Microsoft today released an emergency patch for Internet Explorer versions back to version 5.01 to fix a gaping security hole that was being exploited so massively that security experts were recommending that people stop using Internet Explorer entirely until it was fixed. Since the bad guys were exploiting it before Microsoft knew it existed, the exploit is termed "zero day" because that is how much notice Microsoft got of the problem. It is also termed an "drive-by" exploit since a user could pick up a malware infestation by merely using IE to browse any of thousands of compromised websites. In short, it was really nasty stuff.
This security update resolves a publicly disclosed vulnerability. The vulnerability could allow remote code execution if a user views a specially crafted Web page using Internet Explorer. Users whose accounts are configured to have fewer user rights on the system could be less impacted than users who operate with administrative user rights.
This security update is rated Critical for Internet Explorer 5.01, Internet Explorer 6, Internet Explorer 6 Service Pack 1, and Internet Explorer 7. For information about Internet Explorer 8 Beta 2, please see the section, Frequently Asked Questions (FAQ) Related to This Security Update. For more information, see the subsection, Affected and Non-Affected Software, in this section.
Recommendation. Microsoft recommends that customers apply the update immediately.
Aside from the table of download locations listed at the above link, the emergency patch is also available through Windows Update and Microsoft Update.