Vulnerability Report For Weak Authentication in ATT VNC

Advisory ID Internal

Advisory ID: CORE-2001011501

Bugtraq ID: 2275

CVE Name: 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 ( ):

"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 architectures".

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 performed 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 transmitted, 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 cryptographically strong end-to-end authenticated channel.

References for doing so are provided in the VNC FAQ

( and specifics on how to tunnel VNC over SSH are provided at:

Vendor notified on: 2001-01-15


This vulnerability was found by Core SDI.

This advisory was drafted with the help of the
Vulnerability Help Team. For more information or assistance drafting advisories please mail [email protected].

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 perform a man in the middle attack and obtain unauthorized access to the VNC server.

The client authentication mechanism is described below:

Assuming 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 challenge-response.
- '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 denied.

A classical man-in-the-middle attack can be performed 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 unauthorized 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 successive 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 second).

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 content 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.