DLL load hijacking updates last month, and one in November.