There is a serious security flaw in Microsoft Word and Excel. (To the best of my knowledge, this flaw has not been reported in public before.) 



The stream cipher RC4 with key length up to 128 bits is used in Microsoft Word and Excel to encrypt the documents. But when an encrypted document gets modified and saved, the initialization vector of RC4 remains the same. Thus the same keystream generated from RC4 is applied to encrypt the different versions of that document. The consequence is disastrous since a lot of information of that document could be recovered easily from those encrypted different versions. 



The details of the flaw are described in the document (PDF):



http://eprint.iacr.org/2005/007.pdf