Blog | G5 Cyber Security

Random Subdomain JS: Security Risks

G5 Cyber Security

TL;DR

Using randomized subdomains to run JavaScript (JS) introduces significant cyber security risks. While it can offer some obfuscation, the benefits are usually outweighed by the potential for Cross-Site Scripting (XSS), subdomain takeovers, and other attacks. It’s generally best avoided unless you have a very specific reason and robust mitigation strategies in place.

Why Random Subdomains Are Risky

The idea behind using randomized subdomains is to make it harder for attackers to predict URLs and target your application. However, this doesn’t address the underlying vulnerabilities that allow attacks in the first place. Here’s a breakdown of the problems:

Solution Guide: Mitigating Risks (or Avoiding Them)

  1. Understand the Core Problem: XSS
    • Random subdomains don’t prevent attackers from injecting malicious JavaScript if your application doesn’t properly sanitise user input.
    • If a user can control any part of the JS that runs on any subdomain, they can compromise it.
  2. Avoid Dynamic Subdomain Generation Where Possible
    • The simplest solution is often to avoid generating subdomains dynamically based on user input or unpredictable values.
    • Use a fixed set of subdomains, and carefully control the content served from each one.
  3. If You Must Use Random Subdomains: Strict Content Security Policy (CSP)

    A strong CSP is essential if you’re running JS on randomized subdomains.

    • Define a strict whitelist of allowed sources for scripts. Don’t allow * or broad wildcards.
    • Example CSP header: 
      Content-Security-Policy: script-src 'self' https://trustedcdn.example.com;
    • Use a nonce for inline scripts. This means each time you serve a page, generate a unique random string and include it in both the CSP header and your inline script tags. 
      <script nonce="{{random_nonce}}">// Your inline script here</script>
  4. Subdomain Takeover Protection
    • Random subdomains can be vulnerable to subdomain takeovers if DNS records aren’t properly managed. Attackers might claim ownership of unused subdomains.
    • Regularly scan your DNS records for unexpected entries. 
      dig +trace example.com
    • Use a tool like subfinder or similar to discover all associated subdomains: 
      subfinder -d example.com
  5. Input Validation and Output Encoding
    • Always validate user input on the server-side before using it to generate anything, including subdomain names or JS code.
    • Encode all output properly to prevent XSS attacks. Use a library appropriate for your programming language (e.g., HTML escaping in Python).
  6. Regular Security Audits and Penetration Testing
    • Have your application regularly audited by security professionals.
    • Conduct penetration testing to identify vulnerabilities before attackers do.
  7. Monitor for Anomalous Activity
    • Implement logging and monitoring to detect unusual patterns of requests or JS execution on your subdomains.

Alternatives

Consider these alternatives instead of random subdomains:

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