Excel and PowerPoint have a macro security framework that controls the execution of macros and prevents macros from running automatically. Under this framework, any time a user opens a document the document is scanned for the presence of macros. If a document contains macros, the user is notified and asked if he wants to run the macros or the macros are disabled entirely, depending on the security setting. A flaw exists in the way macros are detected that can allow a malicious user to bypass macro checking.
A malicious attacker could attempt to exploit this vulnerability by crafting a specially formed Excel or PowerPoint document with macro code that would run automatically when the user opened it. The attacker could carry out this attack by hosting the malicious file on a web site, a file share, or by sending it through email.