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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
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
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.
VNC up to version 3.3.3 on all supported platforms.
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:
1. Man in the middle attack against client/server authentication
VNC authenticates communication between client and server using a
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
- '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
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
- '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
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 $
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Its nature and laws have been exhaustively expounded by Locke,
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- [CORE SDI ADVISORY] Weakl authentication in ATT's VNC Iván Arce (Jan 24)