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BUSINESS POSTURE

Caddy is a powerful, open-source web server written in Go. It's designed for ease of use and security, with automatic HTTPS being a core feature. Based on the GitHub repository, here's a breakdown of the business posture:

Priorities and Goals:

  • Provide a user-friendly web server that simplifies HTTPS configuration.
  • Offer a secure-by-default experience, minimizing manual configuration for security features.
  • Maintain high performance and efficiency.
  • Support a wide range of use cases, from simple static sites to complex reverse proxy setups.
  • Foster a strong community and ecosystem around the project.
  • Offer commercial options for users who need additional features or support.

Business Risks:

  • Vulnerabilities in the Caddy codebase could lead to server compromise, data breaches, or denial-of-service attacks.
  • Misconfiguration by users (despite the focus on ease of use) could expose sensitive data or create security vulnerabilities.
  • Dependence on third-party libraries (Go modules) introduces supply chain risks.
  • Failure to keep up with evolving web standards and security best practices could render Caddy less effective or secure over time.
  • Competition from other web servers (e.g., Nginx, Apache) could impact adoption and market share.
  • Reputational damage from security incidents could erode trust in the project.

SECURITY POSTURE

Existing Security Controls:

  • security control: Automatic HTTPS: Caddy obtains and renews TLS certificates automatically via Let's Encrypt or ZeroSSL. This is a core feature and is described extensively in the documentation.
  • security control: Secure defaults: Caddy is designed with secure defaults, minimizing the need for manual security configuration. This is evident throughout the codebase and documentation.
  • security control: Memory safety: Being written in Go, Caddy benefits from Go's memory safety features, reducing the risk of buffer overflows and other memory-related vulnerabilities.
  • security control: Regular updates: The project has a frequent release cycle, indicating that security patches and updates are likely to be addressed promptly.
  • security control: Community scrutiny: As an open-source project, Caddy benefits from scrutiny by a large community of developers and security researchers.
  • security control: HTTP/2 and HTTP/3 support: Caddy supports modern HTTP protocols, which offer performance and security improvements.
  • security control: Security headers: Caddy allows easy configuration of security headers like HSTS, CSP, etc. (mentioned in documentation).
  • security control: Access logs: Caddy provides access logging capabilities, which are crucial for auditing and incident response.

Accepted Risks:

  • accepted risk: Third-party library vulnerabilities: While Go's package management system helps, there's always a risk of vulnerabilities in dependencies.
  • accepted risk: User misconfiguration: Even with secure defaults, users can still misconfigure Caddy, creating security risks.
  • accepted risk: Zero-day vulnerabilities: Like any software, Caddy is susceptible to unknown vulnerabilities.
  • accepted risk: Denial of Service: While Caddy is performant, it's still vulnerable to DoS attacks, especially if not properly configured with rate limiting or other mitigation techniques.

Recommended Security Controls:

  • Implement a robust vulnerability disclosure program (VDP) to encourage responsible reporting of security issues.
  • Perform regular security audits and penetration testing, both internally and by external experts.
  • Integrate Software Composition Analysis (SCA) tools into the build process to identify and manage vulnerabilities in third-party dependencies.
  • Implement fuzzing to proactively discover potential vulnerabilities.
  • Consider developing a formal security model and threat model for Caddy.

Security Requirements:

  • Authentication:

    • Caddy itself primarily deals with serving content and doesn't inherently manage user authentication for websites. Authentication is typically handled by applications behind Caddy or through Caddy modules (plugins).
    • Requirement: If authentication is implemented via modules, ensure secure storage of credentials (e.g., using appropriate hashing algorithms) and protection against common authentication attacks (e.g., brute-force, credential stuffing).
    • Requirement: Support for multi-factor authentication (MFA) should be considered where appropriate.

  • Authorization:

    • Caddy can control access to resources based on various criteria (e.g., IP address, request path).
    • Requirement: Access control mechanisms should follow the principle of least privilege.
    • Requirement: Ensure that authorization checks are performed consistently and correctly, avoiding bypass vulnerabilities.

  • Input Validation:

    • Caddy itself handles incoming HTTP requests and must be robust against malformed or malicious input.
    • Requirement: All input from clients (headers, request bodies, query parameters) should be strictly validated and sanitized to prevent injection attacks (e.g., XSS, command injection).
    • Requirement: Caddy modules should also perform thorough input validation.

