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Project Design Document: Mosquitto MQTT Broker for Threat Modeling

Project Name: Mosquitto MQTT Broker

Project Repository: https://github.com/eclipse-mosquitto/mosquitto

Document Version: 1.1 Date: 2023-10-27 Author: AI Software Architecture Expert

1. Introduction

This document provides a detailed design overview of the Mosquitto MQTT Broker project, specifically tailored for threat modeling activities. It outlines the system architecture, key components, data flow, and security considerations. This document serves as a foundation for identifying potential threats and vulnerabilities within the Mosquitto ecosystem.

Mosquitto is a lightweight, open-source message broker that implements the MQTT (Message Queuing Telemetry Transport) protocol versions 5.0, 3.1.1, and 3.1. It enables machine-to-machine communication and is widely used in IoT (Internet of Things), mobile messaging, and other applications requiring efficient and reliable message delivery.

This document focuses on the core broker functionality and its interactions with clients and backend systems. It aims to provide a clear and comprehensive understanding of the system's architecture to facilitate effective threat modeling and security analysis.

2. System Overview

Mosquitto acts as a central hub for MQTT communication. It receives messages published by MQTT clients and routes them to subscribed clients based on topic-based filtering. The broker manages client connections, sessions, subscriptions, and message persistence (optional).

Key functionalities of Mosquitto include:

  • MQTT Protocol Implementation: Supports MQTT v5.0, v3.1.1, and v3.1, ensuring compatibility with a wide range of MQTT clients and devices.
  • Message Routing: Efficiently routes messages based on topic subscriptions using a hierarchical topic structure and wildcard subscriptions.
  • Client Management: Handles concurrent client connections, manages client session states (clean session vs. persistent session), and gracefully handles client disconnections.
  • Subscription Management: Manages client subscriptions, including shared subscriptions for load balancing message delivery among multiple subscribers.
  • Quality of Service (QoS): Supports QoS levels 0, 1, and 2, providing flexibility in message delivery guarantees, from "at most once" (QoS 0) to "exactly once" (QoS 2).
  • Retained Messages: Stores and delivers the last message published on a topic to new subscribers, useful for providing current status information.
  • Last Will and Testament (LWT): Allows clients to define a message to be published by the broker if the client disconnects unexpectedly, enabling monitoring of client availability.
  • Persistence (Optional): Can persist messages (for QoS 1 and 2) and broker state to disk or database for durability and recovery after restarts. Supports various persistence backends.
  • Authentication and Authorization: Supports multiple authentication methods (username/password, TLS client certificates, external authentication via plugins) and fine-grained authorization using Access Control Lists (ACLs) or plugins.
  • TLS/SSL Encryption: Provides secure communication channels using TLS/SSL for encrypting data in transit between clients and the broker. Supports mutual TLS authentication.
  • Bridging: Allows connecting to other MQTT brokers or message queues, enabling hierarchical or distributed MQTT deployments and integration with other messaging systems.
  • Plugin Support: Extensible architecture through plugins for authentication, authorization, persistence, bridging, logging, and custom functionalities, allowing for tailored deployments.
  • WebSockets Support: Supports MQTT over WebSockets, enabling browser-based MQTT clients to connect to the broker.

3. System Architecture

The following diagram illustrates the high-level architecture of the Mosquitto MQTT Broker and its interactions with external entities.

graph LR
    subgraph "MQTT Clients"
        A["MQTT Publisher Client"]
        B["MQTT Subscriber Client"]
        C["MQTT Client (Pub/Sub)"]
    end
    D["Mosquitto Broker Core"]
    E["Authentication/Authorization Plugin (Optional)"]
    F["Persistence Storage (Optional)"]
    G["Bridge to External Broker (Optional)"]
    H["External Monitoring System (Optional)"]
    I["Configuration Files"]

    A -->|MQTT Publish Message| D
    B -->|MQTT Subscribe Request| D
    C -->|MQTT Connect, Publish, Subscribe, etc.| D

    D -->|Authentication Request| E
    D -->|Authorization Request| E
    D -->|Message Persistence| F
    D -->|Message Forwarding (Bridge)| G
    D -->|Metrics & Logs| H
    D -->|Read Configuration| I

    style D fill:#f9f,stroke:#333,stroke-width:2px
Loading

Components Description:

  • "MQTT Clients": Represent external applications, devices, or services that interact with the Mosquitto broker using the MQTT protocol. Examples include sensors, actuators, mobile apps, backend services, and web applications.

