It's a shock for those of us using a proper modern browser such as Chrome or Firefox or IE9, but there are still lots of people using older versions of Internet Explorer, including the dreaded Internet Explorer 6. If you know someone who falls into that category, tell them that they need to patch their systems pronto to avoid a major potential vulnerability.
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The vulnerability is a familiar kind often seen in browsers: if a user visits a web page containing specifically-crafted malicious code, an attacker could gain control of that user's machine. Microsoft has rated the issue as severe enough to issue an "out of band" update, rather than waiting for its normal monthly Patch Tuesday set of updates.
Machines that are still running IE 6, 7 or 8 will fall into two main categories: users who are so slack about patching their Windows system that they haven't yet been forced into a compulsory upgrade to a newer version oF IE, and businesses which have stuck with an older version because of a compatibility issue with other software. The individual users are arguably a lost cause (and likely to already be part of a botnet). Companies that have made a conscious decision are rather more likely to deploy the patch, which will download via automatic updates if those are enabled.