Wednesday, August 26, 2009

PDF signing and beyond

Today I got an advertising email from GlobalSign (where I previously bought a code signing certificate for Vista kernel drivers some years ago) highlighting their new (?) type of certificates for signing of Adobe PDF files. It made me curious, because, frankly, I've been recently more and more missing this feature. After a quick online research it turned out that this whole Adobe Certified Documents Services (CDS) seem to be nothing new, as apparently even Adobe Reader 6.0 had support for verifying those CDS certificates. The certificates are also available from other popular certification authorities like e.g. Entrust and Verisign, and a couple of others.

So, I immediately felt stupid that I haven't been aware of such a great feature, which apparently is out there for a few years now. Why I thought it was so great a feature? Consider the following scenario…

At our Invisible Things Lab resources page we offer a handful of files to download — slides and some proof of concept code. The website is served over a plaintext HTTP. This means that if you're downloading anything over a public WiFi (hotel, airport lounge, etc) you never know if the PDF you actually get has not been infected somewhere in the middle, e.g. by a guy in the lobby that is messing with the hotel WiFi.

So, one might argue that I should have paid a few hundred bucks and get an SSL certificate for my website and start serving it over HTTPS. But here's the problem — I, as zillions of other small businesses and individuals, host my website on some 5-dollar-a-month one-of-the-thousands hosting provider. I have zero knowledge about what people work there and if they can be trusted, and I also know nothing (and have zero impact) on how secure (or not, for that matter) the server is. (Same applies to my cell phone carrier, ISP, etc, BTW).

Now, the SSL certificate for the website "knows" nothing about how the files on my website should look like, in particular if they are compromised or not. All the SSL certificate does is to give assurance to the remote client that he or she downloaded the actual files that were on the server in the moment of downloading — whether they were the original ones authored by me, or perhaps maliciously modified by somebody who got access to the server.

So, the solution with an SSL certificate would work only if I trusted my web server, which could be assumed only if I run my own dedicated server. That, however, would be an overkill for a small company like ITL, especially that our business is not based on our web presence — in fact the website is maintained mainly for other researchers and students, who can easily download our papers and code from there, and also for the reporters so they can e.g. download a press release from there.

Surprisingly, the website has never been compromised, probably because it doesn't present an interesting target for any skilled person (or maybe exceptionally skilled people work at the hosting provider?). But I cannot know for sure, as I don't constantly monitor all the hashes of all the files, as this would require… well a dedicated server that would be running an SHA1 calculating script in a loop for 24/7 :)

Of course, zillions of other websites works this very same way and present the very same problems.

Now, ability to sign PDFs would be just a great solution here, because I could sign all those files with my certificate, and then all the people downloading stuff from ITL could know they are getting original PDFs that were created on one of the ITL members desktop computers, no matter how compromised the web server or the network connection is.

For the same reasons, I would welcome if others started doing the same, as currently I simply must assume every PDF I download from the net (and PDFs account for the majority of file downloads I do) to be potentially malicious. So, I always open them in my Red or Yellow VM (depending on the source of the download), and only if it "looks good" (very fuzzy term, I know), I might decide to move it to my host desktop (it's easier to work with PDFs on your host, and actually you should use your host desktop for something).

(Yes, I know, Kostya Kortchinsky, or Rafal, can sometimes escape from VMWare, but still I believe that today the best isolation I can get on a desktop, without sacrificing much convince, is via a type II hypervisor. It's horribly inelegant, but well, that's life).

So, I read some more about this Adobe CDS, being all excited about it, and ready to spend a few hundred euros on a certificate, only to realize that it doesn't look as good as I thought.

First disappointment comes from the fact that you must create a PDF using Adobe Acrobat software (not the Reader, but the commercial one). I've created all my PDFs using either Office (in the past) or iWork (today), and none of them seem to offer a way to digitally sign the PDF. I would like to get a simple tool, say pdfsign.exe, that I could use to sign any PDF I have, no matter how I generated it. Also, not surprisingly, the Mac native PDF viewer (Preview) doesn't seem to recognize the digital signature, and I bet some Linux PDF viewers do not as well.

Worst of all, even the Acrobat Reader 9, that I tested under Windows, and that correctly displayed all the CDS information, does one unbelievably stupid thing — it parses and renders the whole PDF before displaying the signature info. So, if you downloaded a malicious PDF, Acrobat Reader will happily open it and parse, without asking you a question of whether you would like to open it (as it is perhaps unsigned). At least I was unable to find an option that would force it to do that. So, if this PDF contained an exploit for the reader, it surely would get executed. Compare this with the (correct) behavior of Vista UAC where it presents the executable signature details before executing it.

You can see how your software works with Adobe PDF signatures, e.g. by looking at this exemplary file signed by GlobalSign.

So, Adobe CDS, in the form they are today, seem to be pretty useless, as far as protection from potentially malicious PDFs is considered (they surely have other positive applications, e.g. to certify about authenticity of e.g. a diploma).

But wouldn't it be great to have such a file signing mechanism globally adopted and not only for PDFs, but for any sort of files, including ZIPs, tgz's, heck, even plain text files? And have our main OSes generically recognize those signatures and display unified prompts of whether we want to allow an application to to open the file or not? Perhaps, in some situations, we could even define policies for specific applications. This seems easy to do from the technical point of view — we just need to "hook" (oh, God, did I say "hook"?) high-level OS API's like e.g. open() or CreateFile().

