What is SSRF (Server-Side Request Forgery)? – Complete Guide

Introduction

In the digital age, web applications interact with multiple services on behalf of users—fetching data, connecting to APIs, or pulling content from URLs. This flexibility can be dangerous when not implemented securely. One such vulnerability that exploits this behavior is SSRF, or Server-Side Request Forgery.

🧠 What is SSRF?

SSRF (Server-Side Request Forgery) is a web security vulnerability that allows an attacker to force the server to make unauthorized requests on its behalf. This can lead to sensitive data exposure, unauthorized access to internal systems, and even remote code execution in some cases.

Instead of attacking the client browser, SSRF targets the server itself.

💡 How SSRF Works

Here's a simplified flow:

1. The application allows users to input a URL (e.g., to fetch a profile image).
2. The server fetches that URL and returns the result.
3. If the input is not validated properly, an attacker can provide an internal IP address (like http://localhost:80).
4. The server unknowingly makes a request to an internal resource.

🧪 Real-life Example

Let’s say a web app accepts a URL to fetch an image:

An attacker replaces the URL:

Now, the server is fetching its own admin panel, possibly exposing sensitive data.

🎯 What Can Attackers Do With SSRF?

1. 🏠 Access internal services not exposed to the public
2. 🔐 Expose cloud metadata (like AWS EC2: http://169.254.169.254)
3. 📂 Scan internal networks
4. 📥 Read files or data behind firewalls
5. 💣 Exploit other services for privilege escalation
6. 
   

🛑 Real-World Incidents

1. Capital One (2019) – A famous SSRF vulnerability allowed attackers to access sensitive AWS metadata, exposing personal data of over 100 million users.
2. Alibaba Cloud – In 2020, a disclosed SSRF bug allowed attackers to exploit internal APIs and steal credentials.
3. 
   

🧰 Common SSRF Targets

1. http://localhost
2. http://127.0.0.1
3. http://169.254.169.254 (cloud metadata)
4. http://internal-service/api

These internal endpoints are not meant to be accessed by users, but SSRF tricks the server into doing it.

🔍 Detecting SSRF Vulnerabilities

Use tools and techniques like:

1. 🔗 Intercept HTTP requests (using Burp Suite or OWASP ZAP)
2. 👨‍💻 Inject test URLs (localhost, 127.0.0.1)
3. 🛰️ Monitor DNS requests (using tools like Burp Collaborator)
4. 🔄 Use internal IP address variations (2130706433 instead of 127.0.0.1)
5. 
   

🛡️ How to Prevent SSRF

1. Input Validation
2. Allow only whitelisted domains or IPs.
3. Reject requests to localhost, 127.0.0.1, etc.
4. Disable Unused Services
5. Turn off internal services not meant for public use.
6. Use Network-level Controls
7. Implement firewall rules to block internal requests.
8. Avoid Blind Fetching
9. Don’t fetch content blindly based on user input.
10. Metadata Protection
11. On cloud platforms, disable public access to metadata endpoints.

📌 Summary

Server-Side Request Forgery (SSRF) is a powerful vulnerability that can have severe consequences if not handled correctly. Modern web applications often trust user input too much, and attackers take advantage of this trust. Always validate and sanitize inputs, restrict internal resource access, and monitor network requests.

📢 Final Thoughts

SSRF attacks are increasing as more applications interact with internal microservices and third-party APIs. Secure your backend like you secure your frontend. One blind URL fetch could open a gateway to your entire infrastructure.