Microsoft releases temporary fix for new IE zero-day exploited in the wild

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Microsoft is warning that a new zero-day flaw in its Internet Explorer (IE) browser is being leveraged in targeted attacks.

On Tuesday, the company posted details about the bug in a security advisory and released a “Fix it” solution – a temporary workaround for the flaw in IE 8 and 9.

The vulnerability, CVE-2013-3893, already being exploited in the wild, may lead to memory corruption, which could allow a remote attacker to execute malicious code in IE, Microsoft revealed.

“The vulnerability is a remote code execution vulnerability,” the advisory said. “The vulnerability exists in the way that Internet Explorer accesses an object in memory that has been deleted or has not been properly allocated…[and] may corrupt memory in a way that could allow an attacker to execute arbitrary code in the context of the current user within Internet Explorer.”

To attack, a saboteur could easily convince users to view a malicious page designed to exploit the flaw. Simply by targeting victims with phishing emails, attackers could scale an attack.

In a Tuesday blog post, Dustin Childs, group manager of response communications at Microsoft Trustworthy Computing, said that hackers could also opt to carry out watering hole attacks to target users: “This would typically occur when an attacker compromises the security of trusted websites regularly frequented [by users],” he wrote.  

So far, customers have reported a “limited number” of targeted attacks directed at IE 8 and 9, though the issue could impact users running “all supported versions” of the browser, Childs warned.

Upon applying the Fix it solution, users are also advised to set their internet and local intranet security zone settings to “high” to prevent exploitation of the bug, Childs wrote. While browsing, users should also disable Active Scripting – a Windows feature used to implement component-based scripting support – or configure IE so they are prompted when Active Scripting runs.

On Wednesday, Dana Tamir, director of enterprise security at Trusteer, an IBM company, blogged that implementing the temporary fix could “limit some functionalities of IE,” and that the workaround couldn't be applied to certain versions of the browser.

“The ‘Fix it' applies only to 32-bit versions of Internet Explorer,” Tamir wrote.  “If you are running 64-bit, the ‘Fix it' cannot be applied.”

In its advisory, Microsoft said that it was actively working to release a patch for the issue, either in its next monthly security update – due out Oct. 8 – or in an out-of-cycle release.

[This story has been updated to reflect that Trusteer, an IBM company, is the official name of the security firm since its acquisition.]
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