Microsoft has released security update 12.2.4 for Office 2008 to address stability issues and CVE-2010-0263 in it Office products including Office 2008 for Mac. There is a flaw in the decompression of XLSX files, the headers are not validated properly. This could result in the execution of initialized blocks of memory, thus an attacker could execute remote code and execute it with that users privileges. The exploit could only be carried out with user interaction.
This update needs to be applied, select check for updates in Office or follow the reference link attached to download the file.