This vulnerability allows remote attackers to execute arbitrary code on vulnerable installations of Microsoft Office. Exploitation requires that the attacker coerce the target into opening a malicious .XLS file.
The specific flaw exists within the rebuilding of malformed cell comments. When Excel encounters a malformed record it attempts to rebuild the broken meta-data. A flaw in this rebuilding process allows the user to specify critical data offsets eventually leading to code execution with the credentials of the current user.