Re: [Full-disclosure] PcwRunAs Password Obfuscation Design Flaw

["Thor (Hammer of God)" <thor@xxxxxxxxxxxxxxx>]

You've well-articulated a problem most (if not all) of the implementations I've 
seen just dance over.   The application accomplishes the encryption 
requirements stipulated by policy or regulation, but the key is easily 
available to the application and of course to attackers.  

I have no idea what mitigation techniques are available for PHP, but in 
.NET/Win applications there are a couple of "first step" attempts to at least 
address the problem.  First you've got DPAPI, which in its simplest form is an 
API that allows the application to encrypt/decrypt data by way of keys stored 
in the system certificate store which is protected by a different set of 
credentials.  The attacker would have to get system to get to the keys.  Of 
course, if the attacker could alter code as you've outlined, then they could 
very well just use the API to decrypt data without worrying about getting the 
keys themselves.  The problem with DPAPI is that it is system-based.  The data 
encrypted by that system can only be decrypted by that system.  That won't work 
in distributed environments, and it can be problematic in system failure 
scenarios.

To get passed single-machine issues, there is another method called DKM, which 
as the name infers is a distributed key management system based on AD and the 
machines' membership is appropriate groups given access to the keys.  So in a 
similar fashion, keys are protected by secondary credentials at the machine 
level.  Again, this requires an attacker to gain system access to get the keys, 
but again, gives an attacker with file-level access where code can be changed 
access to the procedure calls to get the data if they have to. 

It is a very difficult problem to solve, but it all comes down to risk 
management.   If you are protecting against off-line attacks or attacks from 
other systems, DPAPI or DKM will definitely help.  If you are protecting 
against attacks where SYSTEM access is granted, it gets far more difficult (one 
may even say quite improbable) to protect the keys.  

What I've been doing myself is a bit different, but it ultimately suffers from 
the same potential issues:  As part of my TGP "suite,"  I've extended 
functionality to WinMobile so that data encrypted on the PC can be exchanged 
and decrypted on the phone.  It's pretty cool actually...  to get around the 
key management issues, I wrote an API where the devices authenticate to, and 
use certificates to gain access to the encrypted keys on a centralized key 
store.   On the phone, the calls are made each time keys are needed.  On my web 
servers, the call is actually made on application startup, storing the keys in 
memory.    At any time one becomes aware of some breach, you can cut off access 
to the keys.  Not ideal of course, but it works.

I'm not worried about the BSOD scenario.  The remote attacker would have to 
cause a BSOD, and then somehow access the dump.  In production systems, the 
page files are typically kept on another drive (well, maybe not "typically" but 
that is up to the admins) in which case the dump won't exist.   But to your 
point, I just looked at my own web VM, and it is indeed set to do a full dump 
to the system drive.   I still don't think there is enough of a risk of that 
type of breach to warrant changing the paradigm from system-only access to keys 
in memory.

I too would be interested in hearing what processes others are using to address 
this. 

t



> -----Original Message-----
> From: full-disclosure-bounces@xxxxxxxxxxxxxxxxx [mailto:full-disclosure-
> bounces@xxxxxxxxxxxxxxxxx] On Behalf Of b
> Sent: Tuesday, March 27, 2012 6:16 PM
> To: full-disclosure@xxxxxxxxxxxxxxxxx
> Subject: Re: [Full-disclosure] PcwRunAs Password Obfuscation Design Flaw
> 
> So this brings up an interesting problem to tackle:
> 
> How can you encrypt things for use by applications such as service account
> credentials for authentication to other systems and database access
> credentials on disk in such a way that the application can obtain the
> encrypted information without prompting the end user for a decryption
> passphrase and an attacker with local filesystem access cannot obtain the
> encryption key to the credentials by simple computation (like if you had the
> encryption key calculated based on some static/easily guessable
> information)?
> 
> 
> 
> 
> I have seen some attempts to solve this problem, such as with setting up SSL
> certs for apache's use if the certs private key is encrypted, you have to
> supply the private key decryption password at application startup and the
> key then lives in memory.
> 
> This isn't very practical, in my opinion, in most environments I have seen
> because an unplanned outage could require system reboots by your
> monitoring personnel who might not know the application decryption
> passphrase and higher administrators have to be called in to start the
> services up properly.
> 
> With web pages written in something like PHP, it would be even less practical
> because then you would have to design some sort of daemon that stays
> running in memory and the php code would ask the daemon to decrypt the
> database credentials for each request. Or another approach would be to
> thing like that like using the IPC capabilities of your operating system and
> using shared memory. To use this shared memory you would have some
> program that runs on system startup and you enter the credentials and they
> get saved to this shared memory location and your other apps or php pages
> can use the IPC interface to read this memory.
> 
> Even these things have their issues because if the attacker can trigger a
> memory dump that gets written to disk (like causing a BSOD and you have full
> memory dumps enabled) and the attacker has local filesystem access they
> could then read the dump from disk (after the system comes back up) and
> obtain the key like that.
> 
> 
> 
> 
> I'd like to hear you all's thoughts on this so I can learn something new.
> 
> 
> 
> 
> -B

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