MS Office 2007: Digital Signature does not protect Meta-Data
Affects: Microsoft Office 2007 (12.0.6015.5000)
MSO (12.0.6017.5000)
possibly older versions
I. Background
Microsoft Office is a suite containing several programs to
handle Office documents like text documents or spreadsheets.
The latest version uses an XML based document format.
Microsoft Office allows documents to be digitally signed by
authors using certified keys, allowing viewers to verify the
integrity and the origin based on the author's public key.
The author's public key certificate, which can come from a
trusted third party, is embedded in the signed document.
It is XML DSig based.
II. Problem Description
Microsoft Office documents carry meta data information
according to the DublinCore metadata in the file
docProps/core.xml . Among these meta data information
are the fields "LastModifiedBy", "creator" together with
several others that can be displayed/changed through the
following menu "Office Button -> Prepare -> Properties".
These entries can be changed without invalidating the signature.
At least under Windows Operating Systems these information are
also shown in the Window's file systems properties.
III. Impact
The meta data of signed Microsoft Office documents can be
changed. An attacker can change the values to spoof the origin
of signed documents, hoping to induce trust or otherwise
deceive the user.
III.1. Proof of Concept
Open the OOXML ZIP container of a signed document.
Change the values in the docProps/core.xml file.
For example set the value between "<dc:creator>*</dc:creator>"
to "<dc:creator>FooBar</dc:creator>".
The changes will be displayed in the document's properties
dialog as described above. The signature will still be valid.
IV. Workaround
The meta data information of a signed OOXML document
can be changed without invalidating the signature, thus
information about the real author of a signed document can
only be retrieved from the certificate.
The signed file's meta data can not be trusted as the
meta data is not covered by the signature.
V. Solution
No possible solution.
VI. Correction details
A closer look into the references section of the XML signature
used by Microsoft Office (stored in the File
_xmlsignatures\sig1.xml) reveals that the file core.xml is
not in the list of references. Thus it is not covered by the
signature.
As a solution the scope of the signature needs to be extended
to cover all the relevant information contained in the whole
document, thus also the meta data in core.xml.
Include core.xml, and probably other files in the signature's
list of references.
VII. Time line
2007-10-24: Vendor contacted
2007-10-25: Vendor acknowledged receipt
2007-11-14: 1st Deadline reached
2007-11-27: Reminder sent
2007-12-12: No response received until today
Yours,
Henrich C. Poehls, Dong Tran, Finn Petersen, Frederic Pscheid
SVS - Dept. of Informatics - University of Hamburg