Announced last weekend was an Interent Explorer vulnerabilty of such a serious nature that Homeland Security has recommended not to use this browser.

It affects versions 6 to 11. Targets appear to be US defense and financial sectors according to FireEye as quoted in the link below. Recommendations besides not using IE, are to turn off Adobe Flash for IE and use EMET(Enhanced Mitigation Experience Toolkit), a Microsoft security tool.

http://www.theglobeandmail.com/technology/tech-news/microsoft-rushes-to-fix-internet-explorer-after-attacks/article18297817/

http://www.forbes.com/fdc/welcome_mjx.shtml

http://www.fireeye.com/blog/uncategorized/2014/04/new-zero-day-exploit-targeting-internet-explorer-versions-9-through-11-identified-in-targeted-attacks.html