[Noisebridge-discuss] usb 'dead drops' at noisebridge?

[epsas at evil-wire.org]

My first thought is having a pc emulate a flash drive via linux usb-gadget and removing the need for building a decider circuit.  

The usb jack would be eletronically isolated by an optocoupler for safety.  

On Sat, Oct 30, 2010 at 09:47:45PM -0700, Seth David Schoen wrote:
> Mitch Altman writes:
> 
> > Too cool an idea!
> >  
> > Too bad that there's a high risk of trojans by connecting a Windows computer to one (autorun.inf files can run an EXE file upon plugging a USB drive in).
> 
> I wonder what it would take to make a circuit that switched the
> storage device back and forth between the public connector and
> the USB port of a (hidden) "integrity check PC" installed by
> the same person who installed the whole setup:
> 
> public connector -----\             /----- integrity check PC
>                       |             |
>                       \-- decider --/
> 			     |
> 			     |
> 			  storage
> 			   volume
> 
> The "decider" circuit would contain a couple of solid state relays
> that switched the storage volume between the public connector and
> the integrity check PC whenever the previous connection disconnected
> somehow (I don't know if the integrity check PC could actually
> electrically disconnect after unmounting, which seems like the
> easiest way to alert the decider circuit that it's time to switch;
> otherwise the integrity check PC could have its own separate
> connection to the decider to actively tell it that it's OK to
> switch back the other way).
> 
> The integrity check PC could do three things:
> 
> (1) run a vfat fsck on the storage volume to make sure that what
> was left was still a valid VFAT filesystem (in case of accidental
> or intentional corruption) -- especially important in case some
> operating systems' vfat implementations contain exploitable bugs
> that can be triggered by trying to mount a maliciously-crafted
> invalid filesystem;
> 
> (2) reformat the disk as an empty vfat volume if the fsck fails too
> badly; and
> 
> (3) mount the resulting filesystem and unconditionally delete any
> autorun file that exists at the top level.
> 
> It wouldn't have to be much of a PC to do this stuff.  Any old
> embedded system with a USB implementation should suffice.  The
> challenging question for me is whether dosfsck can actually
> catch all of the potential forms of invalidity that could harm
> users (and whether dosfsck itself is exploitable!).
> 
> Other ideas:
> 
> . the integrity check PC could keep a backup of the "last valid
>   mountable" state of the storage volume, and under certain
>   conditions where a user appeared to have erased or corrupted the
>   disk too badly, it would simply restore this state.
> 
> . the integrity check PC could run a scan for Windows viruses,
>   though arguably this is mostly useless and/or the responsibility
>   the system's end-users.
> 
> The biggest problem with this that I see is that it requires
> power, which the original concept doesn't.
> 
> -- 
> Seth David Schoen <schoen at loyalty.org> | Qu? empresa f?cil no pensar en
>      http://www.loyalty.org/~schoen/   | un tigre, reflexion?.
>      http://vitanuova.loyalty.org/     |            -- Borges, El Zahir
> _______________________________________________
> Noisebridge-discuss mailing list
> Noisebridge-discuss at lists.noisebridge.net
> https://www.noisebridge.net/mailman/listinfo/noisebridge-discuss