What about PGP and possibility of using this for signing any sort of files? Well, we use PGP a lot at ITL, but mainly for securing peer-to-peer communication (e.g. between us and our clients). There really is no good way to publish one's PGP key — the concept of Web of Trust might be good for some closed groups of people, but not for publishing files "to the world". And, of course, the first thing that an attacker who subverted PDFs on our website will do is to also subvert the PGP key displayed on the website. I also tried once to publish a PGP key to a key server, but got discouraged immediately after I noticed it didn't use SSL for submission. BTW, anybody knows if the key servers today use SSL? If not, how the trust is established? Maybe email clients, e.g. Thunderbird, come with built in PGP keys for select key servers?

So, I guess that was the main point of writing this post — to express how madly I would welcome a generic, OS-based, non-obligatory, signature verification for files, based on PKI :)

Ah, before a dozen of people jumps to the comment box to tell me that digital signatures do not assure non-maliciousness of anything — please don't do that, because I actually know that. In fact, it is not possible to assure non-maliciousness of pretty much anything, especially without strictly defining an ethical system we would like to use first. What the signatures provide is the liability, so that I know who to sue, in case my naked holiday pictures got leaked to the public because of some malicious PDF exploiting my system. In that case I can sue either the actual person who signed the PDF (if this person is identifiable) or the certification authority who issued the certificate to a wrong (unidentifiable) person.

Tuesday, August 25, 2009

Vegas Toys (Part I): The Ring -3 Tools

We've just published the proof of concept code for the Alex's and Rafal's "Ring -3 Rootkits" talk, presented last month at the Black Hat conference in Vegas. You can download the code from our website here. It's highly recommended that one (re)reads the slides before playing with the code.

In short, the code demonstrates injection of an arbitrary ARC4 code into a vPro-compatible chipset AMT/ME memory using the chipset memory reclaiming attack. Check the README and the slides for more information.

The actual ARC4 code we distribute here is very simple: it sets a DMA write transaction to the host memory every ca. 15 seconds in order to write the "ITL" string at the predefined physical addresses (increased by 4 with every iteration). Of course one can do DMA read as well.

The ability to do DMA from the ARC4 code to/from the host memory is, in fact, all that is necessary to write a sophisticated rootkit or any sort of malware, from funny jokers to sophisticated secret sniffers. Your imagination (and good pattern searching) is the only limit here.

The OS, nor any software running on the host OS, cannot access our rootkit code, unless, of course, it used the same remapping attack we used to insert our code there :) But the rootkit might even cut off this way by locking down the remapping registers, so fixing the vulnerability on the fly, after exploiting it (of course it would be insane for any AV to use our remapping attack in order to scan ME space, but just for completeness;)

An OS might attempt to protect itself from DMA accesses from the rootkit in the chipset by carefully setting VT-d protections. Xen 3.3/3.4, for example, sets VT-d protections in such a way that our rootkit cannot access the Xen hypervisor memory. We can, however, access all the other parts of the system which includes all the domains memory (i.e. where all the interesting data are located). Still, it should be possible to modify Xen so that it set VT-d mappings in such a strict way, that the AMT code (and the AMT rootkit) could not access any useful information in any of the domains. This, in fact, would be a good idea anyway, as it would also prevent any sort of hardware-based backdoors (except for the backdoors in the CPU).

An AMT rootkit can, however, get around such a savvy OS because it can modify the OS's VT-d initialization code before it sets the VT-d protections. Alternatively, if the protections are set before the rootkit was activated, the rootkit can force the system to reboot and boot it from the AMT Virtual CDROM (In fact AMT has been designed to be able to do exactly that), which would contain rootkit agent code that would modify the OS/VMM to-be-loaded image, so that it doesn't setup VT-d properly.

Of course, the proper solution against such an attack would be to use e.g. Intel TXT to assure trusted boot of the system. In theory this should work. In practice, as you might recall, we have already shown how to bypass Intel TXT. This TXT bypass attack still works on most (all?) hardware, as there is still no STM available in the wild (all that is needed for the attack is to have a working SMM attack, and last month we showed 2 such attacks — see the slides for the BIOS talk).

Intel has released a patch a day before our presentation at Black Hat. This is a cumulative patch that is also targeting a few other, unrelated, problems, like e.g. the SMM caching attack (also reported by Loic), the SMM nvacpi attack, and the Q45 BIOS reflashing attack (for which the code will be also published shortly).

Some of you might remember that Intel has patched this very remapping bug last year, after our Xen 0wning Trilogy presentations, where we used the very same bug to get around Xen hypervisor protections. However, Intel forgot about one small detail — namely it was perfectly possible for malware to downgrade BIOS to the previous, pre-Black-Hat-2008 version, without any user consent (after all this old BIO file was also digitally signed by Intel). So, with just one additional reboot (but without a user intervention needed) malware could still use the old remapping bug, this time to get access to the AMT memory. The recent patch mentioned above solves this problem by displaying a prompt during reflash boot, if reflashing to an older version of BIOS. So now it requires user intervention (a physical presence). This "downgrade protection" works, however, only if we have administrator password enabled in BIOS.

We could get into the AMT memory on Q35, however, even if the downgrade attack was not possible. In that case we could use our BIOS reflashing exploit (the other Black Hat presentation).

However, this situation looks differently on Intel latest Q45 chipsets (that also have AMT). As explained in the presentation, we were unable to get access to the AMT memory on those chipsets, even though we can reflash the BIOS there, and consequently, even though we can get rid of all the chipset locks (e.g. the remapping locks). Still, the remapping doesn't seem to work for this one memory range, where the AMT code resides.

This suggest Intel added some additional hardware to the Q45 chipset (and other Series 4 chipsets) to prevent this very type of attacks. But we're not giving up on Q45 yet, and we will be trying other attacks, as soon as we recover from the holiday laziness ;)

Finally, the nice picture of the Q35 chipset (MCH), where our rootkit lives :) The ARC4 processor is somewhere inside...