HSTS Bypass: Risks & Mitigation

TL;DR

While HSTS (HTTP Strict Transport Security) significantly improves website security by forcing browsers to use HTTPS, it’s not foolproof. Attacks like preloading issues, subdomain takeovers, and timing vulnerabilities can bypass HSTS. Regularly audit your configuration, monitor for changes, and implement robust subdomain control to minimize risk.

Understanding the Risks

HSTS tells a browser: “Always connect to this website using HTTPS”. However, several scenarios can allow attackers to circumvent this protection:

1. HSTS Preloading Issues

1. What it is: Websites can submit their domains to the HSTS preload list (managed by Chrome and other browsers). This means the browser knows to enforce HTTPS *before* even receiving an HSTS header from your server.
2. The risk: If a subdomain isn’t correctly included in the preload list, or if it’s incorrectly configured, users might be vulnerable via that subdomain. Changes to the preload list take time to propagate.
3. Mitigation:
   • Carefully review your HSTS configuration before submitting to the preload list.
   • Use a tool like hstspreload.org to validate your setup.
   • Monitor for changes in subdomain ownership or DNS records that could lead to misconfiguration.

2. Subdomain Takeovers

1. What it is: An attacker gains control of a subdomain (e.g., blog.example.com) and points its DNS records to their own server.
2. The risk: If the main domain has an HSTS policy that includes all subdomains, the browser will enforce HTTPS even when connecting to the malicious subdomain. This can lead to phishing attacks or man-in-the-middle exploits.
3. Mitigation:
   • Robust Subdomain Control: Implement strict controls over who can create and modify DNS records for your subdomains. Use multi-factor authentication wherever possible.
   • Regular Audits: Regularly scan your DNS records to identify any unauthorized or unexpected changes. Tools like dig (Linux/macOS) or online DNS record checkers are helpful.

     dig blog.example.com +trace

   • HSTS Policy Scope: Be cautious about using a broad HSTS policy that includes all subdomains unless you have complete control over them. Consider specifying only the necessary subdomains.

3. Timing Vulnerabilities & Downgrade Attacks

1. What it is: Exploiting timing differences in how a server handles HTTP and HTTPS requests, or attempting to force a downgrade to HTTP before the HSTS policy takes effect.
2. The risk: An attacker might be able to intercept initial unencrypted traffic before HSTS is enforced, potentially stealing credentials or injecting malicious content.
3. Mitigation:
   • Redirect all HTTP traffic to HTTPS: Ensure that any incoming HTTP requests are immediately redirected to the HTTPS equivalent.

     # Example .htaccess redirect (Apache)

     RewriteEngine On
     RewriteCond %{HTTPS} off
     RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

   • Preload HSTS: Using the preload list minimizes the window of opportunity for timing attacks.
   • Strict Transport Security Header Configuration: Configure your HSTS header with appropriate directives:
     • max-age: Specifies how long the browser should remember to enforce HTTPS (e.g., max-age=31536000 for one year).
     • includeSubDomains: Includes all subdomains in the policy (use with caution!).
     • preload: Indicates that the domain is included in the HSTS preload list.

     Strict-Transport-Security: max-age=31536000; includeSubDomains; preload

4. Certificate Issues

1. What it is: Expired, revoked or misconfigured SSL/TLS certificates.
2. The risk: Browsers will refuse to connect if the certificate isn’t valid, but attackers can exploit temporary issues or use self-signed certificates in some cases.
3. Mitigation:
   • Automated Certificate Management: Use tools like Let’s Encrypt with automated renewal (e.g., Certbot) to ensure your certificates are always valid.
   • Regular Monitoring: Monitor certificate expiry dates and revocation status.

5. Browser Bugs

Although rare, vulnerabilities in browsers themselves can sometimes bypass HSTS enforcement. Keep your browser up-to-date to benefit from the latest security patches.