Unmasking Server-Side Request Forgery (SSRF): How Attackers Exploit Internal Systems

Web applications often serve as the bridge between users and the server, providing essential resources or communicating with other services. But what if an attacker could hijack this communication and manipulate the server into making requests on their behalf? This is exactly what happens in a Server-Side Request Forgery (SSRF) attack — a serious vulnerability that many web applications overlook.

SSRF

In this post, we’ll dive deep into what SSRF is, how attackers exploit it, and the severe consequences of failing to mitigate this threat.

Server-Side Request Forgery (SSRF) is a security flaw that occurs when an attacker tricks a vulnerable server into making requests to unintended destinations. These destinations could be internal services or external systems, allowing the attacker to exploit functionality that the server is authorized to access but shouldn’t be exposed to user input.

In many cases, SSRF vulnerabilities occur when a server-side application takes user-supplied input (such as URLs or hostnames) and makes requests without adequately validating or sanitizing the input.

Let’s break down SSRF with a simplified real-world example. Imagine a web application where users can provide URLs to fetch remote images or data. If the server does not validate whether the provided URL is safe, attackers can trick the server into making requests to internal services that are not publicly accessible, or worse, to services holding sensitive data.

Here’s a step-by-step guide on how a basic SSRF attack can unfold:

Step 1: Identify a Potential SSRF Vulnerability

Navigate to the URL https://asmb.org. You’ll see an input field (in this case, a chat box) where user input is taken and processed by the server.

Step 2: Open Burp Suite Collaborator

Fire up Burp Suite, a common penetration testing tool, and go to the Collaborator tab. Here, you can generate a custom Collaborator URL that will act as a payload to trigger SSRF. Copy the generated URL.

Step 3: Modify the Collaborator URL

Next, modify the URL to ensure it’s interpreted as a valid URL by the server. You can prepend http:// to the Collaborator URL, resulting in something like:
http://collaborator-url.

Step 4: Inject the URL into the Application

Paste the modified URL into the message input box of the application. Click “Send Message” to submit your payload.

Step 5: Monitor Burp Suite for Results

Now, return to the Collaborator tab in Burp Suite and monitor the incoming traffic. If the server makes a request to your Collaborator URL, you’ll see an HTTP or DNS query coming from the target application’s server — proving that the SSRF attack succeeded.

Step 6: Understand the Impact

In this scenario, the server is making requests based on user input, which can be redirected to any internal or external resources the server has access to. This allows attackers to go far beyond mere mischief — they can use this technique to access private data, perform internal network reconnaissance, or even manipulate external services on behalf of the compromised server.

SSRF vulnerabilities can have catastrophic consequences, especially if the compromised server has access to internal systems or sensitive data. Here are some of the potential impacts:

Access to Internal Networks
Attackers can exploit SSRF to gain access to internal services, often hidden behind firewalls. By targeting resources like localhost or private IP ranges, an attacker could compromise other services that are not exposed to the public internet.Data Exfiltration
With SSRF, attackers can query internal APIs or databases to extract sensitive data. For instance, in cloud environments like AWS, attackers may query the EC2 metadata service to retrieve access tokens or credentials, leading to further breaches.Port Scanning and Network Mapping
SSRF can also be used to perform internal network reconnaissance. By sending requests to various internal IP addresses and ports, attackers can map out network services and identify other vulnerable systems, paving the way for more advanced attacks.Abuse of Third-Party Services
Attackers may abuse SSRF to make requests to third-party services, such as APIs or external servers. This could result in Denial-of-Service (DoS) attacks, unintended service interactions, or even the exploitation of vulnerabilities in external systems.

Mitigating SSRF vulnerabilities requires a multi-layered defense strategy that prevents untrusted input from being used to manipulate server-side requests.

1. Input Validation and Sanitization

Always validate and sanitize user input. Restrict URL schemes (e.g., only allow http and https), and block requests to localhost (127.0.0.1), private IP addresses (10.0.0.0/8, 192.168.0.0/16, etc.), or other internal networks.

2. Implement Allowlisting

Create an allowlist of trusted external resources that the server is allowed to communicate with. By doing so, you prevent the server from making requests to unauthorized or malicious destinations.

3. Block Access to Internal Systems

Ensure that internal systems are not accessible via SSRF. This can be done by blocking access to sensitive resources such as internal services, private IP addresses, and cloud metadata services.

4. Authenticate Internal Resources

Internal APIs or resources should require authentication to ensure that even if an SSRF attack occurs, sensitive resources cannot be accessed without proper credentials.

Here are some notable SSRF vulnerabilities reported in the wild:

HackerOne Report #1736390: A real-world example of SSRF being used to access internal services, leading to data exposure.HackerOne Report #310036: Another case of SSRF used for internal port scanning and network enumeration, demonstrating the potential scope of such attacks.

Server-Side Request Forgery is a powerful vulnerability with the potential for far-reaching consequences. Whether it’s accessing sensitive internal systems, exfiltrating confidential data, or manipulating external services, SSRF opens up a dangerous attack vector that should not be ignored. By understanding the nature of SSRF and implementing robust defenses, organizations can better protect their systems from this pervasive threat.