Greetings fellow hackers and red team enthusiasts! Welcome to another thrilling article in our advanced web attack techniques series. Today, we’ll dive deep into the fascinating world of Server-Side Request Forgery (SSRF) - an often overlooked but extremely powerful attack vector.

As you know, web applications are now more complex and feature-rich than ever before. However, with great power comes great responsibility - and unfortunately, this complexity introduces new security challenges for us to exploit. In this article, we’ll explore the intricacies of SSRF, and show you how to harness this technique to breach even the most secure web applications.

Let’s dive in!

What is SSRF?

Server-Side Request Forgery (SSRF) is a type of web application vulnerability that allows an attacker to make HTTP requests on behalf of the server. This may not seem like a big deal at first, but in reality, SSRF can have devastating consequences. By leveraging this vulnerability, attackers can bypass firewalls, perform port scanning, and access sensitive internal resources - all from the perspective of the vulnerable server.

SSRF can be a goldmine for attackers, as it not only exposes sensitive data but also grants them the ability to potentially compromise the entire internal infrastructure of the target organization.

How does SSRF work?

SSRF typically occurs when a web application makes server-side HTTP requests to a user-defined URL. The application may use this functionality for various purposes, such as displaying remote images, processing external resources, or interacting with APIs. However, if the application fails to properly validate and sanitize the user-supplied URL, an attacker can exploit this to craft malicious requests.

These malicious requests, which appear to originate from the server, can be used to access internal resources or perform unauthorized actions on behalf of the server. Since these requests are made server-side, they are not subject to the same-origin policy, which would otherwise prevent cross-origin requests from a client-side script.

Types of SSRF attacks

There are two primary types of SSRF attacks:

  1. Basic SSRF: In a basic SSRF attack, the attacker directly targets the server hosting the vulnerable application. By crafting malicious requests, the attacker can access local files, perform port scanning, or exploit other vulnerabilities present on the server.

  2. Remote SSRF: In a remote SSRF attack, the attacker targets other systems within the target’s internal network. This is especially dangerous, as it allows the attacker to bypass network defenses and potentially compromise other systems within the network.

Let’s look at an example to illustrate the difference between these two types of SSRF attacks:

Imagine a web application that allows users to fetch and display images from a user-supplied URL. The application fetches the image server-side and returns it to the client for display. An attacker can exploit this functionality by supplying a malicious URL, such as:

http://localhost:8080/internal/admin_panel

In this case, the server would fetch the contents of the internal admin panel and return it to the attacker, effectively bypassing authentication and exposing sensitive information (Basic SSRF).

Now, let’s assume the target organization has an internal GitLab instance running on a separate server. The attacker can modify the malicious URL to target this internal resource, such as:

http://192.168.0.10:3000/internal/gitlab_api

In this scenario, the vulnerable server would fetch the contents of the internal GitLab API on behalf of the attacker, exposing sensitive information and potentially allowing the attacker to compromise the GitLab instance (Remote SSRF).

Identifying SSRF vulnerabilities

To identify SSRF vulnerabilities, you must first locate server-side functionality that fetches external resources based on user input. This could include functionality for embedding images, processing remote files, or interacting with APIs. Once you’ve identified such functionality, the next step is to test for proper input validation and sanitation.

Here are some common techniques for testing SSRF vulnerabilities:

  1. Test with well-known internal IP addresses, such as localhost, 127.0.0.1, or 0.0.0.0.
  2. Attempt to access internal network resources using private IP address ranges, such as 192.168.x.x, 172.16.x.x, or 10.x.x.x.
  3. Use different URL schemes, such as file://, gopher://, or dict://, to bypass input validation.
  4. Test with domain names that resolve to internal IP addresses, such as internal.example.com.

Remember, always test on a system that you have permission to access and avoid causing harm to others.

