Telnet (Telecommunication Network) For Penetration Testing | Quick Guide

In the world of cybersecurity, Telnet might seem outdated and was replaced by SSH protocol but it's still a powerful tool for penetration testers and hackers. Despite its age, this remote access protocol is valuable for network testing which allows you to connect to servers and devices. With the right techniques, ethical hackers can use Telnet to identify vulnerabilities, gather information, and launch attacks on networks—making it a valuable skill for penetration testing.

In this guide, we’ll cover the essentials of using Telnet for testing purposes which includes Port Version Detection, Banner Grabbing, and OS Detection. In case you're an advanced hacker or just starting out, Telnet can be a key tool to help you. So, let's dive into how you can use this simple yet effective protocol for network penetration testing.

What we are going to learn:

1.What is Telnet in Penetration Testing?

2.Port Version Detection with Telnet

3.Banner Grabbing with Telnet

4.Operating System Detection with Telnet

5.The Value of Telnet in Penetration Testing

What is Telnet in Penetration Testing?

Telnet stands for Telecommunication Network. It is a protocol that allows users to remotely access another device over a network. It enables hackers and penetration testers to control a remote system through commands. Telnet has been largely replaced by SSH (Secure Shell) due to its lack of encryption, it’s still in use, especially in older systems or poorly secured networks.

For penetration testers, Telnet is valuable because it operates on port 23, allowing you to establish communication with a server or device and analyze the services it offers. Using Telnet in testing can help uncover vulnerabilities like unencrypted traffic or misconfigurations, which makes it attractive to both ethical hackers and cybersecurity professionals. We can also access different protocols like HTTP, FTP, SMTP, POP3 and many more.

Port Version Detection with Telnet

In penetration testing, the first step to understand a network or service is - port scanning. After identifying open ports, version detection is critically next step. By using tools like Nmap or manually connecting via Telnet, you can identify the services running on a given port, such as FTP, HTTP, or SSH.

How to Use Telnet for Port Version Detection:

>Lets scan a Target system using tool like Nmap. In this example case our target system IP address is 10.10.192.161. The standard command used is:

sudo nmap -sS [Target IP]

Once we press enter we can see information about different protocols and ports.

>Lets use telnet to connect to the target. In our example we have taken 10.10.192.161 IP address and we will use Port 21. The command used is:

telnet [target Ip] [port]

This connection gives you a banner (more on this later), which typically includes the software version and service type running on that port. In the our case when we press enter we can the service running on the port is vsFTPd (Very Secure FTP Daemon), and the version of this software is 3.0.5.

Why is this important? Knowing the exact version of a service helps hackers exploit known vulnerabilities related to that specific version. For example in example case we can check if any vulnerability exits on FTP 3.0.5. BUT How to do this ? This question must be coming in your mind. Don't worry we have created a full tutorial on this which includes which site to use and how to find. You can checkout our video HERE.

Banner Grabbing with Telnet

One of the most common uses of Telnet in penetration testing is banner grabbing. When you connect to a service via Telnet, the server often responds with a welcome message, or banner, that contains valuable information. This banner can reveal the operating system, service versions, and sometimes even system-specific information.

Lets perform banner grabbing using telnet:

>Open a terminal and connect to the desired port. In our example case we will use same IP address as previously used. The port we are using in this example is 80 because it is the default port for HTTP (Hypertext Transfer Protocol) traffic. HTTP is the protocol used for transferring web pages on the internet. When you access a website using a web browser, your browser sends an HTTP request to the server, typically on port 80, to retrieve the web page content. The standard command used is:

telnet [target Ip] [port]

Press enter. Once connected, you can send an HTTP request to retrieve the banner. In our test case or example the command we have used is:

GET /index.html HTTP/1.1
host: telnet

The standard command used is:

HEAD / HTTP/1.1
Host: <target-ip>

When you press enter twice, you will see information which will be like 

HTTP/1.1 200 OK

Date: Sun, 22 Sep 2024 13:45:00 GMT

Server: Apache/2.4.41 (Ubuntu)

.....

A banner typically contains information about the software and services running on a server. When you perform banner grabbing, you might retrieve details such as:

• Server Type and Version: Information about the web server software (e.g., Apache, Nginx) and its version.

• Operating System: Sometimes, the banner includes details about the operating system running on the server.

• Service Information: Details about other services running on the server, such as FTP, SSH, or SMTP.

• Additional Metadata: Other information that the server might disclose, such as configuration details or security features.

 How to exploit it? Check it out HERE.

Operating System Detection with Telnet

The final step in using Telnet for penetration testing is Operating System (OS) Detection. Knowing the OS running on a system can provide crucial information for selecting the right attack methods. For instance, an unpatched Windows server might be vulnerable to specific exploits that wouldn’t work on a Linux system.

Using Telnet for OS Detection:

>After connecting to port using above command we can get information about the operating system. In our example case we get

Server: nginx/1.18.0 (Ubuntu)

>Combine this information with a detailed port scan (using Nmap’s -O option) to confirm the operating system:

nmap -O [target IP]

Knowing the OS helps penetration testers and ethical hackers tailor their attacks. For example, if the target is running Windows Server 2008, you could look for known vulnerabilities or misconfigurations specific to that OS.

The Value of Telnet in Penetration Testing

Despite its age and insecurity, Telnet still serves as a valuable tool for ethical hackers and penetration testers. Its simplicity allows you to gather critical information about a network’s infrastructure without any complex setups. Whether you're grabbing banners, detecting OS versions, or identifying open ports, Telnet’s raw functionality makes it perfect for reconnaissance and testing.

However, always remember that Telnet is unencrypted, meaning it is not safe to use over public networks without tunneling the traffic through a more secure protocol like SSH. As a hacker or penetration tester, you should also be aware of the legal implications of using Telnet for testing—make sure you have explicit permission from the system owner.

Telnet might seem like a relic of the past, but it remains an essential tool for ethical hackers and cybersecurity professionals alike. From port version detection to banner grabbing and OS detection, this protocol provides an array of options for gathering information and uncovering vulnerabilities.

If you're looking to expand your penetration testing skills, learning how to use Telnet effectively is crucial. And remember, always test responsibly—ethical hacking is about securing systems, not breaking into them without permission.

For more tutorials on penetration testing, check out our other guide on Metasploit.

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