[tor-announce] Tor security advisory: "relay early" traffic confirmation attack

Roger Dingledine arma at mit.edu
Wed Jul 30 09:28:52 UTC 2014

  On July 4 2014 we found a group of relays that we assume were trying
  to deanonymize users. They appear to have been targeting people who
  operate or access Tor hidden services. The attack involved modifying
  Tor protocol headers to do traffic confirmation attacks.

  The attacking relays joined the network on January 30 2014, and we
  removed them from the network on July 4. While we don't know when they
  started doing the attack, users who operated or accessed hidden services
  from early February through July 4 should assume they were affected.

  Unfortunately, it's still unclear what "affected" includes. We know
  the attack looked for users who fetched hidden service descriptors,
  but the attackers likely were not able to see any application-level
  traffic (e.g. what pages were loaded or even whether users visited
  the hidden service they looked up). The attack probably also tried to
  learn who published hidden service descriptors, which would allow the
  attackers to learn the location of that hidden service. In theory the
  attack could also be used to link users to their destinations on normal
  Tor circuits too, but we found no evidence that the attackers operated
  any exit relays, making this attack less likely. And finally, we don't
  know how much data the attackers kept, and due to the way the attack
  was deployed (more details below), their protocol header modifications
  might have aided other attackers in deanonymizing users too.

  Relays should upgrade to a recent Tor release ( or, to close the particular protocol vulnerability the
  attackers used -- but remember that preventing traffic confirmation in
  general remains an open research problem. Clients that upgrade (once
  new Tor Browser releases are ready) will take another step towards
  limiting the number of entry guards that are in a position to see
  their traffic, thus reducing the damage from future attacks like this
  one. Hidden service operators should consider changing the location of
  their hidden service.

  We believe they used a combination of two classes of attacks: a traffic
  confirmation attack and a Sybil attack.

  A traffic confirmation attack is possible when the attacker controls
  or observes the relays on both ends of a Tor circuit and then compares
  traffic timing, volume, or other characteristics to conclude that the
  two relays are indeed on the same circuit. If the first relay in the
  circuit (called the "entry guard") knows the IP address of the user,
  and the last relay in the circuit knows the resource or destination
  she is accessing, then together they can deanonymize her. You can read
  more about traffic confirmation attacks, including pointers to many
  research papers, at this blog post from 2009:

  The particular confirmation attack they used was an active attack where
  the relay on one end injects a signal into the Tor protocol headers,
  and then the relay on the other end reads the signal. These attacking
  relays were stable enough to get the HSDir ("suitable for hidden
  service directory") and Guard ("suitable for being an entry guard")
  consensus flags:
  Then they injected the signal whenever they were used as a hidden
  service directory, and looked for an injected signal whenever they
  were used as an entry guard.

  The way they injected the signal was by sending sequences of "relay"
  vs "relay early" commands down the circuit, to encode the message they
  want to send. For background, Tor has two types of cells: link cells,
  which are intended for the adjacent relay in the circuit, and relay
  cells, which are passed to the other end of the circuit.
  In 2008 we added a new kind of relay cell, called a "relay early"
  cell, which is used to prevent people from building very long paths
  in the Tor network (very long paths can be used to induce congestion
  and aid in breaking anonymity):
  But the fix for infinite-length paths introduced a problem with
  accessing hidden services:
  and one of the side effects of our fix for bug 1038 was that while
  we limit the number of outbound (away from the client) "relay early"
  cells on a circuit, we don't limit the number of inbound (towards the
  client) relay early cells:

  So in summary, when Tor clients contacted an attacking
  relay in its role as a Hidden Service Directory to publish
  or retrieve a hidden service descriptor (steps 2 and 3 on
  https://www.torproject.org/docs/hidden-services), that relay would
  send the hidden service name (encoded as a pattern of relay and
  relay-early cells) back down the circuit. Other attacking relays,
  when they get chosen for the first hop of a circuit, would look for
  inbound relay-early cells (since nobody else sends them) and would
  thus learn which clients requested information about a hidden service.

  There are three important points about this attack:

  A) The attacker encoded the name of the hidden service in the injected
  signal (as opposed to, say, sending a random number and keeping a local
  list mapping random number to hidden service name). The encoded signal
  is encrypted as it is sent over the TLS channel between relays. However,
  this signal would be easy to read and interpret by anybody who runs
  a relay and receives the encoded traffic. And we might also worry
  about a global adversary (e.g. a large intelligence agency) that
  records Internet traffic at the entry guards and then tries to break
  Tor's link encryption. The way this attack was performed weakens Tor's
  anonymity against these other potential attackers too -- either while
  it was happening or after the fact if they have traffic logs. So if
  the attack was a research project (i.e. not intentionally malicious),
  it was deployed in an irresponsible way because it puts users at risk
  indefinitely into the future.

