[tor-relays] 90% of exits vulnerable to TCP off-path attack

[dawuud]

yeah that paper Spying in the Dark: TCP and Tor Traffic Analysis
was also a very good read. i believe their rate reduction confirmation attack
(as described in section 5.1) is applicable using the TCP challenge ACK
inference side channel to gain the ability to perform injection attacks
which manipulate the congestion management!

an adversary with the ability to passively monitor a given server should be
able to use the rate reduction attack + challenge ACK side channel to discover
the guard relay of a tor connection to the server given enough time and bandwidth.

the challenge ACK side channel can be used to discover TCP connections from the
exit relay to a middle relay. Further the rate reduction attack could be used to confirm if a
given TCP connection to a middle relay belongs to the target Tor circuit.
this process is repeated to collect the rest of the relays in the circuit.

in Off-Path TCP Exploits: Global Rate Limit Considered Dangerous section 4.2
they talk about their TCP 4-tuple inference attack which discovers connections
from a known client to a known server's TCP port. they describe a clever way to
utilize a binary or multi-bin search by parallelizing probes to many different ports and repeatedly
observing side channel messages to confirm a given port range has a connection or not.

this same parallelizing of probes definitely applies here. although our number of probes is much
greater since it's multiplied by a large fraction of the tor relay consensus (exit, middle, guard relays).


david

On Fri, Aug 12, 2016 at 12:10:49PM -0400, Roger Dingledine wrote:
> On Fri, Aug 12, 2016 at 12:01:03PM -0400, Zack Weinberg wrote:
> > Tor's use of TLS _should_ mean that the worst an attacker can do here
> > is denial-of-service.  The Register article suggests that they might
> > also be able to force the use of specific exit relays (by disrupting
> > connections that don't go through those relays) but weaponizing that
> > against specific users (rather than everyone trying to use an exit the
> > attacker doesn't like) strikes me as nontrivial.
> 
> Agreed. I attended the talk at Usenix Security this week, and the
> presenter seemed to think that if a TCP connection between two relays
> gets cut, then the first relay will redirect all those circuits to some
> other relay. If that were true, then you could do the "keep cutting the
> connection until your target circuit goes the way you want it to" attack.
> 
> But Tor clients are the ones that choose their paths, and when a circuit
> fails, they throw it out and choose a new path, without memory of what
> links inside the Tor network did or didn't work in the past. So clients
> will effectively fail closed, meaning they will keep trying a circuit of
> their choice and you will keep trying to find it and tear it down, but
> you won't be able to "direct" their path in the way the authors thought.
> 
> That said, this kind of TCP level attack is indeed neat, and people
> should keep thinking about ways to apply it, or something like it, to Tor.
> 
> For an earlier paper in the same area, see also
> http://freehaven.net/anonbib/#tcp-tor-pets12
> 
> > Right now I think one should not panic and should wait for the kernel
> > people to do a proper fix.
> 
> Yes, this makes sense.
> 
> Another option is to encourage people to remember that operating system
> diversity is important. :)
> https://torbsd.github.io/
> 
> --Roger
> 
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