Office 2007 documents security question

In previous versions of MS Office the office documents are stored in a
proprietary format. In Office 2007 they have switched to this new open xml
format. which is good for user manipulation and other cool stuff you can do
with XML. But it also opens the door to potential vandalism. What has been
done to protect the integrity and security of the documents? Can a malicious
hacker possibly write a script that parses through all documents and add his
mark? and Office wouldn't detect it because it's no different than regular
user input.

your thoughts please

Howard