Microsoft Office 2007 has a design flaw that allows outbound HTTP requests to be made when a document is opened that has a digital signature.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/65088/AKLINK-SA-2008-004.txt
Source: https://packetstormsecurity.com/files/65088/AKLINK-SA-2008-004.txt.html