Subdomain Security: Protecting Your Main Site

TL;DR

Yes, an attacker can exploit your main site from a compromised subdomain. This is because subdomains share the same domain name and often rely on shared resources like cookies. Secure your subdomains as rigorously as your main site to prevent attacks like session hijacking and cross-site scripting (XSS).

Understanding the Risk

Think of your website as a house with multiple doors (subdomains). If one door is unlocked, an intruder can potentially access the entire house. Here’s why:

• Shared Cookies: Many websites use cookies that are valid across all subdomains. A compromised subdomain could steal these cookies, giving an attacker access to your main site as a legitimate user.
• Cross-Site Scripting (XSS): An XSS vulnerability on a subdomain can inject malicious scripts into pages viewed by users of the main site.
• Session Hijacking: Attackers can steal session IDs from subdomains, allowing them to impersonate logged-in users on your primary domain.
• Shared Infrastructure: If both your main site and subdomains use the same server or database, a compromise in one area could lead to wider access.

How to Protect Your Main Site from Subdomain Attacks

1. Implement Strong Authentication & Authorization:
   • Use strong passwords and multi-factor authentication (MFA) for all accounts, including those managing subdomains.
   • Follow the principle of least privilege – grant users only the permissions they need.
2. Secure Your Subdomains:
   • Treat each subdomain as a separate application and apply the same security measures as your main site. This includes regular vulnerability scanning, penetration testing, and code reviews.
   • Keep all software (CMS, plugins, libraries) up to date on all subdomains.
3. Cookie Security:
   • HttpOnly Flag: Set the HttpOnly flag on cookies to prevent client-side scripts from accessing them, mitigating XSS attacks.

     Set-Cookie: sessionid=abcdefg; HttpOnly

   • Secure Flag: Use the Secure flag to ensure cookies are only transmitted over HTTPS.

     Set-Cookie: sessionid=abcdefg; Secure

   • SameSite Attribute: Control how cookies are sent with cross-site requests. Consider using SameSite=Strict or SameSite=Lax to prevent CSRF attacks.

     Set-Cookie: sessionid=abcdefg; SameSite=Strict

   • Domain Attribute: Be specific with the domain attribute. Avoid setting cookies for the entire domain (e.g., domain=.example.com) if not necessary. Use subdomains specifically.

     Set-Cookie: sessionid=abcdefg; domain=subdomain.example.com

4. Content Security Policy (CSP):

   Implement a CSP to control the resources that your website is allowed to load. This can help prevent XSS attacks by restricting the execution of malicious scripts.

   Content-Security-Policy: default-src 'self'; script-src 'self' https://trustedcdn.com;

5. Input Validation & Output Encoding:
   • Validate all user input on both the client and server sides to prevent injection attacks (XSS, SQL Injection).
   • Encode output properly to ensure that malicious data is not interpreted as code.
6. Regular Monitoring & Logging:
   • Monitor your website for suspicious activity, such as unusual login attempts or unexpected traffic patterns.
   • Enable detailed logging and regularly review logs to identify potential security incidents.
7. Network Segmentation (Advanced):

   Isolate your subdomains from your main site using network segmentation techniques, such as virtual LANs (VLANs) or firewalls. This can limit the impact of a compromise in one area.

Tools to Help

• OWASP ZAP: A free and open-source web application security scanner.
• Burp Suite: A popular commercial web penetration testing tool.
• Qualys SSL Labs: Tests your website’s SSL configuration, including subdomain certificates.