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RealVNC Server does not validate client authentication method

Vulnerability Note VU#117929

Original Release Date: 2006-05-16 | Last Revised: 2008-02-26


The RealVNC Server fails to properly authenticate clients. This may allow a remote attacker to bypass authentication and gain access to the VNC server.


The Virtual Network Computing (VNC) Protocol

According to RealVNC, "The VNC protocol is a simple protocol for remote access to graphical user interfaces."


RealVNC is an implementation of the VNC protocol.

The Problem

The RealVNC Server fails to properly authenticate clients. When a RealVNC client connects to a RealVNC server, the server provides a list of supported authentication methods. By design, the client then selects a method from the list. Due to an implementation flaw, if the client specifies that no (null) authentication should be used, the server accepts this method and authenticates the client, whether or not null authentication was offered by the server.

Note that exploit code for this vulnerability is publicly available.


A remote, unauthenticated attacker could gain access to a system running RealVNC server. If the RealVNC server runs with administrative privileges, the attacker could gain complete control of the system.


This issue is corrected in RealVNC version 4.1.2, RealVNC Personal Edition 4.2.3, and RealVNC Enterprise Edition 4.2.3. Refer to the RealVNC Downloads site to get a patched version.

Prompt Local Users to Accept Connections

Until updates can be applied, selecting the Prompt local user to accept connections option may prevent attackers from gaining a VNC session by exploiting this vulnerability. See the authentication section of the RealVNC user guide for more information.

Vendor Information


RealVNC Affected

Updated:  May 16, 2006



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.


Refer to:

Red Hat, Inc. Not Affected

Updated:  May 17, 2006


Not Affected

Vendor Statement

This issue only affected version 4.1.1 of RealVNC. All Red Hat Enterprise Linux releases shipped with RealVNC versions prior to 4.1.1 and we have
verified with the exploit and by code analysis that they are not vulnerable to this issue.

Vendor Information

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

CVSS Metrics

Group Score Vector



This vulnerability was reported by James Evans.

This document was written by Jeff Gennari.

Other Information

CVE IDs: None
Severity Metric: 30.49
Date Public: 2006-05-15
Date First Published: 2006-05-16
Date Last Updated: 2008-02-26 14:09 UTC
Document Revision: 47

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