SSRF Mitigation - OWASP Top 10

1. Introduction

Server-Side Request Forgery (SSRF) is a security vulnerability that allows an attacker to induce the server-side application to make requests to an unintended location. This can lead to unauthorized access to internal systems, sensitive data exposure, and even complete server takeover.

2. Key Concepts

• Internal Services: Services that are only accessible within the server's network.
• External Requests: Requests made from the server to external resources (e.g., URLs, APIs).
• Input Validation: The process of validating and sanitizing user inputs to prevent malicious data from being processed.

3. Mitigation Techniques

Implementing proper SSRF mitigations involves a combination of techniques:

1. Validate and sanitize all user inputs, especially URLs.
2. Use allow-lists to define which endpoints are accessible.
3. Implement strict response validation to ensure that the data being fetched is as expected.
4. Limit outbound requests from the server to only those that are necessary.
5. Employ network segmentation to restrict access to internal services.

4. Code Examples

Below is an example of how to validate a URL input in a Node.js application:

const { URL } = require('url');

function validateUrl(inputUrl) {
    try {
        const url = new URL(inputUrl);
        // Whitelist example domains
        const allowedDomains = ['example.com', 'api.example.com'];
        if (!allowedDomains.includes(url.hostname)) {
            throw new Error('URL not allowed');
        }
        return url; // Validated URL
    } catch (err) {
        throw new Error('Invalid URL');
    }
}
            

5. Best Practices

Adopting best practices can significantly reduce the risk of SSRF:

• Always use libraries that sanitize and validate user inputs.
• Monitor and log all outgoing requests for suspicious activities.
• Regularly review and update your allow-lists as necessary.
• Conduct security audits and penetration testing to identify vulnerabilities.

6. FAQ