Tor security advisory: hidden services can be located quickly
arma at mit.edu
Thu Jan 12 23:03:40 UTC 2006
Versions affected: all stable versions, and all experimental versions
up through 0.1.1.10-alpha.
Impact: If you offer a Tor hidden service, an adversary who can run a
fast Tor server and who knows some basic statistics can find the location
of your hidden service in a matter of minutes to hours.
Solution: You have three options:
1) Upgrade to Tor 0.1.1.12-alpha from the Tor download page . You're
all set, though be aware that this is an alpha release so there may
be other bugs. You may also want to look through the release notes .
2) Turn off your hidden service until the final 0.1.1.x release is out.
It may be several months.
3) Stick with Tor 0.1.0.16 and manually configure a half dozen
EntryNodes. See the FAQ entry  for some hints about how to do this.
Tor researchers Lasse Øverlier and Paul Syverson have confirmed
that a previously theoretical attack on Tor works very well in
practice. Specifically, they found that a malicious Tor server can locate
a hidden service more quickly than was previously believed. The attack
is simple: access the hidden service repeatedly, and keep track of who
builds circuits through you shortly after each access. Because you can
induce your victim to build a new circuit on demand, eventually one of
his circuits will start at your node.
To slow this attack, our latest experimental release implements a
new feature called "guard nodes": it automatically chooses a handful
of entry nodes and sticks with them for all circuits. This idea is
adapted from the "helper node" concept published by Wright et al ,
but with improved reliability: rather than picking a set of entry nodes
and refusing to access the Tor network if they all become unreachable,
Tor's design dynamically picks new guards as needed, yet switches back
to the old ones when they become reachable again. Therefore Tor users
still have the same level of robustness as before, but the chance of a
successful attack by a limited adversary is greatly reduced.
More details will be presented on January 14 at Shmoocon  and January
26 at Black Hat Federal .
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