Defending from Vulnerabilities: Timing Attack Exploitation via Server-Side Request Forgery (SSRF)

In this week's series, we delve into a more sophisticated server-side attack—exploiting Server-Side Request Forgery (SSRF) using a timing attack. SSRF vulnerabilities occur when an attacker can manipulate server-side logic to make unauthorized network requests to internal or external services. In this scenario, the attacker leverages a timing attack to determine whether specific internal services exist by measuring response times. This type of attack is subtle and can help attackers map internal networks, even when the application does not provide direct feedback (such as error messages). This scenario highlights the importance of securing server-side logic, primarily when the application interacts with other network resources.

Understanding the Attack

1. Server-Side Request Forgery (SSRF):
   • The application allows server-side requests based on user input (e.g., fetching data from a user-specified URL).
   • SSRF vulnerabilities can occur when user input is not sanitised correctly, allowing the attacker to request internal or external services.
2. Attack Vector:
   • The attacker crafts input that forces the server to request internal services.
   • The attacker uses a timing attack by measuring the response time of different requests to infer whether specific services are available.
   • This information can be used to map internal networks, bypass firewalls, and eventually access sensitive data or services.

Defensive Strategy

To defend against this SSRF timing attack scenario, follow these steps:

Input Validation and Whitelisting: Implement strict input validation and only allow requests to trusted, whitelisted URLs. Also, block requests to internal IP ranges (e.g., localhost, 127.0.0.1, private network ranges).

Example:

javascriptCopy codeconst express = require('express');
const axios = require('axios');
const validator = require('validator');
const app = express();

const WHITELISTED_DOMAINS = ['https://api.trusted.com', 'https://www.safe-site.com'];

app.get('/fetch-data', async (req, res) => {
  const { url } = req.query;

  // Validate the URL
  if (!validator.isURL(url)) {
    return res.status(400).send('Invalid URL');
  }

  // Check if the URL is whitelisted
  if (!WHITELISTED_DOMAINS.some(domain => url.startsWith(domain))) {
    return res.status(400).send('URL not allowed');
  }

  try {
    const response = await axios.get(url);
    res.send(response.data);
  } catch (error) {
    res.status(500).send('Failed to fetch data');
  }
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Limit Network Access from the Application: Limit the application’s ability to make outbound network requests by limiting the server's network access through firewall rules or container-level policies.

Example using AWS Security Groups:

• Configure your AWS Security Group only to allow the server to make outbound requests to specific IP addresses or ports. This will block unauthorized access to internal services.

Example using Kubernetes Network Policies:

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: allow-specific-traffic
  namespace: default
spec:
  podSelector:
    matchLabels:
      app: my-app
  policyTypes:
  - Ingress
  - Egress
  egress:
  - to:
    - ipBlock:
        cidr: 203.0.113.0/24 # Allow requests to this specific external IP range

Introduce Random Delays and Uniform Response Times: To mitigate timing attacks, introduce random delays or normalize response times so attackers cannot infer information based on timing.

Example:

app.get('/fetch-data', async (req, res) => {
  const { url } = req.query;

  // Validate the URL
  if (!validator.isURL(url)) {
    return res.status(400).send('Invalid URL');
  }

  // Check if the URL is whitelisted
  if (!WHITELISTED_DOMAINS.some(domain => url.startsWith(domain))) {
    return res.status(400).send('URL not allowed');
  }

  // Introduce a random delay to mitigate timing attacks
  const delay = Math.floor(Math.random() * 200) + 100; // Random delay between 100ms and 300ms
  await new Promise(resolve => setTimeout(resolve, delay));

  try {
    const response = await axios.get(url);
    res.send(response.data);
  } catch (error) {
    // Normalize the response time in case of an error
    await new Promise(resolve => setTimeout(resolve, delay));
    res.status(500).send('Failed to fetch data');
  }
});

Log and Monitor Suspicious Network Requests: Log and monitor all outgoing network requests, flagging any that target internal services or exhibit suspicious patterns.

Example:

const express = require('express');
const axios = require('axios');
const validator = require('validator');
const winston = require('winston');
const app = express();

const logger = winston.createLogger({
  level: 'info',
  format: winston.format.json(),
  transports: [
    new winston.transports.File({ filename: 'ssrf_requests.log' }),
  ],
});

const WHITELISTED_DOMAINS = ['https://api.trusted.com', 'https://www.safe-site.com'];

app.get('/fetch-data', async (req, res) => {
  const { url } = req.query;

  // Validate the URL
  if (!validator.isURL(url)) {
    return res.status(400).send('Invalid URL');
  }

  // Log all outgoing requests
  logger.info(`Outgoing request to URL: ${url}`);

  // Check if the URL is whitelisted
  if (!WHITELISTED_DOMAINS.some(domain => url.startsWith(domain))) {
    logger.warn(`Blocked attempt to access non-whitelisted URL: ${url}`);
    return res.status(400).send('URL not allowed');
  }

  try {
    const response = await axios.get(url);
    res.send(response.data);
  } catch (error) {
    logger.error(`Failed to fetch data from URL: ${url}`, { error });
    res.status(500).send('Failed to fetch data');
  }
});

app.listen(3000, () => {
  console.log('Server running on port 3000');
});

Conclusion

By implementing strict input validation and whitelisting, limiting network access, introducing random delays to mitigate timing attacks, and logging suspicious network requests, you can effectively defend against SSRF vulnerabilities and timing-based attacks in your application.

This detailed approach helps developers and security engineers understand and apply these protections in specific scenarios.

Stay tuned for more specific attack scenarios and defensive strategies in our weekly series.