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[CORE SDI ADVISORY] Weakl authentication in ATT's VNC
From: Iván Arce <core.lists.bugtraq () CORE-SDI COM>
Date: Tue, 23 Jan 2001 19:48:30 -0300

                                    CORE SDI

        Vulnerability report for weak authentication in ATT VNC

Date Published: 2001-01-23

Advisory ID: CORE-2001011501

Bugtraq ID: 2275

CVE CAN: None currently assigned.

Title: Weak authentication in ATT VNC

Class: Design error

Remotely Exploitable: yes

Locally Exploitable: no

Release Mode: USER RELEASE

Vulnerability Description:

 As stated in the VNC home page ( http://www.uk.research.att.com/vnc/ ):

 "VNC stands for Virtual Network Computing. It is, in essence, a remote
 display system which allows you to view a computing 'desktop'
 environment not only on the machine where it is running, but from
 anywhere on the Internet and from a wide variety of machine

 VNC uses a challenge/response mechanism for authenticating
 clients in order to avoid the transmition of clear text passwords
 over insecure channels and prevent unauthorized clients to
 get access to the VNC server.

 A design flaw in the client authentication mechanism permits an attacker
 to obtain legit credentials from a valid client in order to gain
 unauthorized access to the server.
 The attack can be perfomed by an attacker eavesdropping the client/server
 communications with the ability to modify the data flow. NO TCP hijacking
 techniques are required.

 There are other security issues related to the fact that
 VNC does not provide a secure transport protocol that ensures
 confidentiality for the data transmited, those are well known and
 considered design decisions from the VNC development team.
 This advisory does not include them, the advisory addresses a
 security flaw in the design of the authentication mechanism that
 makes it unsuitable to fulfill its design goal.

Vulnerable Packages/Systems:

 VNC up to version 3.3.3 on all supported platforms.

Solution/Vendor Information/Workaround:

 It is advisable to tunnel communications between the VNC server and
 client through a cryptographycally strong end-to-end authenticated
 References for doing so are provided in the VNC FAQ
 (http://www.uk.research.att.com/vnc/faq.html) and specifics on how to
 tunnel VNC over SSH are provided at:


Vendor notified on: 2001-01-15


 This vulnerability was found by Emiliano Kargieman, Agustin Azubel
 and Maximiliano Caceres from Core SDI, http://www.core-sdi.com

 This advisory was drafted with the help of the SecurityFocus.com
 Vulnerability Help Team. For more information or assistance drafting
 advisories please mail vulnhelp () securityfocus com 

 This and other CORE SDI advisories are available at:

Technical Description:

 1. Man in the middle attack against client/server authentication

  VNC authenticates communication between client and server using a
  challenge-response mechanism.
  Due to design flaws in the challenge/response mechanism it is
  possible to perfom a man in the middle attack and obtain
  unauthorized access to the VNC server.

  The client authentication mechanism is described below:

   Asumming that C (the VNC client) is trying to authenticate to
   S (the VNC server), the following protocol is used:

   - A DES key (k) is shared by both endpoints and used for the

   - 'C' connects to 'S' and both endpoints exchange software/protocol
     version information

   - 'S' generates a 16 byte challenge and sends it to 'C'

   - 'C' encrypts the received challenge with 'k' and sends the result
     ('rc') to 'S'

   - 'S' encrypt the challenge with 'k' and compares the result ('rs')
      with the response 'rc' received from the client.

   - If rc==rs access is granted to the client. Otherwise access is

  A classical man-in-the-middle attack can be perfomed against the
  described protocol.

  Assuming that the attacker ('M') has access to the data flowing between
  client and server and is able to modify such data, an attack scenario
  THAT DOES NOT imply a TCP session hijacking attack is outlined:

  - 'M' connects to 'S' and both endpoints exchange software/protocol
     version information

  - 'S' generates a 16 byte challenge ('r1') and sends it to 'M', now
    'M' has a connection established with 'S' with the authentication
     pending a response to the server.

  - 'M' waits for a connection from a legit client 'C' to 'S'

  - Upon connection from the client 'C' to the server 'S', the server
    (as per the protocol design) generates a 16 byte challenge
    ('r2') and sends it to 'C'.

  - 'M' modifies the data traveling from 'S' to 'C' and replaces
     'r2' with 'r1'

  - 'C' receives 'r1' and encrypts it with the shared key 'k', the
     result ('r1c') is sent to the server 'S'

  - 'M' captures the response 'r1c' sent to the server 'S' and uses
    it in its own pending connection.

  - 'S' receives 2 equal responses (r1c), one from 'C' and one from
    'M'. It encrypts with 'k' the challenges (r1 and r2) sent
     and compares the results (r1s and r2s) against the received responses

  - For the legit client connection ( r2s != r1c ) and therefore access
    is not granted

  - For the attacker M connection ( r1s == r1c ) and therefore access
    is granted

  The attacker obtains unauthoraized access to the server using the
  client to generate a valid response to the challenge received.

 2. Weakness in the generation of the random challenge data.

  Additionally, the challenge is generated via rand(3) calls,
  initializing the randseed with a call to time(2).
  The 128 bits which comprises the challenge are generated by sucessive
  calls to rand, each one returning 8 bits of data.
  This actually reduces the useful randomness of the challenge to just
  16 bits, depending on the return value of time() (with precision of a

  The above two facts together render the challenge highly predictable,
  and could enable an attacker eavesdropping connections from a client
  to capture responses and reuse them at a different time in order to
  obtain unauthorized access to the server.


 The contents of this advisory are copyright (c) 2000 CORE SDI Inc.
 and may be distributed freely provided that no fee is charged for this
 distribution and proper credit is given.

$Id: VNC-auth-advisory.txt,v 1.7 2001/01/23 21:28:53 max Exp $

"Understanding. A cerebral secretion that enables one having it to know
 a house from a horse by the roof on the house,
 Its nature and laws have been exhaustively expounded by Locke,
 who rode a house, and Kant, who lived in a horse." - Ambrose Bierce

==================[ CORE Seguridad de la Informacion S.A. ]=========
Iván Arce
PGP Fingerprint: C7A8 ED85 8D7B 9ADC 6836  B25D 207B E78E 2AD1 F65A
email   : iarce () core-sdi com
Florida 141 2do cuerpo Piso 7
C1005AAC Buenos Aires, Argentina.
Tel/Fax : +(54-11) 4331-5402

--- For a personal reply use iarce () core-sdi com

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