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Tor security advisory: cross-protocol http form attack

On Thu, Aug 02, 2007 at 06:19:18PM -0400, Roger Dingledine wrote:
> Tor fixes a critical security vulnerability that allows a
> remote attacker in certain situations to rewrite the user's torrc
> configuration file. This can completely compromise anonymity of users
> in most configurations, including those running the Vidalia bundles,
> TorK, etc. Or worse.

Here are the further details that we promised:

In a nutshell, a malicious website or Tor exit node can give the Tor
user a page that includes a POST element directed to Tor's control port
(localhost:9051). Tor binds its control port only to localhost to avoid
letting untrusted people send it commands, but the attacker skips past
this protection by making the browser do the connection. And the user
doesn't even have to click on anything if she's got javascript enabled.

This particular attack worked because Tor's control protocol gave an
error message on unrecognized commands but didn't hang up. So all the
http headers from the POST were unrecognized commands, and eventually
we got to the payload -- which contains recognized commands -- and it
went bad from there.

Jochen Topf wrote a fine paper describing this attack in 2001:
Thanks to Kyle Williams and Martin Peck who independently rediscovered
the attack in the context of Tor.

The and versions of Tor patched this particular
problem by hanging up if the first command wasn't a successful
'authenticate' command. The recently released and
versions of Tor now enable application-level authentication by default
in the Windows and OS X bundles, which should stop a broad class of
related attacks. Everyone should upgrade.

Yay full disclosure,

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