This vulnerability allows remote attackers to execute arbitrary code on vulnerable installations of Microsoft Excel. User interaction is required to exploit this vulnerability in that the target must visit a malicious page or open a malicious file.
The specific flaw exists within Excel's parsing of Feature11/Feature12 records. The process trusts a supplied counter value without validating its size and proceeds to use it within a copy operation to the stack. An attacker can abuse this to execute arbitrary code under the context of the user running Excel.