    • "MQTT Publisher Client": Clients that send (publish) messages to the broker on specific topics. These clients are the source of data or commands.
    • "MQTT Subscriber Client": Clients that receive (subscribe) messages from the broker for specific topics. These clients consume data or react to events.
    • "MQTT Client (Pub/Sub)": Clients that can both publish and subscribe to messages, acting as both data producers and consumers.

  • "Mosquitto Broker Core": This is the central component and the heart of the MQTT broker. It is responsible for:

    • MQTT Protocol Handling: Full implementation of MQTT protocol specifications (v5.0, v3.1.1, v3.1), including parsing and processing all MQTT control packets.
    • Connection Management: Managing client connections over TCP, WebSockets, and potentially other transports. Handling connection limits, keep-alive mechanisms, and connection termination.
    • Session Management: Managing MQTT sessions, including both clean sessions (no session state persisted) and persistent sessions (session state retained across client reconnects).
    • Subscription Management: Storing and efficiently managing client subscriptions, including topic filter matching and wildcard handling.
    • Message Routing: Routing published messages to all relevant subscribers based on topic subscriptions, applying QoS levels and handling message duplication and delivery guarantees.
    • Retained Message Handling: Storing and retrieving retained messages for topics.
    • Last Will and Testament (LWT) Processing: Monitoring client connections and publishing LWT messages when clients disconnect unexpectedly.
    • Security Enforcement: Enforcing authentication and authorization policies, often delegated to plugins.
    • Plugin Interface: Providing a well-defined API for plugins to extend broker functionality.
    • Metrics and Logging: Generating operational metrics (e.g., connection counts, message rates, error counts) and logging events for monitoring, debugging, and auditing.
    • Configuration Loading: Reading and applying broker configuration from configuration files.

  • "Authentication/Authorization Plugin (Optional)": An optional but highly recommended component for securing the broker. It handles:

    • Client Authentication: Verifying the identity of connecting clients using various methods like username/password, client certificates, or integration with external authentication providers (LDAP, OAuth, etc.).
    • Client Authorization: Controlling client access to topics based on configured Access Control Lists (ACLs) or more complex authorization logic. Plugins can implement fine-grained authorization rules based on client identity, topic, and MQTT actions (publish, subscribe).

  • "Persistence Storage (Optional)": An optional component for ensuring message durability and broker state persistence. It is used for:

    • Message Persistence: Storing messages with QoS 1 and 2 to guarantee delivery even if the broker restarts or fails.
    • Retained Message Persistence: Persisting retained messages across broker restarts.
    • Session Persistence (for Persistent Sessions): Storing session state for persistent clients, allowing them to resume sessions after reconnects.
    • Persistence Backends: Mosquitto supports various persistence backends, including file-based storage and database systems (e.g., SQLite, MySQL, PostgreSQL).

  • "Bridge to External Broker (Optional)": An optional component that enables federation and integration with other MQTT brokers or message queues. It allows:

    • Message Forwarding: Forwarding messages between Mosquitto and other brokers based on topic patterns.
    • Hierarchical Broker Networks: Creating hierarchical MQTT deployments by bridging brokers together.
    • Integration with Enterprise Messaging Systems: Bridging to other messaging systems like Kafka or RabbitMQ.

  • "External Monitoring System (Optional)": External systems used for monitoring the operational status, performance, and security of the Mosquitto broker. Examples include:

    • Metrics Collectors: Systems that collect metrics exposed by Mosquitto (e.g., Prometheus, Grafana).
    • Logging Systems: Centralized logging systems that aggregate and analyze logs from Mosquitto (e.g., ELK stack, Splunk).
    • Alerting Systems: Systems that trigger alerts based on predefined thresholds or events in metrics and logs.

