Security experts are urging users to apply newly released mitigations as a stop-gap while waiting for Microsoft to patch a newly discovered critical vulnerability in Internet Explorer.

Microsoft rushed out a Fix It tool yesterday in lieu of a patch after reports surfaced that attackers were using the vulnerability to target Internet Explorer 8 and 9. According to Microsoft, the vulnerability exists in the way that IE accesses an object in memory that has been deleted or has not been properly allocated. The vulnerability could corrupt memory in a way that could permit an attacker to execute code in the context of the current user within IE.

An attacker could exploit this issue through drive-by downloads, either by compromising a legitimate site or tricking a victim into clicking a malicious link in an email or instant message.

"It's not clear how many legitimate sites, if any, may have been found serving this malware, but Microsoft is definitely taking notice," says Ross Barrett, senior manager of security engineering at Rapid7. "Considering the timing, I would personally expect to see an out-of-band patch from Microsoft."

Noting that the issue is believed to be present in all supported versions of Internet Explorer, he adds that it is possible that the vulnerability has been targeted for some time.

"The fact that it is getting attention now is due to a noticeable volume or impact of active exploitation in the wild," Barrett says. "It may have just been discovered last week, or it may have been in the private toolkit of the world's best malware writers for more than a decade. Hard to say."

Microsoft did not offer any further information about the kinds of websites being used as traps to target victims, and no word has surfaced on when a patch will be available. The company continues to urge customers to apply the Fix It solution, "CVE-2013-3893 MSHTML Shim Workaround," to prevent the vulnerability from being exploited.

According to Websense, an analysis of third-party telemetry feeds from "real-time global Internet requests" suggests as many as 70 percent of Windows business users are susceptible to attackers due to the fact that they are running IE 8 or IE 9 on Windows XP or Windows 7, the systems the attacks are currently focusing on.

"This [attack] is evidence that attackers continue to target low-hanging fruit," says Patrick Thomas, security consultant at Neohapsis. "Address Space Layout Randomization (ASLR) is one of several defensive technologies baked into modern programs and libraries, which makes attacks like these significantly harder. It’s no coincidence that attackers are targeting a dynamic-link library (DLL) that did not get compiled with ASLR."

Enterprise administrators, he adds, should be aware of what software on their networks does or does not use built-in protections such as DEP, ASLR, and stack protections. They should consider upgrade plans or establish patching priorities to mitigate the risks facing those more easily targeted programs.

Besides the Fix It tool, there are some mitigating factors related to the attack. For example, Internet Explorer on Windows Server 2003, Windows Server 2008, Windows Server 2008 R2, Windows Server 2012, and Windows Server 2012 R2 run in a restricted mode that mitigates the vulnerability.

In addition, all supported versions of Microsoft Outlook, Outlook Express, and Windows Mail open HTML email messages in the "Restricted" sites zone, which disables script and ActiveX controls.

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