  • Cryptography:

    • Caddy's core functionality relies on TLS for secure communication.
    • Requirement: Use strong, up-to-date cryptographic algorithms and protocols (e.g., TLS 1.3).
    • Requirement: Ensure proper key management and protection.
    • Requirement: Avoid using weak or deprecated ciphers.
    • Requirement: If custom cryptography is used in modules, it should be implemented by security experts and thoroughly reviewed.

DESIGN

C4 CONTEXT

graph LR
    User(("User"))
    Caddy(("Caddy Web Server"))
    ACME_Provider(("ACME Provider\n(Let's Encrypt, ZeroSSL)"))
    DNS_Provider(("DNS Provider"))
    Backend_App(("Backend Application(s)"))
    Admin_Interface(("Admin Interface"))

    User -- HTTPS --> Caddy
    Caddy -- ACME Protocol --> ACME_Provider
    Caddy -- DNS API --> DNS_Provider
    Caddy -- HTTP/HTTPS --> Backend_App
    Admin_Interface -- Admin API --> Caddy
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Element Descriptions:

  • Element:

    • Name: User
    • Type: Person
    • Description: Represents a user accessing websites or services hosted by Caddy.
    • Responsibilities: Accessing web content, interacting with web applications.
    • Security controls: Browser security features, HTTPS.

  • Element:

    • Name: Caddy Web Server
    • Type: Software System
    • Description: The Caddy web server itself.
    • Responsibilities: Serving web content, managing TLS certificates, routing requests, enforcing security policies.
    • Security controls: Automatic HTTPS, secure defaults, memory safety (Go), HTTP/2 & HTTP/3 support, security headers, access logs.

  • Element:

    • Name: ACME Provider (Let's Encrypt, ZeroSSL)
    • Type: External System
    • Description: An ACME (Automated Certificate Management Environment) provider used for obtaining TLS certificates.
    • Responsibilities: Issuing and revoking TLS certificates.
    • Security controls: ACME protocol security, certificate transparency.

  • Element:

    • Name: DNS Provider
    • Type: External System
    • Description: A DNS provider used for managing DNS records, often required for ACME DNS challenges.
    • Responsibilities: Managing DNS records.
    • Security controls: DNSSEC, API security.

  • Element:

    • Name: Backend Application(s)
    • Type: Software System
    • Description: Applications or services running behind Caddy, which Caddy may proxy requests to.
    • Responsibilities: Handling application-specific logic.
    • Security controls: Application-specific security measures, input validation, authentication, authorization.

  • Element:

    • Name: Admin Interface
    • Type: Person/Software System
    • Description: The interface used to administer Caddy, typically via a command-line interface or a configuration file (Caddyfile).
    • Responsibilities: Configuring Caddy, monitoring its status.
    • Security controls: Access controls, secure configuration practices.

C4 CONTAINER

graph LR
    User(("User"))
    Caddy(("Caddy Web Server"))
    ACME_Provider(("ACME Provider"))
    DNS_Provider(("DNS Provider"))
    Backend_App(("Backend Application(s)"))
    Admin_Interface(("Admin Interface"))

    subgraph Caddy
        Request_Handler(("Request Handler"))
        TLS_Manager(("TLS Manager"))
        Modules(("Modules (Plugins)"))
        Config_Adapter(("Config Adapter"))
        Admin_API_Endpoint(("Admin API Endpoint"))

        Request_Handler -- HTTP --> Backend_App
        TLS_Manager -- ACME --> ACME_Provider
        TLS_Manager -- DNS --> DNS_Provider
        Config_Adapter -- Caddyfile/JSON --> Admin_Interface
        Admin_API_Endpoint -- Admin API --> Admin_Interface
    end

    User -- HTTPS --> Request_Handler
    Request_Handler -- Internal API --> TLS_Manager
    Request_Handler -- Internal API --> Modules
Loading

Element Descriptions:

  • Element:

    • Name: Request Handler
    • Type: Container
    • Description: Handles incoming HTTP requests, routing them to the appropriate modules and backend applications.
    • Responsibilities: Request parsing, routing, header manipulation, connection management.
    • Security controls: Input validation, rate limiting, connection security.

