CVE-2007-4600 - Mathcad Protect Worksheet Vulnerability

[bugtraq () firewraith co uk]

Mathcad Security Vulnerability Briefing - CVE-2007-4600


Synopsis of Vulnerability
==========================
The ‘Protect Worksheet’ functionality, used to protect sections Mathcad sheets from alterations, in versions 12 through 
14 is easily bypassed allowing access to the protected data due to the implementation of the file format used to save 
the files. 


Background on Mathcad
======================
Mathcad (http://www.ptc.com/appserver/mkt/products/home.jsp?k=3901) is used to perform, document and share calculation 
and design work. The unique Mathcad visual format and scratchpad interface integrate standard mathematical notation, 
text and graphs in a single worksheet - making Mathcad ideal for knowledge capture, calculation reuse, and engineering 
collaboration.


Vulnerable Software Versions
=============================
Mathsoft, Mathcad 12 
Mathsoft, Mathcad 13
Mathsoft, Mathcad 13.1
PTC,      Mathcad 14

Running on Microsoft, Windows 2000, Service Pack 4
Running on Microsoft, Windows XP, Service Pack 2


Impact
=======
Access Vector:          Locally exploitable.
Access Complexity:      Low.
Authentication:         Not required to exploit.
Impact type:            Provides unauthorised access. Allows partial confidentiality, integrity, and availability 
violation. Allows unauthorised disclosure of information.


Description of Vulnerability
=============================
According to Mathcad’s online help: 

‘When distributing worksheets, you may wish to restrict user access to most regions. Rather than locking an area, you 
may opt instead to use worksheet protection.
The intent of file protection is to prevent other users from opening the worksheet in a text editor and editing its 
contents by hand. The allowed file formats are either binary (XMCDZ, MCD) or output-only (RTF, HTML). With file 
protection enabled, you can only alter the contents of a worksheet from Mathcad. You can create, edit, and delete 
regions within the worksheet with no restrictions.’



The XMCDZ file format is not a true binary format. It is the standard Mathcad .XMCD XML sheet, which has been GZIPPED. 
For this reason it is a simple matter to get the original plain text XML sheet out of the file, using an archive 
utility.

Once the XML file has been extracted, within the <editor> tag there will be a <protection> tag. This will look like: 
<protection protection-level="low" password="XZEdIlJPXZxa1CQRKn6Sfw=="/> 

There are 2 components to this tag; the level of restrictions places upon the sheet and also an optional password 
needed for un-protecting the sheet. 

There are 3 protection-level settings, high, medium and low. These correspond to Editing, Content and File protection, 
respectively. For example if a sheet was saved with Editing protection enabled, then the <protection> tag would have a 
"high" protection level. This can easily be changed with a text editor before the sheet is reopened in Mathcad.

The password is hashed, however the same hash is always produced for a given string. For example 
"XZEdIlJPXZxa1CQRKn6Sfw==" represents the string "password", and could be used in any sheet. 

Due to these limitations the entire <protection> tag could be removed, the level of protection could be reduced, or the 
password could be changed. 

The MCD format is a proprietary binary type format. It was used in older version of the application, before the XML 
format became the standard. However if this format is selected from a newer version of the application, a warning is 
generated stating that ‘If your worksheet is saved as a Mathcad 11 file, some features and calculations may not be 
preserved’. Selecting either Mathcad 11 or Mathcad 12 MCD file formats produces a warning about potential loss of 
functionality.

The sheets do include an MD5 hash, however this is only used internally by the application to determine if the sheet 
has been changed outside of Mathcad and the equations require recalculating. Changing the <protection> tag in the XML 
file will not be detected by the application, and no exceptions will be raised.


Workaround
===========
None.


Proof of Concept
=================
None required.