  • "Configuration Files": Files that store the broker's configuration settings, including:

    • Listeners: Network interfaces and ports for accepting client connections.
    • Security Settings: Authentication and authorization configurations, TLS/SSL settings.
    • Persistence Settings: Configuration for persistence storage.
    • Plugin Configurations: Settings for loaded plugins.
    • Logging Configuration: Configuration for broker logging.

4. Data Flow

The following sequence diagram illustrates the typical data flow for publishing and subscribing to messages in Mosquitto.

sequenceDiagram
    participant "MQTT Publisher Client" as Publisher
    participant "Mosquitto Broker Core" as Broker
    participant "Authentication/Authorization Plugin" as AuthPlugin
    participant "MQTT Subscriber Client" as Subscriber

    Publisher->>Broker: CONNECT (Client ID, Credentials, etc.)
    activate Broker
    Broker->>AuthPlugin: Authentication Request (Credentials)
    activate AuthPlugin
    AuthPlugin-->>Broker: Authentication Response (Success/Failure)
    deactivate AuthPlugin
    alt Authentication Success
        Broker-->>Publisher: CONNACK (Connection Accepted)
    else Authentication Failure
        Broker-->>Publisher: CONNACK (Connection Refused)
        deactivate Broker
        return
    end

    Subscriber->>Broker: CONNECT (Client ID, Credentials, etc.)
    Broker->>AuthPlugin: Authentication Request (Credentials)
    AuthPlugin-->>Broker: Authentication Response (Success/Failure)
    alt Authentication Success
        Broker-->>Subscriber: CONNACK (Connection Accepted)
    else Authentication Failure
        Broker-->>Subscriber: CONNACK (Connection Refused)
        deactivate Broker
        return
    end

    Subscriber->>Broker: SUBSCRIBE (Topic Filter)
    Broker->>AuthPlugin: Authorization Request (Subscribe, Topic Filter, Client ID)
    AuthPlugin-->>Broker: Authorization Response (Permit/Deny)
    alt Authorization Permitted
        Broker-->>Subscriber: SUBACK (Subscription Acknowledged)
    else Authorization Denied
        Broker-->>Subscriber: SUBACK (Subscription Denied)
        return
    end


    Publisher->>Broker: PUBLISH (Topic, Payload, QoS)
    Broker->>AuthPlugin: Authorization Request (Publish, Topic, Client ID)
    AuthPlugin-->>Broker: Authorization Response (Permit/Deny)
    alt Authorization Permitted
        Broker->>Broker: Route Message based on Topic
        Broker->>Subscriber: PUBLISH (Topic, Payload, QoS)
    else Authorization Denied
        Broker-->>Publisher: PUBACK/PUBREC (Authorization Failure - based on QoS)
        return
    end
    deactivate Broker

    Subscriber->>Broker: DISCONNECT
    Broker-->>Subscriber: DISCONNECT ACK (Implicit)

    Publisher->>Broker: DISCONNECT
    Broker-->>Publisher: DISCONNECT ACK (Implicit)
Loading

Data Flow Description:

  1. Connection Establishment and Authentication:

    • MQTT Publisher Client and MQTT Subscriber Client initiate a CONNECT message to the Mosquitto Broker Core.
    • The Mosquitto Broker Core forwards authentication requests to the "Authentication/Authorization Plugin" (if configured).
    • The "Authentication/Authorization Plugin" validates the provided credentials.
    • The plugin returns an authentication response (success or failure) to the Mosquitto Broker Core.
    • If authentication is successful, the broker responds with a CONNACK (Connection Accepted) message; otherwise, it responds with CONNACK (Connection Refused).

  2. Subscription and Authorization:

    • MQTT Subscriber Client sends a SUBSCRIBE message to the Mosquitto Broker Core.
    • The Mosquitto Broker Core forwards authorization requests to the "Authentication/Authorization Plugin" to check if the client is authorized to subscribe to the requested topic filter.
    • The plugin returns an authorization response (permit or deny).
    • If authorization is permitted, the broker registers the subscription and responds with a SUBACK (Subscription Acknowledged) message; otherwise, it responds with SUBACK (Subscription Denied).