  • Element:

    • Name: TLS Manager
    • Type: Container
    • Description: Manages TLS certificates, including obtaining, renewing, and configuring them.
    • Responsibilities: ACME client, certificate storage, TLS configuration.
    • Security controls: Secure key management, certificate validation, OCSP stapling.

  • Element:

    • Name: Modules (Plugins)
    • Type: Container
    • Description: Extends Caddy's functionality with additional features (e.g., reverse proxy, caching, authentication).
    • Responsibilities: Varies depending on the module.
    • Security controls: Module-specific security measures, input validation.

  • Element:

    • Name: Config Adapter
    • Type: Container
    • Description: Parses and applies configuration from various sources (e.g., Caddyfile, JSON).
    • Responsibilities: Configuration parsing, validation, and application.
    • Security controls: Secure configuration parsing, input validation.

  • Element:

    • Name: Admin API Endpoint
    • Type: Container
    • Description: Provides an API for administering Caddy.
    • Responsibilities: Exposing administrative functions.
    • Security controls: Authentication, authorization, input validation.

  • Element:

    • Name: User
    • Type: Person
    • Description: Represents a user accessing websites or services hosted by Caddy.
    • Responsibilities: Accessing web content, interacting with web applications.
    • Security controls: Browser security features, HTTPS.

  • Element:

    • Name: ACME Provider
    • Type: External System
    • Description: An ACME (Automated Certificate Management Environment) provider used for obtaining TLS certificates.
    • Responsibilities: Issuing and revoking TLS certificates.
    • Security controls: ACME protocol security, certificate transparency.

  • Element:

    • Name: DNS Provider
    • Type: External System
    • Description: A DNS provider used for managing DNS records, often required for ACME DNS challenges.
    • Responsibilities: Managing DNS records.
    • Security controls: DNSSEC, API security.

  • Element:

    • Name: Backend Application(s)
    • Type: Software System
    • Description: Applications or services running behind Caddy, which Caddy may proxy requests to.
    • Responsibilities: Handling application-specific logic.
    • Security controls: Application-specific security measures, input validation, authentication, authorization.

  • Element:

    • Name: Admin Interface
    • Type: Person/Software System
    • Description: The interface used to administer Caddy, typically via a command-line interface or a configuration file (Caddyfile).
    • Responsibilities: Configuring Caddy, monitoring its status.
    • Security controls: Access controls, secure configuration practices.

DEPLOYMENT

Possible Deployment Solutions:

  1. Bare Metal/Virtual Machine: Caddy is downloaded and run directly on a server.
  2. Docker Container: Caddy is run within a Docker container.
  3. Kubernetes: Caddy is deployed as a pod within a Kubernetes cluster, often using an Ingress controller.
  4. Cloud Provider-Specific Services: Using managed services like AWS Fargate, Google Cloud Run, or Azure Container Instances.

Chosen Solution (Detailed Description): Docker Container

Caddy is often deployed as a Docker container due to its ease of use, portability, and reproducibility. The official Caddy Docker image is available on Docker Hub.

graph LR
    Internet(("Internet"))
    Host_Machine(("Host Machine (Docker Engine)"))
    Caddy_Container(("Caddy Container"))
    Backend_Container(("Backend App Container"))

    Internet -- HTTPS --> Host_Machine
    Host_Machine -- Docker Network --> Caddy_Container
    Caddy_Container -- HTTP --> Backend_Container
    Caddy_Container -- Volume Mount --> Caddyfile_Volume(("Caddyfile Volume"))
    Caddy_Container -- Volume Mount --> Data_Volume(("Data Volume"))
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Element Descriptions:

  • Element:

    • Name: Internet
    • Type: External
    • Description: The public internet.
    • Responsibilities: Routing traffic to the host machine.
    • Security controls: Network firewalls, DDoS protection.

  • Element:

    • Name: Host Machine (Docker Engine)
    • Type: Infrastructure Node
    • Description: The physical or virtual machine running the Docker Engine.
    • Responsibilities: Running Docker containers.
    • Security controls: Host OS hardening, firewall, intrusion detection/prevention systems.

  • Element:

    • Name: Caddy Container
    • Type: Container Instance
    • Description: The running instance of the Caddy Docker image.
    • Responsibilities: Serving web content, managing TLS certificates, routing requests.
    • Security controls: Container isolation, resource limits, security context.

