[tor-announce] Tor is released (security patches)

Roger Dingledine arma at mit.edu
Fri Oct 28 00:10:11 UTC 2011

Tor fixes a critical anonymity vulnerability where an attacker
can deanonymize Tor users. Everybody should upgrade.

The attack relies on four components: 1) Clients reuse their TLS cert
when talking to different relays, so relays can recognize a user by
the identity key in her cert. 2) An attacker who knows the client's
identity key can probe each guard relay to see if that identity key
is connected to that guard relay right now. 3) A variety of active
attacks in the literature (starting from "Low-Cost Traffic Analysis
of Tor" by Murdoch and Danezis in 2005) allow a malicious website to
discover the guard relays that a Tor user visiting the website is using.
4) Clients typically pick three guards at random, so the set of guards
for a given user could well be a unique fingerprint for her. This
release fixes components #1 and #2, which is enough to block the attack;
the other two remain as open research problems.

Special thanks to "frosty_un" for reporting the issue to us! (As far
as we know, this has nothing to do with any claimed attack currently
getting attention in the media.)

Clients should upgrade so they are no longer recognizable by the TLS
certs they present. Relays should upgrade so they no longer allow a
remote attacker to probe them to test whether unpatched clients are
currently connected to them.

This release also fixes several vulnerabilities that allow an attacker
to enumerate bridge relays. Some bridge enumeration attacks still
remain; see for example proposal 188.


Changes in version - 2011-10-26
  o Privacy/anonymity fixes (clients):
    - Clients and bridges no longer send TLS certificate chains on
      outgoing OR connections. Previously, each client or bridge would
      use the same cert chain for all outgoing OR connections until
      its IP address changes, which allowed any relay that the client
      or bridge contacted to determine which entry guards it is using.
      Fixes CVE-2011-2768. Bugfix on 0.0.9pre5; found by "frosty_un".
    - If a relay receives a CREATE_FAST cell on a TLS connection, it
      no longer considers that connection as suitable for satisfying a
      circuit EXTEND request. Now relays can protect clients from the
      CVE-2011-2768 issue even if the clients haven't upgraded yet.
    - Directory authorities no longer assign the Guard flag to relays
      that haven't upgraded to the above "refuse EXTEND requests
      to client connections" fix. Now directory authorities can
      protect clients from the CVE-2011-2768 issue even if neither
      the clients nor the relays have upgraded yet. There's a new
      "GiveGuardFlagTo_CVE_2011_2768_VulnerableRelays" config option
      to let us transition smoothly, else tomorrow there would be no
      guard relays.

  o Privacy/anonymity fixes (bridge enumeration):
    - Bridge relays now do their directory fetches inside Tor TLS
      connections, like all the other clients do, rather than connecting
      directly to the DirPort like public relays do. Removes another
      avenue for enumerating bridges. Fixes bug 4115; bugfix on
    - Bridges relays now build circuits for themselves in a more similar
      way to how clients build them. Removes another avenue for
      enumerating bridges. Fixes bug 4124; bugfix on,
      when bridges were introduced.
    - Bridges now refuse CREATE or CREATE_FAST cells on OR connections
      that they initiated. Relays could distinguish incoming bridge
      connections from client connections, creating another avenue for
      enumerating bridges. Fixes CVE-2011-2769. Bugfix on
      Found by "frosty_un".

  o Major bugfixes:
    - Fix a crash bug when changing node restrictions while a DNS lookup
      is in-progress. Fixes bug 4259; bugfix on Bugfix
      by "Tey'".
    - Don't launch a useless circuit after failing to use one of a
      hidden service's introduction points. Previously, we would
      launch a new introduction circuit, but not set the hidden service
      which that circuit was intended to connect to, so it would never
      actually be used. A different piece of code would then create a
      new introduction circuit correctly. Bug reported by katmagic and
      found by Sebastian Hahn. Bugfix on; fixes bug 4212.

  o Minor bugfixes:
    - Change an integer overflow check in the OpenBSD_Malloc code so
      that GCC is less likely to eliminate it as impossible. Patch
      from Mansour Moufid. Fixes bug 4059.
    - When a hidden service turns an extra service-side introduction
      circuit into a general-purpose circuit, free the rend_data and
      intro_key fields first, so we won't leak memory if the circuit
      is cannibalized for use as another service-side introduction
      circuit. Bugfix on; fixes bug 4251.
    - Bridges now skip DNS self-tests, to act a little more stealthily.
      Fixes bug 4201; bugfix on, which first introduced
      bridges. Patch by "warms0x".
    - Fix internal bug-checking logic that was supposed to catch
      failures in digest generation so that it will fail more robustly
      if we ask for a nonexistent algorithm. Found by Coverity Scan.
      Bugfix on; fixes Coverity CID 479.
    - Report any failure in init_keys() calls launched because our
      IP address has changed. Spotted by Coverity Scan. Bugfix on; fixes CID 484.

  o Minor bugfixes (log messages and documentation):
    - Remove a confusing dollar sign from the example fingerprint in the
      man page, and also make the example fingerprint a valid one. Fixes
      bug 4309; bugfix on
    - The next version of Windows will be called Windows 8, and it has
      a major version of 6, minor version of 2. Correctly identify that
      version instead of calling it "Very recent version". Resolves
      ticket 4153; reported by funkstar.
    - Downgrade log messages about circuit timeout calibration from
      "notice" to "info": they don't require or suggest any human
      intervention. Patch from Tom Lowenthal. Fixes bug 4063;
      bugfix on

  o Minor features:
    - Turn on directory request statistics by default and include them in
      extra-info descriptors. Don't break if we have no GeoIP database.
      Backported from; implements ticket 3951.
    - Update to the October 4 2011 Maxmind GeoLite Country database.

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