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Exploiting WMF (tiny) FAQ
From: H D Moore <fdlist () digitaloffense net>
Date: Thu, 5 Jan 2006 10:32:57 -0600

Q) Why did you release an IDS and AV evading exploit module so soon after 
the vulnerability was discovered?

A) The vulnerability was being exploited, in the wild, for at least two 
weeks (based on email reports) prior to the original BT post. The WMF 
structure is widely documented. The AV vendors were providing 
less-than-capable signatures for no reason other than that no public code 
was available that demonstrated alternate encodings. The IDS vendors were 
(and some still are) providing signatures that couldn't survive a single 
legal byte change in the WMF header. The release of a "polymorphic" (not) 
exploit forced the vendors to either fix their products or cry 
"irresponsibility" and give up. IPS vendors realized how SOL they are wrt 
to client-side HTTP attacks (so many encodings, so many ways to DoS an 
IPS that tries to decode them).

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 0x0300 or 0x0100 
WORD Version

# This parameter can be anywhere from 0x20 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

-HD

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