I suspect my app is vulnerable to Server-Side Request Forgery SSRF due to dynamic URL fetching How can I secure it without affecting functionality

Question

My app dynamically fetches URLs based on user input, and I’m concerned it could be vulnerable to Server-Side Request Forgery (SSRF). However, I need to retain this feature for the app’s functionality, so removing it isn’t an option.

Are there specific methods or libraries I can use to filter or validate the URLs securely? I’d appreciate advice on securing this feature without compromising the app’s intended use.

Answer 1

To secure your application from Server-Side Request Forgery (SSRF) vulnerabilities while still allowing dynamic URL fetching based on user input, you can implement several strategies. Here’s a comprehensive approach to mitigate the risks associated with SSRF without losing the functionality your app needs:

1. Input Validation and Whitelisting

Start by validating user input to ensure only expected URLs are processed. Instead of allowing arbitrary URLs, use a whitelist approach to define what URLs or domains can be accessed. This is the first line of defense against SSRF.

const allowedDomains = ['example.com', 'api.example.com'];

function isValidUrl(url) {
    try {
        const { hostname } = new URL(url);
        return allowedDomains.includes(hostname);
    } catch (error) {
        return false; // Invalid URL
    }
}

// Usage
if (!isValidUrl(userInputUrl)) {
    throw new Error('Invalid URL');
}

2. Limit URL Protocols

Restrict the protocols that can be used to fetch URLs. Generally, you want to limit access to HTTP and HTTPS only, as other protocols (like file:// or ftp://) can lead to SSRF.

function isSafeProtocol(url) {
    const safeProtocols = ['http:', 'https:'];
    const { protocol } = new URL(url);
    return safeProtocols.includes(protocol);
}

// Usage
if (!isSafeProtocol(userInputUrl)) {
    throw new Error('Unsupported protocol');
}

3. Use a Proxy Service

If you need to fetch external resources, consider routing requests through a proxy server that you control. This allows you to inspect and filter requests further before they reach the target destination.

Set up a secure proxy that only allows requests to predefined domains.
Log and monitor the requests made through the proxy for suspicious activity.

4. Rate Limiting and Logging

Implement rate limiting to prevent abuse of the URL-fetching functionality. This can help detect and mitigate potential SSRF attempts by limiting the number of requests from a single user.

Additionally, keep detailed logs of URL requests to help identify and investigate any suspicious behavior.

5. Use Security Headers

Make sure to set proper security headers in your application to mitigate risks associated with SSRF. For example, setting the Content-Security-Policy header can help restrict the sources from which content can be loaded.

6. Perform Outbound Request Validation

Before making an outbound request, validate the response. For instance, if you’re expecting a JSON response, ensure the content type matches what you expect. This can help catch any anomalies in the response.

const axios = require('axios');

async function fetchUrl(url) {
    const response = await axios.get(url);
    
    // Validate response content type
    if (!response.headers['content-type'].includes('application/json')) {
        throw new Error('Unexpected content type');
    }
    
    return response.data;
}

7. Regular Security Audits and Penetration Testing

Continuously audit your application for vulnerabilities, including SSRF. Conduct regular penetration testing to identify and mitigate potential security issues proactively.