Proposal: Exit Scanning

[Mike Perry]

Title: Exit Scanning
Version: $Revision$
Last-Modified: $Date$
Author: Mike Perry
Created: 13-Feb-2009

Overview:

This proposal describes the implementation and integration of an
automated exit node scanner for scanning the Tor network for malicious,
misconfigured, firewalled or filtered nodes.

Motivation:

Tor exit nodes can be run by anyone with an Internet connection. Often,
these users aren't fully aware of limitations of their networking
setup.  Content filters, antivirus software, advertisements injected by
their service providers, malicious upstream providers, and the resource
limitations of their computer or networking equipment have all been
observed on the current Tor network.

It is also possible that some nodes exist purely for malicious
purposes.  In the past, there have been intermittent instances of
nodes spoofing SSH keys, as well as nodes being used for purposes of
plaintext surveillance. 

While it is not realistic to expect to catch extremely targeted or
completely passive malicious adversaries, the goal is to prevent
malicious adversaries from deploying dragnet attacks against large
segments of the Tor userbase.


Scanning methodology:

The first scans to be implemented are HTTP, HTML, Javascript, and
SSL scans.

The HTTP scan scrapes Google for common filetype urls such as exe, msi,
doc, dmg, etc. It then fetches these urls through Non-Tor and Tor, and
compares the SHA1 hases of the resulting content. 

The SSL scan downloads certificates for all IPs a domain will locally
resolve to and compares these certificates to those seen over Tor. The
scanner notes if a domain had rotated certificates locally in the
results for each scan.

The HTML scan checks HTML, Javascript, and plugin content for
modifications. Because of the dynamic nature of most of the web, the
scanner has a number of mechanisms built in to filter out false
positives that are used when a change is noticed between Tor and
Non-Tor. 

All tests also share a URL-based false positive filter that
automatically removes results retroactively if the number of failures
exceeds a certain percentage of nodes tested with the URL.


Deployment Stages:

To avoid instances where bugs cause us to mark exit nodes as BadExit
improperly, it is proposed that we begin use of the scanner in stages.

1. Manual Review:

  In the first stage, basic scans will be run by a small number of
  people while we stabilize the scanner. The scanner has the ability
  to resume crashed scans, and to rescan nodes that fail various
  tests.

2. Human Review:

  In the second stage, results will be automatically mailed to 
  an email list of interested parties for review. We will also begin 
  classifying failure types into three to four different severity 
  levels, based on both the reliability of the test and the nature of 
  the failure.

3. Automatic BadExit Marking:

  In the final stage, the scanner will begin marking exits depending
  on the failure severity level in one of three different ways: by
  node idhex, by node IP, or by node IP mask. A potential fourth, less
  severe category of results may still be delivered via email only for
  review.
 
  BadExit markings will be delivered in batches upon completion
  of whole-network scans, so that the final false positive
  filter has an opportunity to filter out URLs that exhibit
  dynamic content beyond what we can filter.


Specification of Exit Marking:

Technically, BadExit could be marked via SETCONF AuthDirBadExit over
the control port, but this would allow full access to the directory
authority configuration and operation.

The approved-routers file could also be used, but currently it only
supports fingerprints, and it also contains other data unrelated to
exit scanning that would be difficult to coordinate.

Instead, we propose that a new badexit-routers file that has three
keywords:

  BadExitNet 1*[exitpattern from 2.3 in dir-spec.txt]
  BadExitFP 1*[hexdigest from 2.3 in dir-spec.txt]

BadExitNet lines would follow the codepaths used by AuthDirBadExit to
set authdir_badexit_policy, and BadExitFP would follow the codepaths
from approved-router's !badexit lines.

The scanner would have exclusive ability to write, append, rewrite,
and modify this file. Prior to building a new consensus vote, a
participating Tor authority would read in a fresh copy.
 

Security Implications:

Aside from evading the scanner's detection, there are two additional
high-level security considerations:

1. Ensure nodes cannot be marked BadExit by an adversary at will

It is possible individual website owners will be able to target certain
Tor nodes, but once they begin to attempt to fail more than the URL
filter percentage of the exits, their sites will be automatically
discarded. 

Failing specific nodes is possible, but scanned results are fully
reproducible, and BadExits should be rare enough that humans are never
fully removed from the loop.

State (cookies, cache, etc) does not otherwise persist in the scanner
between exit nodes to enable one exit node to bias the results of a
later one.

2. Ensure that scanner compromise does not yield authority compromise

Having a separate file that is under the exclusive control of the
scanner allows us to heavily isolate the scanner from the Tor
authority, potentially even running them on separate machines.




-- 
Mike Perry
Mad Computer Scientist
fscked.org evil labs
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