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\title{The Faithless Endpoint}
\subtitle{How Tor puts certain users at greater risk}

\author {Len Sassaman\inst{1}}


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\institute{Katholieke Universiteit Leuven\\
Kasteelpark Arenberg 10, B-3001 Leuven-Heverlee, Belgium\\
\email{len.sassaman@esat.kuleuven.be}
}
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\date{}

\begin{document}

\maketitle

\begin{abstract}

We demonstrate that the decision to employ certain security solutions must
be balanced against the additional risks that these security solutions
introduce for a given user's situation, in the context of its specific
threats. As an example case, we consider the anonymity network Tor and
examine scenarios where the use of Tor decreases one's overall security.
We then show that such trade-offs are reasonable for some, but not all,
potential users of Tor. We then consider possible ways to mitigate these
risks.

\end{abstract}


\section{Introduction}

Tor~\cite{tor-design} is a distributed, low-latency anonymous
communication service. Tor operates by use of a client application which
tunnels the user's TCP traffic through a network of volunteer-operated Tor
servers for the purpose of providing anonymity and
unlinkability~\cite{terminology} for the user's network communications.

The Tor network consists of Tor servers operated by volunteer
administrators. There is no central validation of an operator's
trustworthiness; distributed-trust anonymity systems such as Tor assume
that some portion of the network may consist of nodes operated by an
adversary as part of the threat model they address. 

% Discuss the concepts of distributed-trust systems, and why it's okay
% that bad-guys can operate some of the nodes.

Tor and other anonymity systems based on the mix-net
primitive~\cite{chaum-mix} route traffic from the client to its destination
on the network through several nodes in the system, encrypting the traffic
in a nested fashion such that the first node in the chain only sees
encrypted data coming from the client, the next nodes in the chain see
only encrypted data coming from the previous node and know neither the
source nor the destination of this traffic, and the final node decrypts
the plain-text network request, which it is able to read, but does not
know the origin of the data.

Should a node in a user's chain be compromised, as long as enough of the
other nodes in the selected chain are honest, the user's anonymity is
still maintained.

This approach offers better anonymity protection than the ``trust-base''
approach offered by systems such as Anonymizer~\cite{anonymizer}, where a
single compromise of the trusted entity is enough to reveal the identity
of its users.

\section{Opportunistic end-point attacks}
\label{mitm}

%Man-in-the middle attacks on any susceptible victim
% MITM was theory, now with rogue APs is more popular for things like
% phishing and ID theft
%
% Phishers, etc., can run Tor nodes and perform this attack as easily!

The problem of man-in-the-middle attacks on network protocols has been
known for many years. Since performing a man-in-the-middle attack requires
the ability to gain access to some point on the victim's network
connection, indiscriminate attacks against large numbers of users have
been uncommon due to the attention that network operators have paid to the
security of their networks.

In recent years, as the use of wireless network connectivity has
increased, the prevalence of this class of attack has also increased.
Attackers looking for opportunities to steal user credentials,
particularly for financial websites, now have a way to control a point in
the user's network which facilitates active man-in-the-middle attacks: the
wireless access point. Rogue access points can intercept network requests
to sites of interest and redirect the traffic to impostor sites under the
control of the attacker, allowing the attacker to learn login-credentials
and other private information.

While users of the Tor system are protected against manipulation of their
traffic by rogue access points due to the network traffic being encrypted
to the Tor servers, Tor users are similarly susceptible to
easily-performed man-in-the-middle attacks due to the untrusted nature of
the Tor network. Should an opportunistic attacker want the ability to view
and manipulate the traffic of large numbers of users without compromising
key parts of the Internet infrastructure, he need only operate a Tor exit
node. Existing man-in-the-middle and identity-harvesting toolkits
currently used for rogue access points could be easily adapted to work on
Tor exit traffic, putting Tor users at risk.

\section{Threat analysis}

% People who really need Tor won't think this is a flaw in Tor -- after
% all, who doesn't know that all traffic that isn't end-to-end encrypted
% is snoopable? Use SSL, or IPsec, or PGP, or OTR.
%
% Describe the threats those people face.