  3. Publishing and Authorization:

    • MQTT Publisher Client sends a PUBLISH message to the Mosquitto Broker Core.
    • The Mosquitto Broker Core forwards authorization requests to the "Authentication/Authorization Plugin" to check if the client is authorized to publish to the specified topic.
    • The plugin returns an authorization response (permit or deny).
    • If authorization is permitted, the broker proceeds with message routing.
    • Message Routing: The broker determines which clients are subscribed to the published topic and routes the message accordingly.
    • Message Delivery: The broker forwards the PUBLISH message to all authorized and subscribed MQTT Subscriber Clients, respecting the QoS level.
    • If authorization is denied, the broker sends a PUBACK or PUBREC (depending on QoS) indicating authorization failure to the publisher.
    • Persistence (Optional): If persistence is enabled and required for the QoS level or retained message flag, the broker interacts with the "Persistence Storage".

  4. Disconnection:

    • MQTT Clients can send a DISCONNECT message to gracefully disconnect from the Mosquitto Broker Core.
    • The broker acknowledges the disconnection implicitly.

5. Deployment Scenarios

Mosquitto deployment scenarios remain as described in the previous version (Single Broker, Clustered Broker, Cloud-Based, Embedded, Edge Computing). These scenarios influence the threat landscape and security requirements.

6. Security Considerations for Threat Modeling

This section details security considerations, categorized for threat modeling using a STRIDE-like approach, focusing on potential threats and vulnerabilities.

6.1. Spoofing (Identity)

  • Threat: Unauthorized clients impersonate legitimate clients to gain access or publish malicious messages.
    • Example: An attacker uses stolen credentials or a compromised client ID to connect to the broker as a trusted device.
    • Mitigation:
      • Strong Authentication: Implement robust authentication mechanisms like TLS client certificates or strong password policies.
      • Client ID Validation: Enforce client ID uniqueness and validation to prevent hijacking.
      • Mutual TLS (mTLS): Use mTLS for client authentication, ensuring both client and server verify each other's identities.

6.2. Tampering (Data Integrity)

  • Threat: Messages or data in transit or at rest are modified without authorization.
    • Example: An attacker intercepts MQTT traffic and modifies published messages, leading to incorrect data or commands being delivered to subscribers.
    • Mitigation:
      • TLS/SSL Encryption: Encrypt all communication channels using TLS/SSL to protect data in transit from eavesdropping and tampering.
      • Message Signing (Application Layer): Implement message signing at the application layer for critical data to ensure integrity beyond transport layer security.
      • Secure Persistence: If persistence is used, ensure the persistence storage is securely configured and protected from unauthorized access and modification.

6.3. Repudiation (Non-Accountability)

  • Threat: Actions performed by clients cannot be reliably traced back to them, hindering accountability and auditing.
    • Example: A malicious client publishes harmful messages, but without proper logging and auditing, it's difficult to identify the source and take corrective action.
    • Mitigation:
      • Detailed Logging: Enable comprehensive logging of client connections, disconnections, publish/subscribe actions, and authorization events.
      • Audit Trails: Implement audit trails to track client activities and message flows for forensic analysis and accountability.
      • Client Identification: Ensure each client is uniquely identifiable and logged for all actions.

6.4. Information Disclosure (Confidentiality)

  • Threat: Sensitive information is exposed to unauthorized parties.
    • Example: MQTT traffic is not encrypted, allowing attackers to eavesdrop and read sensitive data being transmitted (e.g., sensor readings, control commands, credentials). Configuration files containing sensitive information are exposed.
    • Mitigation:
      • TLS/SSL Encryption: Mandatory TLS/SSL for all client connections to protect data confidentiality in transit.
      • Secure Configuration Management: Securely store and manage configuration files, protecting them from unauthorized access. Avoid storing sensitive information directly in configuration files; use secrets management solutions if necessary.
      • Access Control to Logs and Metrics: Restrict access to broker logs and metrics to authorized personnel only, as they may contain sensitive operational information.

