WMF Exploitation FAQ

You think that every possible bit of information about the WMF issue was already posted ? What about an exploitation FAQ ?

Here it goes, H.D. Moore, creator of Metasploit posted one on the [funsec] list.

Exploitation FAQ:

Q) The Windows Meta File format has a number of optional headers, can any
of these be used to trigger the arbitrary code execution flaw via
SetAbortProc?

A) No. The CLP headers (16 bit and 32 bit) cause the Picture and Fax
Viewer (PFV) and Internet Explorer to throw an error when trying to
render the image. Internet Explorer will only display an image internally
if the “placeable” header has been prepend to the bare WMF header. If the
“placeable” header exists, a device context check will fail during the
call to Escape() and the SetAbortProc() function is not reached. This
effectively prevents IE or the PFV from executing the SetAbortProc() call
when any optional header has been prepended. This may not hold true for
Explorer’s preview and icon view.

Q) What about the Enhanced Meta File format? Does this format allow access
to the exploitable function?

A) No. The EMF format has a separate API (which may or may not have its
own problems), but it does not allow access to the WMF Escape() function.
A WMF file can be delivered with the EMF extension however, which will
cause it to be processed with the vulnerable API.

Q) Are there any other ways to obtain code execution besides via WMF files
viewed by PFV or Explorer?

A) Yes. Any application that accepts WMF files and calls PlayMetaFile with
the supplied data can be exploited. Some of these only recognize WMF
files with the placeable header, which may prevent the application from
reaching the SetAbortProc function. There are *many* other places where
standard (ie. included with the OS) applications call the PlayMetaFile
function, its just a matter of figuring out which ones can be used to
deliver the malicious WMF content. A potential vector includes the
display of icons stored inside of a standard executable. Viewing these
files in an Explorer directory listing could result in the execution of
code in an embedded WMF file. This has yet to be tested.

Q) What WMF header fields are mandatory for code execution through the
PFV ?

A) Not many. The Windows Meta File header and possible field values are
listed below:

# Possible values: 1 or 2 (memory or disk)
WORD FileType

# The HeaderSize must always be 9
WORD HeaderSize;

# The Version field can be 0×0300 or 0×0100
WORD Version

# This parameter can be anywhere from 0×20 to 0xffffffff
DWORD FileSize

# Completely arbitrary
WORD NumOfObjects

# Completely arbitrary
DWORD MaxRecordSize

# Completely arbitrary
WORD NumOfParams

The MSB of the actual MetaFileRecord function field is completely ignored.

Credits: A number of anonymous sources contributed to this information.

More information on the WMF structure can be found at the following sites:
- http://wvware.sourceforge.net/caolan/ora-wmf.html
- http://www.geocad.ru/new/site/Formats/Graphics/wmf/wmf.txt

Share