🛠️SOCKS proxy

Theory

SOCKS (SOCKet Secure) is a network protocol that allows users to route network traffic to a server on a client's behalf. SOCKS is between the application and the transport layer of the OSI model.

This is especially useful for penetration testing engagements where a target is hiding behind one or multiple firewalls. A compromised server connected to two networks can be used as a SOCKS proxy server to pivot from a network to another.

In short, a SOCKS proxy can relay TCP and UDP connections and hence help bypass network segmentation. It's sort a dynamic port forwarding technique.

Practice

There are two types of dynamic port forwarding used during penetration testing engagements.

Dynamic port forwarding: tunnel the whole attacker's network traffic (instead of only one port) through a remote machine.
Reverse dynamic port forwarding: tunnel the whole network traffic from a remote machine through the attacker's machine.

Basic server setup

While setting up port forwarding, it's important to remember that non-admin users can only open ports above 1024.

In practice, there are many ways to turn a controlled machine into a SOCKS proxy server.

One of the most easy is by relying on SSH, however, it requires to have an SSH server running on the controlled machine and a valid account. The tester needs to open an SSH connection to the machine that should be turned into a SOCKS proxy, and supply the -D option along with the port to use for tunneling. The command can also be used with -N option to make sure no command gets executed after the SSH session is opened.

ssh -N -D $PORT $CONTROLLED_TARGET

Once the ssh command exits successful (or once a session opens) the tester can then proceed to the usage part.

A reverse dynamic port forwarding can be also put in place to tunnel a machine's traffic through the attacker machine. It is implemented entirely in the client (i.e. the server does not need to be updated) (since OpenSSH 7.6).

ssh -N -R $PORT $CONTROLLED_TARGET

Chisel is a standalone binary for pivoting on Linux and Windows systems.

Working on a server/client, the binary simply needs to be uploaded on the victim host and executed as server on the attacker machine.

# attacker
chisel server --reverse --socks5 -p $PORT

# victim
chisel client $ATTACKER_MACHINE_IP:$ATTACKER_MACHINE_PORT R:socks

A meterpreter session can be taken advantage of by setting up a SOCKS proxy with the appropriate module.

The first steps consists in creating a route, from a meterpreter shell to the target

meterpreter > run autoroute -s 10.11.1.0/24

The session can then be put in background and, the SOCKS server can be created.

msf > use auxiliary/server/socks_proxy
msf > set SRVPORT $PORT
msf > set VERSION 4a
msf > run

Basic client usage

Once the SOCKS proxy server is set up, network traffic can be tunneled through with proxychains-ng (C), a tool still maintained and more advanced based on the original proxychains (that is not maintained anymore). The port in use by the SOCKS proxy should be supplied in the configuration file (/etc/proxychains.conf) like in the following examples.

# type    ip    port    [user    pass]
socks5    192.168.67.78    1080    lamer    secret
socks4    192.168.11.49    1080
http    192.168.89.33    8080    justu    hidden
http    192.168.39.93    8080

Chaining proxies

In certain scenarios, SOCKS proxies can be chained. This can easily be used with proxychains. In the following example, SSH is used to turn compromised machines into SOCKS proxy servers.