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attack-surface.md

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Attack Surface Analysis for caddyserver/caddy

Attack Surface: Insecure Caddyfile/JSON Configuration

  • Description: Misconfigurations in Caddy's configuration files (Caddyfile or JSON) can expose sensitive information or create security vulnerabilities.
  • How Caddy Contributes: Caddy relies on these files for all its behavior. Incorrect directives or exposed secrets within these files directly impact Caddy's security.
  • Example: Including database credentials directly in the Caddyfile for a reverse proxy to a backend application, and then accidentally making the Caddyfile world-readable.
  • Impact: Exposure of sensitive credentials, potential for unauthorized access to backend systems, compromise of data.
  • Risk Severity: High to Critical (depending on the sensitivity of exposed information).
  • Mitigation Strategies:
    • Secure Storage: Store Caddy configuration files with restrictive permissions (e.g., readable only by the Caddy user).
    • Environment Variables/Secrets Management: Utilize environment variables or dedicated secrets management tools to inject sensitive information (API keys, credentials) into Caddy configuration instead of hardcoding them in files.
    • Configuration Validation: Regularly review and validate Caddy configurations for security best practices and potential misconfigurations.
    • Principle of Least Privilege: Only grant necessary permissions to the Caddy process and configuration files.

Attack Surface: Unauthenticated Caddy Admin API

  • Description: The Caddy Admin API, if enabled without proper authentication, allows unauthorized control over the Caddy server.
  • How Caddy Contributes: Caddy provides this API for dynamic configuration and management, but it must be secured.
  • Example: Enabling the Admin API on a public interface without setting up authentication. An attacker could then use the API to modify Caddy's configuration, potentially redirecting traffic or causing denial of service.
  • Impact: Full compromise of the Caddy server, potential for data exfiltration, denial of service, and further attacks on backend systems.
  • Risk Severity: Critical.
  • Mitigation Strategies:
    • Disable Admin API (if not needed): If dynamic configuration is not required, disable the Admin API entirely.
    • Enable Authentication: Configure strong authentication for the Admin API using API keys or other supported methods.
    • Network Restrictions: Restrict access to the Admin API to trusted networks or specific IP addresses (e.g., localhost or internal management network).
    • Regularly Review Access: Periodically review and audit access controls for the Admin API.

Attack Surface: Open Reverse Proxy Misconfiguration

  • Description: Incorrectly configured reverse_proxy directive can turn Caddy into an open proxy, allowing attackers to proxy requests to arbitrary destinations.
  • How Caddy Contributes: Caddy's reverse_proxy functionality is powerful but requires careful configuration to prevent misuse.
  • Example: Setting up a reverse_proxy without any access controls or destination restrictions. An attacker could use this Caddy instance to bypass firewalls, access internal networks, or launch attacks against other systems (SSRF).
  • Impact: Server-Side Request Forgery (SSRF), open proxy abuse, potential for internal network access and further attacks.
  • Risk Severity: High.
  • Mitigation Strategies:
    • Restrict Proxy Destinations: Carefully define allowed backend destinations for the reverse_proxy directive. Use allowlists or denylists based on domains or IP ranges.
    • Authentication/Authorization: Implement authentication and authorization mechanisms for proxied requests if necessary.
    • Rate Limiting: Implement rate limiting on the reverse proxy to mitigate abuse and DoS potential.
    • Regular Configuration Review: Regularly review and audit reverse proxy configurations to ensure they are secure and aligned with intended use.

Attack Surface: Delayed Security Updates

  • Description: Failing to promptly apply security updates to Caddy leaves it vulnerable to known exploits.
  • How Caddy Contributes: Like any software, Caddy requires regular updates to address discovered vulnerabilities.
  • Example: A new vulnerability is discovered in Caddy, and an exploit is publicly available. If the Caddy instance is not updated, it remains vulnerable to this exploit.
  • Impact: Exploitation of known vulnerabilities, potential for full server compromise, data breaches, denial of service.
  • Risk Severity: High to Critical (depending on the severity of the unpatched vulnerability).
  • Mitigation Strategies:
    • Regular Update Schedule: Establish a regular schedule for checking and applying Caddy updates.
    • Monitoring Release Notes: Subscribe to Caddy's release notes and security announcements to be aware of new updates and security patches.
    • Automated Updates (where possible): Utilize system package managers or container image updates to automate Caddy updates where feasible.
    • Testing Updates: Test updates in a staging environment before applying them to production to ensure compatibility and stability.