A couple years ago, in diary entry "Unprotecting Malicious Documents For Inspection" I explain how .xls spreadsheets are password protected (but not encrypted). And in follow-up diary entry "Maldocs: Protection Passwords", I talk about an update to my oledump plugin plugin_biff.py to crack these passwords using password lists (by default, an embedded password list is used that is taken from the 2011 public-domain default password list used by John The Ripper).