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Server-Side Request Forgery (SSRF)

1. Introduction

Server-Side Request Forgery (SSRF) is a vulnerability that allows an attacker to send crafted requests from a server to unintended locations. SSRF can lead to unauthorized access to internal systems, sensitive data leakage, and other security issues.

2. Key Concepts

  • **Server-Side Request**: A request made by a server on behalf of a user.
  • **Forger**: The attacker who manipulates the server to make unintended requests.
  • **Target**: Typically, internal services or other systems that are not directly exposed to the internet.

3. How SSRF Works

SSRF attacks typically exploit features in web applications that allow users to specify URLs. The server processes these URLs and sends requests. If these requests can target internal services, attackers can retrieve sensitive information.


graph TD;
    A[User Input] --> B[Web Application];
    B --> C{Validate URL?};
    C -->|No| D[Reject Request];
    C -->|Yes| E[Send Request];
    E --> F[Internal Service];
    F --> G[Sensitive Data];
    G --> H[Return Data to Attacker];
            

4. Code Example

Here’s a simple example in Python using Flask that demonstrates how SSRF can be exploited:

from flask import Flask, request
import requests

app = Flask(__name__)

@app.route('/fetch')
def fetch():
    url = request.args.get('url')
    response = requests.get(url)
    return response.text

if __name__ == '__main__':
    app.run(debug=True)
                

In this example, an attacker could exploit the `/fetch` endpoint by providing a URL that points to an internal service.

5. Preventive Measures

To mitigate SSRF vulnerabilities, consider the following best practices:

  • Implement **URL whitelisting** to restrict accessible endpoints.
  • Use **input validation** to filter out unexpected inputs.
  • Employ **network segmentation** to limit server access to internal services.
  • Disable unnecessary **protocols** (e.g., file://, gopher://) in your application.
  • Monitor and log requests for **anomalous behavior**.

6. FAQ

What is SSRF?

SSRF is a type of security vulnerability that allows an attacker to send requests from a vulnerable server to any internal or external service.

How can I test for SSRF vulnerabilities?

You can test for SSRF by finding input fields that accept URLs and attempting to input internal addresses to see if they are accessible.

What are the consequences of SSRF?

Consequences can include data exfiltration, unauthorized access to internal services, and other types of attacks such as remote code execution.