That was fast: Microsoft today released an emergency patch for a previously unknown Internet Explorer vulnerability revealed over the weekend that was discovered being exploited by a cyber espionage group out of China. 

In a surprise twist, Microsoft included a patch for IE on Windows XP, the older operating system it no longer supports as of last month.

Microsoft was under pressure for a quick fix to the flaw (CVE-2014-1776), which came just after it ended support for Windows XP, prompting advice from UK and US CERTs for users to consider using alternative browsers until IE got its patch. The bug, a "critical" memory corruption vulnerability, according to Microsoft, was spotted being used in drive by web attacks. It affects IE versions 6, 7, 8, 9, 10, and 11, and basically allows an attacker to remotely run code on a targeted machine.

"The security of our products is something we take incredibly seriously. When we saw the first reports about this vulnerability we decided to fix it, fix it fast, and fix it for all our customers," said Adrienne Hall, general manager for Microsoft Trustworthy Computing.

Hall said in a blog post that Microsoft decided to include a patch for IE on the Windows XP as well. She downplayed the worries about widespread attacks using the 0day, noting that the number of actual attacks were minimal. Hall said:

Even though Windows XP is no longer supported by Microsoft and is past the time we normally provide security updates, we’ve decided to provide an update for all versions of Windows XP (including embedded), today. We made this exception based on the proximity to the end of support for Windows XP. The reality is there have been a very small number of attacks based on this particular vulnerability and concerns were, frankly, overblown. Unfortunately this is a sign of the times and this is not to say we don’t take these reports seriously. We absolutely do.

IE 10 and 11 users that had the Enhanced Protection Mode in place by default were safe from exploits of the bug, as well as users running Microsoft's Enhanced Mitigation Experience Toolkit (EMET) versions 4.1 and 5.0.

The exploit spotted in the wild used a Flash exploitation method, and bypassed Microsoft's Address Space Layout Randomization (ASLR) and Data Execution Prevention (DEP) protections.

Trey Ford, global security strategist at Rapid7, applauded Microsoft's quick turnaround for the patch. Ford says:

Out-of-band updates are a big deal. Major vendors like Microsoft, Oracle, Adobe and others have highly structured software testing workflows that are expensive in terms of time and resources.  To interrupt a scheduled development cycle for an emergency patch, or out of band release is a noteworthy event where a vendor is placing the public good ahead of their development and delivery lifecycle. One thing particularly of interest is that Microsoft made the decision to issue this patch for Windows XP, which is no longer officially supported. I think this underscores the importance of this patch, and the priority with which it should be deployed. Corporate and private users should prioritize downloading (testing, where required by change controls) and deploying this patch.

Meanwhile, Microsoft's Hall noted that users with Windows Automatic Updates will automatically get the update. "If you are like most people, you have automatic updates turned on, and you’ll get this new update without having to do anything.  If you haven’t turned it on automatic updates yet, you should do so now.  Click the 'Check for Updates' button on the Windows Update portion of your Control Panel to get this going," Hall said.