As of the 12th January, Microsoft have ended support for all but the most recent version of Internet Explorer. This means that the older versions will cease to be updated, and will therefore quickly become susceptible to attack.
In most cases, these means that Internet Explorer 11 becomes the only supported version. The one exception to this is for those users still using Windows Vista. As Internet Explorer 8 is the last version made available for this operating system, this will continue to receive security updates until Vista itself reaches end of life on the 11th of April 2017.
Of course, users of Windows 10 will probably have noticed that Internet Explorer has now been sidelined entirely in favour of Microsoft's new "Edge" browser. While it is still there (albeit hidden away), it is only really being provided for legacy purposes.
Microsoft also ended support for Windows 8 on the same day. Anyone still using this version of Windows should either upgrade to Windows 8.1 or Windows 10 to continue receiving updates.