Another Attack on Code Signing

 
 
By Larry Seltzer  |  Posted 2007-08-02 Print this article Print
 
 
 
 
 
 
 

Opinion: It's all about trust decisions. Most things you do on a computer involve imperfect evaluations of trust. And now the evaluations of signed 64-bit Vista device drivers have become less perfect than they used to be.

I think it was around the introduction of Authenticode that Microsoft talked a lot about how security is all about trust: Who do you trust? Who do you not trust? This is a tautology of security: It has to be true in any environment that you have to trust someone—the operating system kernel, for example. And because theyre part of the kernel, some device drivers have to be trusted as well. This is a problem that is difficult to solve. Authenticode addresses the problem by supporting digital signatures on executable files, including device drivers, and allowing users the opportunity to examine the signature and make a judgment about whether they trust the person or company who signed the file to run code on their system.

Because it requires all new device drivers and programming changes, Microsoft took the opportunity with 64-bit Windows Vista to require that device drivers be signed. Try to load an unsigned driver and you get a big, cant-miss error message.

Code-signing certificates have been abused in the past. Click here to read the story of VeriSign and a company named "CLICK YES TO CONTINUE."

There have been reports before of holes in the driver-signing requirements. But now theres a tool from Linchpin Labs named Atsiv that allows you to load unsigned drivers in 64-bit Vista. The two keys to how it does this are that it is, itself, a signed 64-bit device driver, and it uses a custom loader for other drivers it loads.

At this point its worth giving a little more detail on the certificates used in driver code signing. Such drivers require their own code-signing certificates from a trusted certificate authority. These cost nontrivial money—at least a couple hundred dollars—and the CA is supposed to do some checks on the authenticity of the applicant, so you cant claim youre IBM and use a stolen credit card to pay for it. There have been some abuses in the past, but clearly you run some risks and encounter obstacles trying to skirt the system. And theres more: 64-bit drivers must be signed by a special cross certificate from Microsoft.

So back to Atsiv. I havent actually tested it myself because I dont have a 64-bit Vista system up. Normally, when you load a 64-bit driver, there is an informational dialog box typical of what users have seen from Authenticode for years. See the example, from a Microsoft white paper.

But there are ways to suppress these dialog boxes, and even if there werent, some users just reflexively say yes to such things. In other words, they are too trusting. So if you run Atsiv you may or may not get such a warning for the driver that is a component of the tool. But you will get a warning from User Account Control: Even if you are logged in as administrator, which is required in order to load drivers, you will get a warning that the operation you are attempting to perform requires elevated privileges. This is your chance—perhaps your last chance—to reflect on what you thought the program did and whether you want to trust it to do dangerous things.

Since certificates are involved, its possible for Microsoft or whoever issued Linchpin Labs certificates to revoke them. Im still trying to find out how often Vista 64 checks for certificate revocation. The worst case is at driver load time, which would be bad, since this system Im writing on hasnt been rebooted since last patch Tuesday. Ill get back to you when I have an answer.

If the cert were to be revoked, any malware installed in this way may be disabled, but its a moot point perhaps. The system is already 0wned, so you cant trust it. In fact, since drivers are kernel code, perhaps they can interfere with certificate revocation checks.

So digital signatures on code are about facilitating trust decisions. They arent a way to stop malicious code; they help you to decide what might be malicious code. Make a bad decision, and you shoot yourself in the foot.

The hole Atsiv abuses may be filled one day, but it shows that the driver signature requirement is one more imperfect mechanism for facilitating trust decisions. Atsiv doesnt fly through without warning. Nothing changes the fact that the most important security device is the one sitting at the keyboard.

Security Center Editor Larry Seltzer has worked in and written about the computer industry since 1983. Check out eWEEK.coms for the latest security news, reviews and analysis. And for insights on security coverage around the Web, take a look at eWEEK.com Security Center Editor Larry Seltzers blog Cheap Hack. More from Larry Seltzer
 
 
 
 
Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.
 
 
 
 
 
 
 

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