Use-after-free vulnerability in Microsoft Internet Explorer 9 and 10 allows remote attackers to execute arbitrary code via vectors involving crafted JavaScript code, CMarkup, and the onpropertychange attribute of a script element, as exploited in the wild in January and February 2014.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.