Tuesday, 3 January 2006

Thoughts on the WMF vulnerability

OK, so we know there’s a zero-day vulnerability (i.e. one which was not reported to any security organisation or vendor before being exploited) out there which utilises a malformed WMF file to execute code on a victim’s computer. This is being termed a ‘remote code execution’ vulnerability – this term is now being used to cover any situation where an attacker could cause code to be executed, but doesn’t differentiate between a situation where this can be done by the attacker actively sending data over a network to the victim, and one (such as this) where the victim must request data from the attacker. However, the attacker can cause software executing on the victim computer to automatically request the bad data – in this case, for example, by sending an email message to the victim containing a suitably malformed image file, which will cause some email packages to automatically render (draw) the image when the message is displayed.

Firstly, what is a WMF file? It stands for Windows Metafile. That’s a pretty meaningless name. What it actually contains is just a sequence of commands – that map one-to-one to GDI API calls – for producing a drawing. The easiest way to construct a WMF is to use the CreateMetaFile API which produces a GDI drawing surface, a device context, and returns a handle to it, an HDC. Once you’ve finished drawing – using the regular GDI API calls – you then call CloseMetaFile which gives you a HMETAFILE. You can then draw the metafile again using PlayMetaFile. It appears that this API is the one which contains the vulnerability – that some part of the format is insufficiently checked and the attacker can therefore cause the processor’s instruction pointer to end up pointing at a part of the supplied file.

This does suggest that any application that renders WMF data using the PlayMetaFile API could be an attack vector. Because it is such a venerable format, many applications will support it. You can include WMF drawings in your Word documents. You can process WMF files in Paint Shop Pro.

The current advisory from Microsoft suggests unregistering the shimgvw.dll component. This component is responsible for much more than WMF rendering. It performs all thumbnail rendering in Windows Explorer for all the best-known filetypes. It provides the size and other information for the Task pane and status bar. It also implements the ‘Windows Picture and Fax Viewer’ frame that appears if you click Preview on the context menu for an image. Unregistering this DLL kills all this functionality – but it does not protect against the vulnerability in other applications which call PlayMetaFile (except those which use shimgvw.dll as a proxy, such as Internet Explorer). This is my supposition, anyway – I would be astonished if shimgvw.dll did not render WMF simply by calling PlayMetaFile, and likewise Enhanced Metafiles by calling PlayEnhMetaFile.

While WMF files are most often used in the filesystem for storing vector-based clip-art (one among many other formats), you can also find them used within other formats, because of the native OS support. For example, when copying a diagram from Visio to Word, you will find that the prerendered version of the diagram (used for a linked or embedded diagram when the diagram is not active) is a metafile – although in this case it is most likely an Enhanced Metafile. Whether the Enhanced Metafile format can also be exploited is an unknown.

No comments: