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attack-surface.md

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Attack Surface Analysis for caddyserver/caddy

Attack Surface: 1. Automatic HTTPS (ACME) Misconfiguration/Abuse

  • Description: Exploitation of Caddy's automatic HTTPS provisioning mechanism to obtain fraudulent certificates or disrupt certificate management.
  • Caddy's Contribution: Caddy's core feature is automatic HTTPS. This reliance on external CAs and challenge mechanisms, managed by Caddy, creates this specific attack surface.
  • Example: An attacker uses DNS hijacking to pass the DNS-01 challenge and obtain a valid certificate for a domain they don't control, leveraging Caddy's automated process.
  • Impact: Man-in-the-middle (MITM) attacks, phishing, data breaches, loss of user trust.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Use DNSSEC: Implement DNS Security Extensions (DNSSEC).
    • CAA Records: Configure Certification Authority Authorization (CAA) records.
    • Monitor Certificate Transparency Logs: Regularly monitor CT logs.
    • Secure DNS Provider API Keys: Protect API keys used for DNS-01 challenges.
    • Network Segmentation: Isolate the Caddy server where possible.
    • Use a Reputable ACME CA: Stick with well-known CAs.
    • Regularly Update Caddy: Keep Caddy updated.

Attack Surface: 2. Caddyfile Misconfiguration (Reverse Proxy)

  • Description: Incorrectly configured reverse_proxy directives expose internal services or applications.
  • Caddy's Contribution: Caddy's reverse_proxy directive, a core Caddy component, is the direct source of this risk if misconfigured.
  • Example: A Caddyfile uses reverse_proxy * http://localhost:8080, exposing an internal application on port 8080.
  • Impact: Unauthorized access to internal services, data breaches.
  • Risk Severity: High
  • Mitigation Strategies:
    • Principle of Least Privilege: Only expose necessary paths/resources.
    • Use Specific Path Matching: Avoid broad matches.
    • Implement Authentication/Authorization: Use Caddy's authentication modules.
    • Regularly Review Caddyfile: Conduct code reviews.
    • Use a Staging Environment: Test changes before production.
    • Input Validation: Validate any user input used in proxy configurations.

Attack Surface: 3. Caddyfile Misconfiguration (File Server)

  • Description: Improperly configured file_server directives expose sensitive files.
  • Caddy's Contribution: Caddy's file_server directive, a core Caddy component, is the direct source of this risk.
  • Example: file_server serves a directory containing .git folders.
  • Impact: Information disclosure, source code leakage.
  • Risk Severity: High
  • Mitigation Strategies:
    • Specify a Root Directory: Always define the root.
    • Restrict Access to Sensitive Files: Use handle or route to deny access.
    • Disable Directory Listings: Ensure listings are off.
    • Regularly Review Served Files: Check for exposed sensitive data.

Attack Surface: 4. Vulnerabilities in Caddy or Plugins

  • Description: Exploitable vulnerabilities in Caddy's core code or plugins.
  • Caddy's Contribution: This is a direct risk stemming from Caddy itself and its plugin architecture.
  • Example: A buffer overflow in a Caddy plugin allows code execution.
  • Impact: DoS, RCE, data breaches, system compromise.
  • Risk Severity: Critical (for RCE), High (for DoS)
  • Mitigation Strategies:
    • Keep Caddy Updated: Regular updates are crucial.
    • Use Trusted Plugins: Only install from reputable sources.
    • Review Plugin Code (if possible): For open-source plugins.
    • Monitor Security Advisories: Stay informed.
    • Limit Plugin Usage: Reduce the attack surface.
    • Vulnerability Scanning: Use scanners to identify issues.

Attack Surface: 5. Unsecured Admin API

  • Description: Unauthorized access to Caddy's administrative API.
  • Caddy's Contribution: Caddy's built-in admin API is the direct source of this risk.
  • Example: The admin API is exposed on port 2019 without authentication.
  • Impact: Complete control over the Caddy instance.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Disable the Admin API (if not needed): Best practice if unused.
    • Restrict Access: Use firewall rules or network ACLs.
    • Enable Authentication: Configure strong authentication.
    • Change the Default Port: Use a non-standard port.
    • Monitor API Access: Log and monitor all access.

Attack Surface: 6. Weak TLS Configuration

  • Description: Caddy configured to use outdated or weak TLS protocols and ciphers.
  • Caddy's Contribution: While Caddy defaults to strong settings, it allows configuration of weaker options within the Caddyfile.
  • Example: Caddyfile explicitly allows TLS 1.0 and weak ciphers.
  • Impact: Increased risk of MITM attacks and eavesdropping.
  • Risk Severity: High
  • Mitigation Strategies:
    • Use Caddy's Defaults: Rely on the secure defaults.
    • Explicitly Configure Strong Ciphers: If overriding, be explicit.
    • Regularly Review TLS Configuration: Keep it up-to-date.
    • Use TLS Testing Tools: Assess the configuration's strength.