Proposal to mitigate insecure protocols over Tor

[Kevin Bauer]

Below is a proposal to mitigate insecure protocol use over Tor.

Title: Block Insecure Protocols by Default
Author: Kevin Bauer & Damon McCoy
Date: January 15, 2008

Overview:

	This document 1) demonstrates the extent to which insecure protocols are
	currently used within the Tor network, and 2) proposes a simple solution
	to prevent users from unknowingly using these insecure protocols. By
	insecure, we consider protocols that explicitly leak sensitive user names
	and/or passwords, such as POP, IMAP, Telnet, and FTP.

Motivation:

	As part of a general study of Tor use in 2006/2007 [1], we attempted to
	understand what types of protocols are used over Tor. While we observed a
        enormous volume of Web and Peer-to-peer traffic, we were
surprised by the
	number of insecure protocols that were used over Tor. For example, over an
	8 day observation period, we observed the following number of connections
	over insecure protocols:

	POP and IMAP:10,326 connections
	Telnet: 8,401 connections
	FTP: 3,788 connections

	Each of the above listed protocols exchange user name and password
	information in plain-text. As an upper bound, we could have observed
	22,515 user names and passwords. This observation echos the reports of
	a Tor router logging and posting e-mail passwords in August 2007 [2]. The
	response from the Tor community has been to further educate users
	about the dangers of using insecure protocols over Tor. However, we
	recently repeated our Tor usage study from last year and noticed that the
	trend in insecure protocol use has not declined. Therefore, we propose that
	additional steps be taken to protect naive Tor users from inadvertently
	exposing their identities (and even passwords) over Tor.

Security Implications:

	None. This proposal is intended to improve Tor's security by limiting the
	use of insecure protocols.

Specification:

	As an initial step towards mitigating the use of the above-mentioned
	insecure protocols, we propose that the default ports for each respective
	insecure service be blocked at the Tor client's socks proxy. These default
	ports include:

	  23 - Telnet
	109 - POP2
	110 - POP3
	143 - IMAP

	Notice that FTP is not included in the proposed list of ports to block. This
	is because FTP is often used anonymously, i.e., without any identifying
	user name or password.

	This blocking scheme can be implemented as a set of flags in the client's
	torrc configuration file:
		
	  BlockInsecureProtocols 0|1
	  WarnInsecureProtocols 0|1

	When the warning flag is activated, a message should be displayed to
	the user similar to the message given when Tor's socks proxy is given an IP
	address rather than resolving a host name.

	We recommend that the default torrc configuration file block insecure
	protocols and provide a warning to the user to explain the behavior.

	Finally, there are many popular web pages that do not offer secure
	login features, such as MySpace, and it would be prudent to provide
	additional rules to Privoxy to attempt to protect users from unknowingly
	submitting their login credentials in plain-text.

Compatibility:

	None, as the proposed changes are to be implemented in the client.

References:

	[1] Shining Light in Dark Places: A Study of Anonymous Network Usage.
             University of Colorado Technical Report CU-CS-1032-07. August 2007.

	[2] Rogue Nodes Turn Tor Anonymizer Into Eavesdropper's Paradise.
             http://www.wired.com/politics/security/news/2007/09/embassy_hacks.
	     Wired. September 10, 2007.