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Windows back door rumor is bunk

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Contrary to a recent rumor circulating on the internet, Microsoft did not intentionally back-door the majority of Windows systems by means of the WMF vulnerability. Although it is a serious issue that should be patched straight away, the idea that it's a secret back door is quite preposterous.

The rumor began when popinjay expert Steve Gibson examined an unofficial patch issued by Ilfak Guilfanov, and, due to his lack of security experience, observed behavior that he could not explain by means other than a Microsoft conspiracy. He then went on to speculate publicly about this via a "This Week in Tech" podcast, and on his own web site. Slashdot grabbed the story, and the result is a fair number of Netizens who now mistakenly believe that the WMF flaw was created with malicious intent.

What it is

We think it's time that this irrational fear is put to rest. First, let's look at how the flaw works: A WMF (Windows Metafile) image can trigger the execution of arbitrary code because the rendering engine, shimgvw.dll, supports the SetAbortProc API, which was originally intended as a means to cancel a print task, say when the printer is busy with a very large job, or the queue is very long, or there is a mechanical problem, and so on. Unfortunately, due to a bit of careless coding, it is possible to cause shimgvw.dll (i.e., the Windows Picture and Fax Viewer) to execute code when SetAbortProc is invoked.

A metafile is essentially a script to play back graphical device interface (GDI) calls when a rendering task is initiated. Unfortunately, and due entirely to Microsoft's carelessness whenever security competes with functionality, it is possible to point the abort procedure to arbitrary code embedded in a metafile.

Gibson could not imagine why WMF rendering should need the SetAbortProc API, since, as he mistakenly believed, WMF outputs to a screen, not a printer. In fact, it can output to a printer as well. But following Gibson's erroneous assumption, the question arose: what would be the point of polling the process and allowing the user, or application, to cancel it?

Having exhausted his imagination on that score, he concluded that there's no good reason for SetAbortProc to be involved in handling metafiles. The more logical explanation, Gibson reckoned, was that someone at Microsoft had deliberately back-doored Windows with this peculiar little stuff-up. And besides, the idea of compromising a computer with an image file seemed quite cloak-and-dagger, adding to the supposed "mystery."

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