Tor is designed to combat the threats of network monitoring, traffic
analysis, and data interception posed by an adversary who is able to
observe significant portions of the network. Users who are concerned about
such attacks have traditionally been well-informed about network security
in general, and are aware of the dangers of unencrypted traffic. The
concerns in~\ref{mitm} are mitigated by good data security practices.

Naive users, who may not be fully aware of the various security threats the
Internet poses, and who may be turning to Tor to provide a simple solution to
all of their privacy and security problems, are the most likely to suffer.
These users are, as a whole, less vigilant about ensuring that their web
traffic is SSL encrypted, that their login credentials aren't submitted to
an otherwise secure site via a URL argument, that SSL certificates are
signed by a valid certificate authority, and so forth.


% So who gets hurt?
%
% Naive users, that's who. And they don't face the threats Tor prevents
% anyway.

Ironically, it may be that the users most likely to fall victim to attacks
by a malicious Tor endpoint should consider those opportunistic attacks a
greater threat, both in terms of potential damage as well as likelihood of
occurrence, than the traffic analysis attacks against which Tor aims to
protect. As long as there exist security disadvantages to using Tor, this
trade-off should be evaluated in the context of a given user's threat
model.

Assuming that naive users will have the information necessary to perform a
reasonable threat model analysis, however, is ill-advised. Furthermore, it
is harmful to the anonymity of the Tor network if it has too few
users~\cite{econymics}. A solution to faithless endpoint attacks must
be found.

\section{Partial Solutions}

We propose two partial solutions to this attack. 

\subsection{Aggressive opportunistic encryption}

The Tor client is able to aggressively seek opportunistic end-to-end
encryption for the user's network data. The Tor client is bundled with
Privoxy~\cite{privoxy}, a local proxy which sanitizes web traffic by
removing identifiable information such as browser type. It is possible to
modify Privoxy so that it attempts to connect to requested web sites via
SSL first, reverting back to the requested form if the connection via SSL
fails. If traffic leaving the last node of the Tor network is encrypted to
the actual destination on the Internet, the attacker will not be able to
intercept the data.

Should an opportunistic encryption attempt by the Tor client take place
through an attacker's Tor node, the attacker could artificially cause the
request to fail, forcing a request in the clear. In this case, the client
would be no better off for having attempted opportunistic encryption.
However, if a user obtains an encrypted session through a Tor chain with an
honest exit node, and subsequent network traffic through the Tor system
exits through a node operated by an attacker, the integrity of the data
will be preserved by the existing encryption session.

% Solution? Trusted endpoints.

\subsection{Trusted endpoints}

A more comprehensive solution to this problem is the introduction of
``trusted endpoints'' to the Tor security model. While a user of the Tor
network may be unwilling to trust any individual entity to preserve her
privacy or anonymity in many circumstances, it is reasonable to place some
amount of trust in the assumption that an ISP or other reputable
organization would not harvest information for the purposes of financial
and identity theft.

The policy of permitting anyone willing to operate a Tor node to
participate in the Tor server network should continue; the attack
discussed in this paper is not one which can be performed at any point in
the Tor chain except the exit point, and this policy provides insurance
against the threat that the Tor network could come to be operated by a
single adversary. However, if the Tor client were to be provided with a
list of ``trusted endpoints,'' the potential for the opportunistic attacks
would be mitigated while the benefits of an open network are preserved.

\section{Conclusions and future work}
\label{conclusions}

% Lack of ubiquitous eavesdropping protection sucks, and leads to other
% privacy apps potentially being more dangerous.

We have shown that under certain circumstances, using Tor may put one's
security at greater risk than it would be otherwise. While we have offered
some suggestions to mitigate these risks, the underlying problem remains:
cleartext is dangerous to network users.

It is the lack of ubiquitous cryptography in network communication
protocols that enables these attacks in the first place. Providers of
network services should work to ensure that whenever possible, network
communications are encrypted between the server and the client, and
servers can be authenticated by the client, to avoid the potential for
man-in-the-middle attacks in contexts where they are a likely threat.


\subsection*{Acknowledgments}

We would like to thank George Danezis, Roger Dingledine, and Nick
Mathewson for insightful discussions on Tor security trade-offs. We also
thank Meredith L. Patterson for her comments on this paper.

Len Sassaman's work was supported in part by the EU within the PRIME 
Project under contract IST-2002-507591.

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