Microsoft has released a patch for a critical day-zero vulnerability in Internet Explorer 6, Internet Explorer 7, and Internet Explorer 8. On investigating the vulnerability, Microsoft writes:

The vulnerability is a remote code execution vulnerability that exists in the way that Internet Explorer accesses an object in memory that has been deleted or has not been properly allocated. The vulnerability 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. An attacker could host a specially crafted website that is designed to exploit this vulnerability through Internet Explorer and then convince a user to view the website.

On completion of this investigation, Microsoft will take the appropriate action to protect our customers, which may include providing a solution through our monthly security update release process, or an out-of-cycle security update, depending on customer needs.

Microsoft has released a shim workaround for the issue. More information can be found on the Microsoft Security Advisory.