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Gibson Research Corporation is owned and operated by Steve Gibson. The contents of this page are Copyright c Gibson Research Corporation. Transcript of Episode The Windows MetaFile Backdoor? Description: Leo and I carefully examine the operation of the recently patched Windows MetaFile vulnerability. Given the nature of what it is, this would make it a remote code execution "backdoor.
Leo Laporte: This is Security Now! Leo Laporte here with Steve Gibson. Steve Gibson: Yeah. We have to delay again. Anything you want to update? Steve: Yeah. There were a couple things sort of in my category of errata. Someone made the point that I refer to hackers as "hackers," rather than as "crackers. Leo: Oh, we get this every time. Steve: I know. Leo: I get this so often. Steve: Yeah, exactly. So I wanted to say something about it. And so, you know, I try to say "malicious hacker.
Steve: Exactly. Leo: Yeah. Steve: So, for example, when I talk to people not in the industry, you know, who are not, like, saying, wait a minute, do you mean a malicious hacker or one of those good guy hackers, I mean, they know that hackers are bad.
And so Leo: I think you can tell.
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English is a very subtle language. You can tell from the context. There are both. What I say when people - and I get this all the time still. Steve: Oh, and Leo, if we started saying "cracker" all the time What are they talking about? Leo: So we know there are good hackers.
I consider Steve a hacker of the I mean, you hack away at code. Steve: No. Steve: Okay. Just remove the -u, and it reregisters it. So anybody who wants to get it directly from the web page, just go to the show notes for And Leo, I imagine you might want to put that on your page, too. Leo: I will.
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Now what do I do? Leo: And that was a bad fix from Microsoft. I mean, are we still happy with it? Steve: Oh, yes. What they did was to remove this feature, which is what I think it actually is, from Windows after it was discovered. Are you ready to move on to the Steve: Yeah, yeah.
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Leo: Okay. You have to have, you know, something needs to have registered the handling of Windows MetaFiles. But, you know, the IrfanView program is a known way of displaying metafiles that is vulnerable, and many people use that. So you Steve: You know, and again Is that right? You are vulnerable to the WMF flaw. Leo: If you have something that can view Windows MetaFiles. Leo: Oh. Well, it turns out that, in the process of doing that, it became a non-critical vulnerability by their definition.
I mean, and this is from a page on their website. They say a vulnerability in Windows is critical only if its exploitation could allow the propagation of an Internet worm without user action. In other words, anything else is not critical.
Leo: Why are they fudging it like that? And so, get this, the next level down from "critical" is an "important severity" rating. Leo: Ah.
Steve: Very good point. So, I mean, they But if, in fact, these machines are vulnerable, then I had committed, and I believe I should, to fix them for people because Microsoft, it was very clear then last week, was not going to. So over the weekend I rolled up my sleeves and sort of switched into what was really hacker mode.
And I wanted to acquire an understanding of exactly what this problem was in order to determine for myself first if, in fact, these older versions of Windows were actually vulnerable. And then, if so, I would certainly have a head start on how to cure that vulnerability.
So I started with what was known, which was the vulnerability in our existing versions of Windows, you know, , XP, and so forth, and basically created from scratch my own GRC-style vulnerability testing tool. And, you know, there was, you know, code snippets from the hacking sites. And Ilfak had in fact published the source for his tester.
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Mine ends up working differently because, again, I wrote it from scratch. I have a different approach. But I had a hard time getting this vulnerability to trigger.
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I was creating metafiles. Steve: No, this is Windows Steve: I removed the patch from my system, and I could not get the exploit to trigger using a metafile that I created with my own code. So, you know, I scratched my head. I looked at, you know, at the other samples of malicious metafiles. Then you have a series of metafile records where each one starts out with a four-byte size of that record in words, then a two-byte function number which is what type of metafile record this is, then followed by between zero or however many data that function requires.
But, you know, everyone will be able to follow along. It creates something called a Printer Device Context where things like the thickness of the pen, the color of the pen, the size of the paper, sort of all the things that are about the context of this printing page are stored. So once the application has a page ready, it turns it over to Windows and says, okay, here, go print this.
The problem is, what if the user aborted that page, that is, aborted the printing of the page, after it had been handed over to Windows? It makes complete sense in a printer device context. Leo: So my understanding of it and the general understanding of it has changed a little bit.
Leo: Right. Steve: Well, okay.
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First of all, it makes no sense at all in a metafile device context. That is, it was no longer interpreting my metafile records record by record, which is the way metafiles are supposed to be processed.
But what Windows did when it encountered this particular nonsensical sequence was to start executing the next byte of code in the metafile. Leo: Hmm. Leo: Why? As I said before, each record in a metafile begins with a four-byte length, followed by a two-byte function number. So in other words, each metafile record has six bytes minimum that it can possibly be in size. Oh, and since the size is in words, the smallest possible size for a metafile record would be three words long, or six bytes.
Look, the reason I had problems making this exploit happen initially is I was setting the length correctly. It turns out that the only way to get Windows to misbehave in this bizarre fashion is to set the length to one, which is an impossible value.