[bitcoin-dev] Bitcoin XTs Tor IP blacklist downloading system has significant privacy leaks.

Peter Todd pete at petertodd.org
Wed Aug 19 01:36:45 UTC 2015

On Tue, Aug 18, 2015 at 09:08:01PM -0400, Christophe Biocca via bitcoin-dev wrote:
> So I checked, and the code described *does not* run when behind a
> proxy of any kind, including tor:
> https://github.com/bitcoinxt/bitcoinxt/commit/73c9efe74c5cc8faea9c2b2c785a2f5b68aa4c23#diff-11780fa178b655146cb414161c635219R265
> At least based on my admittedly weak understanding of how the internal works.
> Hopefully I save the next reader of your post from also having to dig
> around to find the code and realize this is a false alert.

That's not entirely correct.

The code does disable downloading of the Tor exit node list if fListen
is false, or if there is a proxy setup, this means the statement:

>  Connections are made over clearnet even when using a proxy or
>  onlynet=tor,

is false. However, in the common scenario of a firewalled node, where
the operator has neglected to explicitly set -listen=0, the code does
still download the Tor exit node list, revealing the true location of
the node. This is contrary to the previous behavior of not revealing any
IP information in that configuration.

FWIW Gregory Maxwell removed the last "call home" feature in pull-req
#5161, by replacing the previous calls to getmyip.com-type services with
a local peer request. Similarly the DNS seeds use the DNS protocol
specifically to avoid leaking IP address information.

tl;dr: Yes, Bitcoin XT has a privacy problem with the automatic Tor exit
node list download.

> On Tue, Aug 18, 2015 at 6:36 PM F L via bitcoin-dev <
> bitcoin-dev at lists.linuxfoundation.org> wrote:
> > Bitcoin XT contains an unmentioned addition which periodically downloads
> > lists of Tor IP addresses for blacklisting, this has considerable privacy
> > implications for hapless users which are being prompted to use the
> > software.  The feature is not clearly described, is enabled by default, and
> > has a switch name which intentionally downplays what it is doing
> > (disableipprio).  Furthermore these claimed anti-DoS measures are trivially
> > bypassed and so offer absolutely no protection whatsoever.
> >
> > Connections are made over clearnet even when using a proxy or onlynet=tor,
> > which leaks connections on the P2P network with the real location of the
> > node.  Knowledge of this traffic along with uptime metrics from
> > bitnodes.io can allow observers to easily correlate the location and
> > identity of persons running Bitcoin nodes.  Denial of service can also be
> > used to crash and force a restart of an interesting node, which will cause
> > them to make a new request to the blacklist endpoint via the clearnet on
> > relaunch at the same time their P2P connections are made through a proxy.
> > Requests to the blacklisting URL also use a custom Bitcoin XT user agent
> > which makes users distinct from other internet traffic if you have access
> > to the endpoints logs.
> >
> >
> >
> > https://github.com/bitcoinxt/bitcoinxt/commit/73c9efe74c5cc8faea9c2b2c785a2f5b68aa4c23
> >
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