6.5. Denial of Service (Availability)

  • Threat: Attackers disrupt the broker's availability, preventing legitimate clients from communicating.
    • Example: A DDoS attack floods the broker with connection requests or publish messages, overwhelming its resources and causing it to become unresponsive. Exploiting vulnerabilities in the broker software or plugins to crash the service.
    • Mitigation:
      • Rate Limiting and Connection Limits: Implement rate limiting for connection requests and publish/subscribe actions. Configure connection limits to prevent resource exhaustion from excessive connections.
      • Resource Management: Properly configure broker resource limits (CPU, memory, file descriptors) to prevent resource exhaustion.
      • Input Validation: Implement robust input validation to prevent exploitation of vulnerabilities through malformed MQTT messages.
      • Regular Security Updates and Patching: Keep the Mosquitto broker and its dependencies up-to-date with the latest security patches to address known vulnerabilities.
      • WebSockets DDoS Protection: If WebSockets are enabled, implement specific DDoS protection mechanisms for WebSocket connections.

6.6. Elevation of Privilege (Authorization Bypass)

  • Threat: Attackers gain unauthorized access to resources or functionalities beyond their intended permissions.
    • Example: Exploiting vulnerabilities in authentication or authorization plugins to bypass access controls and gain administrative privileges or access to restricted topics. Misconfiguration of ACLs leading to unintended access.
    • Mitigation:
      • Robust Authorization Mechanisms: Implement and properly configure strong authorization mechanisms (ACLs or plugins) to enforce least privilege access control. Regularly review and audit ACL configurations.
      • Secure Plugin Development and Review: If using plugins, ensure they are developed securely and undergo security reviews. Use plugins from trusted sources.
      • Principle of Least Privilege: Configure authorization policies based on the principle of least privilege, granting clients only the necessary permissions.
      • Regular Security Audits: Conduct regular security audits and penetration testing to identify and address potential authorization bypass vulnerabilities.

6.7. Plugin Vulnerabilities

  • Threat: Vulnerabilities in plugins can compromise the security of the entire broker.
    • Example: A vulnerable authentication plugin allows attackers to bypass authentication or gain unauthorized access. A malicious plugin could be installed to exfiltrate data or disrupt broker operations.
    • Mitigation:
      • Plugin Security Audits: Thoroughly audit and review the security of any plugins before deployment.
      • Trusted Plugin Sources: Use plugins from trusted and reputable sources.
      • Plugin Updates: Keep plugins updated to the latest versions to patch known vulnerabilities.
      • Plugin Sandboxing (if available): Explore plugin sandboxing or isolation mechanisms to limit the impact of plugin vulnerabilities.
      • Minimize Plugin Usage: Only use necessary plugins and avoid installing unnecessary plugins to reduce the attack surface.

6.8. Configuration Vulnerabilities

  • Threat: Insecure configurations can introduce vulnerabilities and weaken the broker's security posture.
    • Example: Using default credentials, disabling authentication, misconfiguring TLS/SSL, or exposing management interfaces to the public network.
    • Mitigation:
      • Secure Configuration Practices: Follow security best practices for configuring Mosquitto, including changing default credentials, enabling strong authentication and authorization, enforcing TLS/SSL, and properly configuring listeners and network interfaces.
      • Configuration Validation: Implement configuration validation and automated checks to detect misconfigurations.
      • Regular Configuration Reviews: Regularly review and audit broker configurations to ensure they remain secure and aligned with security policies.
      • Principle of Least Functionality: Disable unnecessary features and functionalities to reduce the attack surface.

7. Conclusion

This revised design document provides a more detailed and structured overview of the Mosquitto MQTT Broker, specifically enhanced for threat modeling. The expanded security considerations section, categorized using a STRIDE-like approach, offers a more comprehensive starting point for identifying and analyzing potential threats. By considering these security aspects in conjunction with the system architecture and data flow, security professionals can conduct more effective threat modeling exercises and develop appropriate mitigation strategies to secure Mosquitto deployments. This document should be used as a living document, updated as the system evolves and new threats emerge.