Exploiting SSRF vulnerabilities

Once you’ve identified an SSRF vulnerability, it’s time to exploit it. The exploitation process varies depending on the specific vulnerability and the target’s internal infrastructure. However, here are some general techniques to get you started:

  1. Access sensitive files: Exploit SSRF to access sensitive files on the server, such as /etc/passwd, /etc/shadow, or application configuration files.
  2. Perform port scanning: Use SSRF to perform port scanning on the server or other internal systems. This can help you identify additional vulnerabilities or expose hidden services.
  3. Bypass authentication: Leverage SSRF to access restricted resources or administrative interfaces, bypassing authentication mechanisms.
  4. Chain vulnerabilities: Combine SSRF with other vulnerabilities, such as Local File Inclusion (LFI) or Remote Code Execution (RCE), to achieve full system compromise.
  5. Exfiltrate data: Exploit SSRF to send sensitive data to an external server under your control.

Real-world SSRF attack examples

SSRF vulnerabilities have been discovered and exploited in various high-profile web applications. Here are some notable examples:

  • Capital One data breach (2019): A misconfigured AWS WAF allowed an attacker to exploit an SSRF vulnerability in a Capital One web application, resulting in the exfiltration of over 100 million customer records.

  • Google Cloud Platform SSRF (2019): Security researcher Ezequiel Pereira discovered an SSRF vulnerability in Google Cloud Platform’s metadata service, which could have allowed an attacker to access sensitive information and potentially compromise internal Google infrastructure.

  • Snapchat SSRF (2014): In 2014, Snapchat suffered an SSRF vulnerability that allowed attackers to access sensitive user data and internal infrastructure. The vulnerability was discovered by researcher Aviv Raff and later fixed by Snapchat.

SSRF defense mechanisms

To protect against SSRF attacks, developers must implement proper input validation, sanitation, and access controls. Here are some best practices for defending against SSRF:

  1. Validate and sanitize user input: Always validate and sanitize user-supplied URLs to ensure they conform to expected patterns and do not contain malicious payloads.
  2. Use allowlists: Implement allowlists for remote resources that can be accessed by the application, preventing unauthorized access to internal resources.
  3. Restrict network access: Configure your server’s firewall to block outbound traffic to internal IP addresses, limiting the potential impact of an SSRF attack.
  4. Use secure coding practices: Follow secure coding practices, such as using secure libraries and frameworks, to minimize the risk of introducing SSRF vulnerabilities in your application.
  5. Monitor and log requests: Monitor and log all server-side requests made by your application to detect and investigate potential SSRF attacks.
  6. Implement server-side request throttling: Limit the rate at which server-side requests can be made to prevent attackers from using SSRF for resource exhaustion or Denial of Service (DoS) attacks.

Tools for SSRF detection and exploitation

Several tools and frameworks can help you detect and exploit SSRF vulnerabilities. Here are some popular options:

  1. Burp Suite: Burp Suite is a comprehensive web application security testing platform that includes various features for identifying and exploiting SSRF vulnerabilities, such as the Repeater and Intruder tools.

  2. SSRFMap: SSRFMap is a Python-based tool designed specifically for detecting and exploiting SSRF vulnerabilities. It supports various attack techniques, including file reading, port scanning, and remote command execution.

  3. Collaborator: Burp Suite’s Collaborator is a server-side component that helps you identify SSRF vulnerabilities by monitoring and logging server-side interactions resulting from your testing.

  4. Nuclei: Nuclei is a fast and customizable vulnerability scanner that includes templates for detecting SSRF vulnerabilities in web applications.

  5. Custom scripts: You can also write your custom scripts or tools to automate the detection and exploitation of SSRF vulnerabilities, using languages like Python or Ruby and leveraging existing libraries and frameworks.

Conclusion

SSRF is a powerful and often overlooked attack vector that can lead to severe consequences for the target organization. As a red teamer or penetration tester, understanding and exploiting SSRF vulnerabilities is crucial for assessing the security posture of web applications and internal infrastructure.

By following the techniques and best practices outlined in this article, you’ll be well-equipped to identify, exploit, and defend against SSRF attacks. Remember to always conduct your tests ethically and responsibly, and never cause harm to others.

Happy hacking!