tor/doc/spec/proposals/129-reject-plaintext-ports

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.