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threat-modeling.md

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Threat Model Analysis for caddyserver/caddy

Threat: ACME Account Compromise

  • Threat: ACME Account Compromise

    • Description: An attacker gains access to the credentials (API keys, account details) used by Caddy to interact with ACME Certificate Authorities (like Let's Encrypt or ZeroSSL). The attacker could then issue certificates for domains they don't own, revoke existing certificates, or otherwise disrupt the certificate management process, all through Caddy's automated systems.
    • Impact: Loss of confidentiality and integrity. The attacker could potentially launch man-in-the-middle attacks, impersonate the legitimate site, or cause service outages.
    • Affected Component: Caddy's tls app, specifically the ACME client functionality within the automation module.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Use strong, unique, and randomly generated passwords/API keys for ACME accounts.
      • Store credentials securely using environment variables or a secure secrets management system (e.g., HashiCorp Vault, AWS Secrets Manager). Never hardcode credentials in the Caddyfile.
      • Regularly rotate API keys.
      • Implement monitoring and alerting for unauthorized certificate issuance or revocation attempts (using Certificate Transparency logs).
      • Restrict permissions on the ACME account to the minimum necessary.

Threat: DNS Hijacking for ACME Challenge Validation (where Caddy manages DNS)

  • Threat: DNS Hijacking for ACME Challenge Validation (where Caddy manages DNS)

    • Description: If Caddy is configured to directly manage DNS records for ACME challenges (using a DNS provider plugin), and an attacker compromises the credentials for that DNS provider through Caddy's configuration, the attacker can manipulate DNS records to obtain fraudulent certificates. This is distinct from general DNS hijacking; this threat focuses on the compromise via Caddy's integration.
    • Impact: Loss of confidentiality and integrity. The attacker can intercept traffic and impersonate the legitimate site.
    • Affected Component: Caddy's tls app, specifically the ACME client and the configured DNS provider plugin.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Use strong, unique credentials for the DNS provider API, stored securely (environment variables, secrets manager).
      • Enable two-factor authentication for the DNS provider account.
      • Regularly rotate API keys used by the Caddy DNS provider plugin.
      • Monitor DNS records for unauthorized changes.
      • Restrict the permissions of the API key used by Caddy to the minimum necessary (only allow modification of specific records related to ACME challenges).

Threat: Certificate Issuance/Renewal Failure (due to Caddy misconfiguration)

  • Threat: Certificate Issuance/Renewal Failure (due to Caddy misconfiguration)

    • Description: Caddy fails to obtain or renew a TLS certificate due to a misconfiguration within Caddy itself. This could be an incorrect ACME endpoint, a misconfigured DNS provider, an invalid challenge type, or other errors in the tls app configuration. The website becomes inaccessible via HTTPS or serves an expired certificate.
    • Impact: Loss of confidentiality, integrity, and availability. Users will see browser warnings.
    • Affected Component: Caddy's tls app, specifically the automation module and the configured ACME client.
    • Risk Severity: High
    • Mitigation Strategies:
      • Thoroughly review and validate the tls app configuration in the Caddyfile.
      • Test certificate issuance and renewal in a staging environment before deploying to production.
      • Use Caddy's logging features to diagnose any errors during the certificate process.
      • Ensure that all required parameters for the chosen ACME client and challenge provider are correctly configured.

Threat: Weak TLS Configuration (within Caddy)

  • Threat: Weak TLS Configuration (within Caddy)

    • Description: The Caddyfile is explicitly configured to use weak ciphers, outdated TLS protocols (e.g., TLS 1.0, TLS 1.1), or insecure TLS features, overriding Caddy's secure defaults. An attacker could exploit these weaknesses.
    • Impact: Loss of confidentiality. Potential for man-in-the-middle attacks.
    • Affected Component: Caddy's tls app, specifically the protocols, ciphers, and other TLS-related directives within the global options or site blocks.
    • Risk Severity: High
    • Mitigation Strategies:
      • Rely on Caddy's default TLS settings. Avoid manual configuration unless absolutely necessary and you have a deep understanding of TLS.
      • If manual configuration is required, explicitly specify only strong ciphers and protocols (TLS 1.2 and TLS 1.3).
      • Regularly review and update the TLS configuration.
      • Use online tools (e.g., SSL Labs' SSL Test) to assess the configuration.

Threat: Unintended Reverse Proxy Exposure (due to Caddy misconfiguration)

  • Threat: Unintended Reverse Proxy Exposure (due to Caddy misconfiguration)

    • Description: reverse_proxy directives in the Caddyfile are misconfigured, causing requests to be routed to unintended backend services or applications that Caddy is proxying. This exposes internal services that were not meant to be publicly accessible.
    • Impact: Loss of confidentiality and potentially integrity. Attackers could gain access to internal systems.
    • Affected Component: Caddy's http.handlers.reverse_proxy module.
    • Risk Severity: High
    • Mitigation Strategies:
      • Thoroughly test all reverse_proxy configurations. Use a staging environment.
      • Use specific to addresses and avoid overly broad wildcards.
      • Implement authentication and authorization on backend services, even if they are behind Caddy.
      • Regularly review and audit the reverse_proxy configuration.

Threat: Zero-Day Vulnerability in Caddy Core

  • Threat: Zero-Day Vulnerability in Caddy Core

    • Description: An attacker exploits a previously unknown vulnerability (zero-day) in the core Caddy codebase itself. This is a direct threat to Caddy, not a misconfiguration or external factor.
    • Impact: Potentially severe, ranging from denial of service to remote code execution (RCE).
    • Affected Component: The core Caddy server (various modules could be affected).
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Keep Caddy updated to the latest stable version.
      • Monitor for security advisories from the Caddy project.
      • Consider using a WAF (though this is a mitigation for the impact, not the vulnerability itself).
      • Implement IDS/IPS.

Threat: Denial of Service (DoS) targeting Caddy directly

  • Threat: Denial of Service (DoS) targeting Caddy directly

    • Description: An attacker floods the Caddy server with requests, aiming to overwhelm Caddy's resources (CPU, memory, network handling capabilities) and make it unavailable. This is distinct from a DoS targeting a backend application; this targets Caddy's ability to function as a web server.
    • Impact: Loss of availability.
    • Affected Component: The core Caddy server and potentially any configured modules (e.g., http, tls).
    • Risk Severity: High
    • Mitigation Strategies:
      • Configure rate limiting using Caddy's built-in features or plugins.
      • Use a CDN (this helps mitigate the impact, but the attack still targets Caddy).
      • Monitor server resource usage.
      • Configure connection timeouts appropriately within Caddy.