WMF FAQ by HD Moore

The creator of the Metasploit Framework has released his own WMF FAQ. This was posted to the FD Security Mailing list this morning. Good information.

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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