Guardeonic Solutions AG
Thorsten Delbrouck <tdk guardeonic com>
http://www.guardeonic.com/
Security Advisory #01-2004
Advisory Name: Microsoft Word Form Protection Bypass
Release Date: 2004-01-02
Affected Product: Microsoft Word
Platform: Microsoft Windows, probably Apple Mac OS
Version: tested on 2000, 2002 (XP), 2003,
probably other versions vulnerable as well
Severity: Document ("Form") protection can be easily removed
Author: Thorsten Delbrouck <tdk guardeonic com>
Vendor Communication: 2003-11-27, 10:30 UTC Microsoft notified
to: secure microsoft com
2003-11-27 confirmed receipt
from: secure microsoft com
2003-12-03 Note from Microsoft, Form
protection "is not intended as a full-proof
protection for tampering or spoofing, this is
merely a functionality to prevent accidental
changes of a document", request additional
time to update Microsoft Knowledge Base
article. Targetting beginning of January 2004
for release of this advisory.
from: "Magnus" <secure microsoft com>
2003-12-08 Microsoft has already released the
KB article (or added a warning to an existing
article). Read the KB article at
http://support.microsoft.com/?id=822924
from: "Magnus" <secure microsoft com>
Overview:
---------
Word provides an option to protect "forms" by password. This is used
to ensure that unauthorized users can not manipulate the contents of
documents except within specially designed "form" areas. This feature
is also often used to protect documents which do not even have form
areas (quotations/offers etc.).
(Word users will find this option on the "Tools" menu, entry
"Protection", select "Forms" there and provide a password)
If a Word document is "protected" by this mechanism, users cannot
select parts of the text or place the cursor within the text --- thus
they cannot make any changes to the document.
Description:
------------
When saving protected Word-documents as html-files, Word adds a
"checksum" of the password (enclosed in a proprietary tag) to the
code. The checksum format looks somewhat like CRC32 but currently
there are no further details available. The same checksum can be
found within the original Word document (hexadecimal view). If this
"checksum" is replaced by 0x00000000 the password equals an empty
string.
Example:
--------
1.) Open a protected document in MS Word
2.) Save as "Web Page (*.htm; *.html)", close Word
3.) Open html-document in any Text-Editor
4.) Search "<w:UnprotectPassword>" tag, the line reads something like
that: <w:UnprotectPassword>ABCDEF01</w:UnprotectPassword>
5.) keep the "password" in mind
6.) Open original document (.doc) with any hex-editor
7.) search for hex-values of the password (reverse order!)
8.) Overwrite all 4 double-bytes with 0x00, Save, Close
9.) Open document with MS Word, Select "Tools / Unprotect Document"
(password is blank)
Variation:
----------
If the 8 checksum bytes are replaced with the checksum of a known
password it should be fairly easy to unprotect the document, make any
necessary changes, save, close and reset the password to the original
(unknown!) password by simply restoring the original values. Document
changed without even knowing the password. Nasty.
(Note: Take care to get file properties (author, organisation,
date/time etc.) right.)
Solution:
---------
No solution is currently available. Do not rely on the "Protect
Forms" mechanism to protect a Word document against changes.
Credits:
--------
Magnus from the Microsoft Security Response Center for his fast
responses and for showing a decent sense of humour. :-)
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