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AT&T WinVNC client authentication process vulnerable to man-in-the-middle attack

Vulnerability Note VU#303080

Original Release Date: 2001-06-13 | Last Revised: 2001-06-18


WinVNC's challenge/response mechanism can allow an intruder to obtain legitimate credentials from a valid client in order to gain unauthorized access to the server.


AT&T WinVNC is a free package available from AT&T Labs Cambridge that allows an existing desktop of a PC to be available on the desktop of a remote host. If an intruder is able to eavesdrop traffic between the client and server with the ability to modify the data, they can gain access to the target system desktop, allowing local access to the system.


This vulnerability could allow a remote attacker to gain unauthorized access to the WinVNC service.


Tunnel WinVNC through software which provides strong authentication and secure communication. There is an example of this at

Vendor Information


ATT Affected

Updated:  June 14, 2001



Vendor Statement

We have not received a statement from the vendor.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.


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Our thanks to CORE SDI for the information contained in their bulletin.

This document was written by Ian A. Finlay.

Other Information

CVE IDs: None
Severity Metric: 3.95
Date Public: 2001-01-23
Date First Published: 2001-06-13
Date Last Updated: 2001-06-18 23:41 UTC
Document Revision: 34

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