As a red teamer or pen tester, you must be prepared to face firewalls that protect the target network. Firewalls are the first line of defense against unauthorized access, and they are designed to restrict incoming and outgoing network traffic. However, these restrictions can be bypassed using different techniques and tools, and this is where you come in. In this article, we will explore some of the most effective firewall bypass techniques and best practices that you can use to penetrate your target network.

Firewall Basics

Before we dive into the techniques, let’s first understand what a firewall is and how it works. A firewall is a network security device that monitors and controls incoming and outgoing network traffic based on a set of predefined rules. The firewall can be either a software or hardware device that sits between the internal network and the external network (usually the Internet).

Firewalls can be configured to block or allow traffic based on the source IP address, destination IP address, port number, protocol, and other criteria. For example, a firewall may be configured to block all traffic from a specific IP address or only allow traffic on specific ports. Firewalls can also be configured to use different security policies for different types of traffic.

Firewalls can be classified into three main types: packet filtering, stateful inspection, and application-layer.

  1. Packet Filtering Firewalls: These are the simplest and earliest form of firewall. They examine the packet header and decide whether to block or allow traffic based on the source and destination IP addresses, ports, and protocols. Packet filtering firewalls are easy to configure and operate, but they are not very effective against more sophisticated attacks.
  2. Stateful Inspection Firewalls: These firewalls keep track of the state of network connections and allow or block traffic based on the state information. Stateful inspection firewalls are more effective than packet filtering firewalls because they can distinguish between legitimate traffic and traffic that is part of an ongoing attack.
  3. Application-layer Firewalls: These firewalls operate at the application layer of the network stack and can examine the contents of network packets to decide whether to allow or block traffic. Application-layer firewalls are the most sophisticated type of firewall and can detect and block many types of attacks, but they are also the most complex and resource-intensive.

Now that we have an understanding of firewalls let’s dive into some techniques that you can use to bypass them.

Techniques for Firewall Bypass

Port Scanning

Port scanning is the process of scanning a target system or network for open ports. A port is a logical connection point through which a computer communicates with another computer or network device. Each port is assigned a unique number, and different applications use different ports to communicate. For example, the HTTP protocol uses port 80, and the HTTPS protocol uses port 443.

Firewalls can be configured to block incoming traffic on specific ports. Therefore, scanning for open ports can help you identify open ports that you can use to bypass the firewall. There are several tools available for port scanning, including Nmap, Masscan, and Zmap.

Example:

nmap -p 1-65535 -sV -sS -T4 target_ip_address

This command will scan all the ports (1-65535) of the target IP address and attempt to determine the service and version running on each open port.

Protocol Tunneling

Protocol tunneling involves encapsulating traffic in another protocol to bypass firewalls that block the original protocol. For example, if a firewall is blocking traffic on port 80, you can tunnel your traffic through port 443, which is usually open for HTTPS traffic.

There are several tools available for protocol tunneling, including Iodine, Chisel, and Ptunnel.

Example:

Iodine is a tool that can be used to tunnel IPv4 traffic over DNS. To use Iodine, you first need to set up a DNS server that can resolve subdomains to the IP address of the target system. Then, you can use Iodine to establish a tunnel between your system and the target system.

To set up a DNS server, you can use a tool like dnsmasq or BIND. Once you have set up the DNS server, you can use the following command to establish a tunnel using Iodine:

iodine -f -P password subdomain.yourdomain.com

This command will establish a tunnel using Iodine and will use the password “password” to encrypt the traffic.

Application Layer Protocol Manipulation

Application layer protocol manipulation involves manipulating the application layer protocol to bypass the firewall. For example, you can use HTTP smuggling to bypass a firewall that blocks HTTP traffic. HTTP smuggling involves manipulating the HTTP protocol to make the firewall think that the traffic is legitimate HTTP traffic.

There are several tools available for HTTP smuggling, including Burp Suite and Httptunnel.

Example:

To use Burp Suite for HTTP smuggling, you can follow these steps:

  1. Set up Burp Suite as a proxy server and configure your browser to use the proxy server.
  2. Send an HTTP request to the target system using Burp Suite.
  3. Modify the request to include a payload that is not recognized by the firewall.
  4. Send the modified request to the target system.
  5. Use Burp Suite to intercept and modify the response from the target system to include the payload.

Encrypted Communication

Encrypted communication involves using encryption to protect the traffic from being intercepted by the firewall. If the firewall cannot decrypt the traffic, it will not be able to block it.

There are several tools available for encrypted communication, including OpenVPN, PPTP, and L2TP.

Example:

To use OpenVPN for encrypted communication, you can follow these steps:

  1. Set up an OpenVPN server on a system that is not blocked by the firewall.
  2. Install the OpenVPN client on your system.
  3. Connect to the OpenVPN server using the client.
  4. Once the connection is established, all the traffic between your system and the OpenVPN server will be encrypted and will not be blocked by the firewall.

Best Practices

Use a VPN

Using a VPN can help you bypass firewalls by encrypting your traffic and routing it through a server that is not blocked by the firewall. There are several VPN services available, including NordVPN, ExpressVPN, and Private Internet Access.

Use Stealth Techniques

Stealth techniques involve using techniques that are designed to evade detection. For example, you can use tools like Nmap and Masscan with the “-sS” flag to perform stealth scans that do not send any packets to the target system.

Use Port Forwarding

Port forwarding involves forwarding traffic from one port to another. For example, you can forward traffic from port 80 to port 443 to bypass a firewall that blocks traffic on port 80. There are several tools available for port forwarding, including SSH and netcat.

Conclusion

Firewalls are an essential part of network security, but they can be bypassed using different techniques and tools. As a red teamer or pen tester, it is your job to identify these weaknesses and exploit them. By using the techniques and tools we have discussed in this article, you can bypass firewalls and penetrate your target network. However, it is important to remember that these techniques should only be used for legitimate purposes and with the consent of the target organization.