Friday, February 5, 2010

The USB Password Vulnerability

In early January Heise Security reported that a German security firm had discovered a vulnerability in the password authentication process of several USB sticks that are rated as being highly secure. The discovery has been widely reported, and led to various responses from USB vendors Sandisk, Verbatim and Kingston, including patching and recalling their devices from the field. The full list of effected sticks has been reported by Simon Hunt for example. Steve Ragan of the TechHerald has commented that the whole incident is “"quickly becoming the first FUD-based news cycle for 2010”.

What was the vulnerability?

Well when a user plugs in a password-protected USB stick their desktop starts the stick by launching a popup application prompting the user for their password. You would expect that the user supplied password is then transferred to the stick for verification, and the stick grants access if the password is correct.

What German security company SySS, discovered is that the password verification is actually performed in the popup application itself , and an acknowledgment code is sent back to the stick indicating if the candidate password is correct or not. By sniffing this traffic SySS determined that the acknowledgment code granting access is static, and in particular does not depend on the password entered by the user. Essentially the desktop popup verifies the user supplies password and then returns "yes" or "no" to the stick.

SySS captured the acknowledgment code, and then wrote  proof-of-concept exploit which injects the acknowledgement code into the memory space of the desktop popup so that the value returned to the stick is always the positive acknowledgement code. Thus regardless of what password the user enters the hack ensures that the stick will always grant access.

What was the impact?

Given the injection code, the password-protection can be defeated on sticks susceptible to the attack, which turns out to be a reasonably large class of commercial sticks that are marketed as being highly secure. All things being equal, the risk of a data breach from lost sticks is therefore increased, since the password-protection of the sticks can be bypassed with the right software. And losing sticks is increasing. CSO Online recently reported on a UK survey conducted by Credant which revealed that 4,500 memory sticks have been forgotten in people's pockets as they take their clothes to be washed at the local dry cleaners.

The impact is not limited to a single vendor product. The vulnerability exists in several families of secure USB devices across the major USB vendors because they all rely on a common USB chipset whose security properties have not been properly vetted.

FIPS Certification

The incident is all the more telling in that the vulnerability impacts devices that use AES 256-bit encryption and are rated as secure by the FIPS 140-2 certification process. Users are paying quite a premium over vanilla sticks for the advertised additional assurance that their data are protected by a certified device using strong cryptography, and for some US government agencies such purchases are mandatory. The relative ease with which the password protection was bypassed calls into question the value of the FIPS 140-2 process.

In Computerworld NIST is quoted as saying "From our initial analysis, it appears that the software authorizing decryption, rather than the cryptographic module certified by NIST, is the source of this vulnerability", and then also "Nevertheless, we are actively investigating whether any changes in the NIST certification process should be made in light of this issue”.

To be fair, the FIPS 140-2 focuses on verification of cryptographic modules and not the supporting software, however the incident highlights the narrowness of the approach and the expectation that certification is more than secure cryptography. Chris Merrit at Lumension has a good post on the fine print of the certification FIPS 140-2 process, and he concludes

So, bottom line: while this discovery seems to suggest an area to which NIST might want to bring some clarity and rigor, it does not mean that FIPS 140-2 is fatally flawed. It’s up to you, as the buyer, to understand what (potentially critical) functions occur inside & outside the cryptographic boundary, and how that might impact the security of the device in your case. And since what you’re looking for is what’s not certified, it might be useful to have an expert review the vendor security policy (posted with the certification on the NIST website) to help you understand the nuances.

AES-256 and Passwords

As I explained in Are AES 256-bit keys too Large? it is very unrealistic to equate password security with the security of AES-256. To achieve the equivalent of 256-bit security users would need to select 40 character passwords at random, and we are a long way from that. In fact so far away that we will never get there. So USB devices that protect their data using AES-256 encryption sound impressive, but when access control to those devices and the underlying keys is controlled by a password, then this setup sounds a lot less secure. The SySS vulnerability now shows that the whole AES-256 encryption process can be bypassed in the presence of weak password handling.


Is there a useful conclusion from this incident? There is a lot of embarrassment all round and we have little confidence that a similar issue will not arise in the future. Security is just done poorly in general, and blatant examples are uncovered whenever someone takes the time to look under the hood. Some  articles and posts have focussed on verifying passwords in software as the culprit, which is partly true, but the real issue is not software but insecure programming of software - the password verification should never have been done on the desktop, and a static acknowledgement code should never have been used to unlock the USB device.

A trusted path should be established between the desktop keyboard and the USB device, and for smart cards this needs to be done with a secure reader. But this is at odds with the plug-and-play semantics of USB sticks where the portability of the ubiquitous USB connector is the selling point.


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