  (This concern is in addition to the general issue that it's probably
  unwise from a legal perspective for researchers to attack real users
  by modifying their traffic on one end and wiretapping it on the
  other. Tools like Shadow are great for testing Tor research ideas out
  in the lab: http://shadow.github.io/ )

  B) This protocol header signal injection attack is actually pretty neat
  from a research perspective, in that it's a bit different from previous
  tagging attacks which targeted the application-level payload. Previous
  tagging attacks modified the payload at the entry guard, and then
  looked for a modified payload at the exit relay (which can see the
  decrypted payload). Those attacks don't work in the other direction
  (from the exit relay back towards the client), because the payload
  is still encrypted at the entry guard. But because this new approach
  modifies ("tags") the cell headers rather than the payload, every
  relay in the path can see the tag.

  C) We should remind readers that while this particular variant of
  the traffic confirmation attack allows high-confidence and efficient
  correlation, the general class of passive (statistical) traffic
  confirmation attacks remains unsolved and would likely have worked
  just fine here. So the good news is traffic confirmation attacks
  aren't new or surprising, but the bad news is that they still work. See
  https://blog.torproject.org/blog/one-cell-enough for more discussion.

  Then the second class of attack they used, in conjunction with their
  traffic confirmation attack, was a standard Sybil attack -- they
  signed up around 115 fast non-exit relays, all running on
  or Together these relays summed to about 6.4% of the
  Guard capacity in the network. Then, in part because of our current
  guard rotation parameters:
  these relays became entry guards for a significant chunk of users over
  their five months of operation.

  We actually noticed these relays when they joined the network, since
  the DocTor scanner reported them:
  We considered the set of new relays at the time, and made a decision
  that it wasn't that large a fraction of the network. It's clear there's
  room for improvement in terms of how to let the Tor network grow while
  also ensuring we maintain social connections with the operators of all
  large groups of relays. (In general having a widely diverse set of relay
  locations and relay operators, yet not allowing any bad relays in,
  seems like a hard problem; on the other hand our detection scripts did
  notice them in this case, so there's hope for a better solution here.)

  In response, we've taken the following short-term steps:

  1) Removed the attacking relays from the network.
  2) Put out a software update for relays to prevent "relay early" cells
     from being used this way.
  3) Put out a software update that will (once enough clients have
     upgraded) let us tell clients to move to using one entry guard
     rather than three, to reduce exposure to relays over time.
  4) Clients can tell whether they've received a relay or relay-cell.
     For expert users, the new Tor version warns you in your logs if
     a relay on your path injects any relay-early cells: look for the
     phrase "Received an inbound RELAY_EARLY cell".

  The following longer-term research areas remain:

  5) Further growing the Tor network and diversity of relay operators,
     which will reduce the impact from an adversary of a given size.
  6) Exploring better mechanisms, e.g. social connections, to limit the
     impact from a malicious set of relays. We've also formed a group to
     pay more attention to suspicious relays in the network:
  7) Further reducing exposure to guards over time, perhaps by extending
     the guard rotation lifetime:
  8) Better understanding statistical traffic correlation attacks and
     whether padding or other approaches can mitigate them.
  9) Improving the hidden service design, including making it harder
     for relays serving as hidden service directory points to learn what
     hidden service address they're handling:

  Q1) Was this the Black Hat 2014 talk that got canceled recently?
  Q2) Did we find all the malicious relays?
  Q3) Did the malicious relays inject the signal at any points besides
      the HSDir position?
  Q4) What data did the attackers keep, and are they going to destroy it?
      How have they protected the data (if any) while storing it?

  Great questions. We spent several months trying to extract information
  from the researchers who were going to give the Black Hat talk, and
  eventually we did get some hints from them about how "relay early"
  cells could be used for traffic confirmation attacks, which is how
  we started looking for the attacks in the wild. They haven't answered
  our emails lately, so we don't know for sure, but it seems likely that
  the answer to Q1 is "yes". In fact, we hope they *were* the ones doing
  the attacks, since otherwise it means somebody else was. We don't yet
  know the answers to Q2, Q3, or Q4.

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