  • Element:

    • Name: Backend App Container
    • Type: Container Instance
    • Description: A container running a backend application served by Caddy.
    • Responsibilities: Handling application-specific logic.
    • Security controls: Container isolation, application-specific security measures.

  • Element:

    • Name: Caddyfile Volume
    • Type: Volume
    • Description: A Docker volume containing the Caddyfile (configuration).
    • Responsibilities: Persisting Caddy's configuration.
    • Security controls: File system permissions.

  • Element:

    • Name: Data Volume
    • Type: Volume
    • Description: A Docker volume for persisting Caddy's data (e.g., TLS certificates).
    • Responsibilities: Persisting Caddy's data.
    • Security controls: File system permissions.

BUILD

Caddy's build process is automated using GitHub Actions. The process involves building binaries for various platforms, running tests, and creating Docker images.

graph LR
    Developer(("Developer"))
    GitHub_Repo(("GitHub Repository"))
    GitHub_Actions(("GitHub Actions"))
    Go_Compiler(("Go Compiler"))
    Linters(("Linters"))
    Tests(("Tests"))
    SAST(("SAST (Static Analysis)"))
    Docker_Build(("Docker Build"))
    Docker_Hub(("Docker Hub"))
    Caddy_Binaries(("Caddy Binaries"))

    Developer -- Push Code --> GitHub_Repo
    GitHub_Repo -- Trigger --> GitHub_Actions
    GitHub_Actions -- Checkout Code --> Go_Compiler
    GitHub_Actions -- Run --> Linters
    GitHub_Actions -- Run --> Tests
    GitHub_Actions -- Run --> SAST
    Go_Compiler --> Caddy_Binaries
    GitHub_Actions -- Build --> Docker_Build
    Docker_Build --> Docker_Hub
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Security Controls in Build Process:

  • Automated Build: GitHub Actions provides a consistent and reproducible build environment.
  • Linters: Code linters (like golangci-lint) are used to enforce code style and identify potential issues.
  • Tests: Comprehensive tests are run to ensure code quality and prevent regressions.
  • Static Analysis (SAST): Go's built-in security checks and potentially additional SAST tools are used to identify potential security vulnerabilities in the code.
  • Software Composition Analysis (SCA): While not explicitly shown in the diagram, SCA should be part of the build process to identify vulnerabilities in dependencies. This is likely done using tools like go mod's vulnerability checking or dedicated SCA tools.
  • Signed Releases: Caddy releases are likely signed to ensure their integrity.
  • Docker Image Security: The Docker build process should follow best practices for creating secure Docker images (e.g., using minimal base images, avoiding running as root).

RISK ASSESSMENT

Critical Business Processes:

  • Serving web content securely and reliably.
  • Automatic HTTPS certificate management.
  • Providing a user-friendly and extensible web server platform.

Data to Protect:

  • TLS certificates and private keys (Highest sensitivity): Compromise would allow attackers to impersonate the server.
  • Website content (Variable sensitivity): Depending on the website, this could range from public information to highly sensitive data.
  • User data (Variable sensitivity): If Caddy is used to proxy requests to applications that handle user data, this data is also at risk.
  • Configuration files (High sensitivity): Contain sensitive information about the server setup, including potentially credentials for backend services.
  • Access logs (Medium sensitivity): Contain information about website visitors, which could be subject to privacy regulations.

QUESTIONS & ASSUMPTIONS

Questions:

  • What specific SAST and SCA tools are used in the Caddy build process?
  • Is there a formal vulnerability disclosure program (VDP) in place?
  • Are regular penetration tests conducted?
  • What are the specific procedures for handling security incidents?
  • What are the plans for addressing potential future vulnerabilities in the ACME protocol or its implementations?
  • Are there any specific compliance requirements (e.g., PCI DSS, GDPR) that Caddy deployments need to meet?

Assumptions:

  • BUSINESS POSTURE: The Caddy project prioritizes security and ease of use.
  • SECURITY POSTURE: The development team follows secure coding practices.
  • SECURITY POSTURE: Regular security updates are released to address vulnerabilities.
  • DESIGN: The Caddyfile is the primary configuration mechanism.
  • DESIGN: The official Docker image is the preferred deployment method for many users.
  • DESIGN: GitHub Actions is the primary CI/CD platform.