Microsoft is reporting an unpatched vulnerability in all versions of Internet Explorer. All versions of IE, other than those running on Windows Server, are vulnerable. This includes Internet Explorer 11 on Windows 8.1 and RT.
The vulnerability comes from a memory corruption bug which could lead to remote code execution. Microsoft says that they are aware of targeted attacks exploiting this vulnerability on Internet Explorer 8 and 9. Exploits such as these are often version-specific, even if the vulnerability affects multiple versions.
Attacks may be blocked by running a Microsoft "Fix it" solution for an earlier vulnerability: CVE-2013-1347 MSHTML Shim Workaround.
The company has not decided how to respond to the vulnerability. Certainly they will write a patch, but whether they schedule it for a Patch Tuesday or go "out of band" is not yet clear.
Microsoft's advisory also says that EMET (the Enhanced Mitigation Experience Toolkit) may be used to mitigate against the vulnerability.