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Authorization in MQTT: Using ACLs to Control Access to MQTT Messaging

Ernest Russell
Aug 2, 2023
Authorization in MQTT: Using ACLs to Control Access to MQTT Messaging

What is Authorization?

Authorization is the process of granting or denying access to specific resources, actions, or information based on the identity and privileges of a user, device, or application. It is crucial to security and access control in computer systems, networks, web applications, and other environments where sensitive data and resources need protection.

The primary reasons we need authorization are:

  1. Data Security: Authorization helps protect sensitive data from unauthorized access. By enforcing access controls, only authorized users or entities are allowed to view, modify, or delete sensitive information.
  2. Preventing Unauthorized Actions: Authorization ensures that users can only perform actions they are explicitly permitted to do. This prevents unauthorized users from performing malicious actions or interfering with critical operations.
  3. Privacy Protection: In applications and systems that store personal or sensitive information, authorization ensures that only authorized personnel can access and manage that data, protecting users' privacy.
  4. Resource Protection: Authorization helps prevent unauthorized access to valuable resources such as files, databases, devices, or services. Unauthorized access to these resources could lead to data breaches, system compromises, or service disruptions.
  5. Compliance and Regulatory Requirements: Many industries and sectors have strict compliance and regulatory requirements regarding data protection and access controls. Proper authorization mechanisms help organizations meet these requirements and avoid legal consequences.
  6. Business Logic Enforcement: Authorization allows organizations to enforce business rules and logic by controlling access to specific features or functionalities within an application or system.
  7. Limiting Damage from Compromised Accounts: If an account is compromised, proper authorization can limit the damage by restricting the actions the attacker can perform.
  8. Multi-Tenancy Support: In multi-tenant environments, authorization helps ensure that each tenant can only access and manage their own data and resources without interfering with other tenants' data.
  9. Scalability and User Management: Authorization systems enable centralized control of user access across a large number of users and resources, simplifying user management and access control administration.

Overall, authorization is essential for maintaining data and resources' confidentiality, integrity, and availability within an organization's infrastructure. Ensuring that only trusted and authorized entities can access specific resources and perform actions necessary for their roles or privileges is critical.

The Difference Between Authentication and Authorization

Before we dive into Access Control Lists (ACLs), it is vital to understand the difference between authorization and authentication. Authentication is the process of verifying the identity of a user or system. This is typically done using a username and password but can also include biometric identification or other methods. Authorization, conversely, is the process of determining what actions a user or system is allowed to perform. Authorization is often based on the user's role or group membership.

Common Authorization Methods

Here are some standard authorization methods used to control access to resources in computer systems and web applications. These methods help ensure that only authorized users or entities can perform certain actions or access specific information:

  1. Role-Based Access Control (RBAC): RBAC is a widely used method where access permissions are assigned to specific roles, and users are assigned to these roles based on their job functions or responsibilities. Users gain access to resources based on their associated roles, making it easier to manage permissions at scale.
  2. Attribute-Based Access Control (ABAC): ABAC is a more fine-grained authorization method that considers various attributes or characteristics of the user, resource, and environment when making access control decisions. These attributes can include user roles, location, time of access, and other user-defined factors.
  3. Discretionary Access Control (DAC): DAC is a simple authorization model where each resource owner can decide who gets access to their resources and their access level. This approach allows users to control access to their files and directories with granularity.
  4. Mandatory Access Control (MAC): MAC is a more rigid authorization model often used in high-security environments. Access decisions are based on system-level policies defined by administrators, and users cannot change access permissions. This model is typically found in government and military settings.
  5. Rule-Based Access Control: This method defines access control rules explicitly using if-then statements. These rules determine which users or entities can access specific resources or actions.
  6. Attribute-Based Based Access Control (XACML): XACML is an authorization standard that uses ABAC principles to define access control policies based on attributes. It provides a standardized way to express complex access control decisions.
  7. OAuth: OAuth is an authorization framework widely used for granting third-party applications limited access to a user's resources on a server without sharing the user's credentials. It is commonly used for authorization in modern web and mobile applications.
  8. OpenID Connect: OpenID Connect is an authentication and authorization protocol built on top of OAuth 2.0. It allows applications to authenticate users and obtain basic profile information while delegating user authentication to an identity provider.
  9. JSON Web Tokens (JWT): JWT is a compact and self-contained way of representing authorization information between parties as JSON objects. It is commonly used in modern web applications for stateless authentication and authorization.
  10. Biometric Authorization: This method uses biometric characteristics (e.g., fingerprints, facial recognition) to grant access to specific resources or actions.

It's important to note that the authorization methods used can vary depending on the system or application's specific requirements and security needs.

What is an ACL?

Access control lists (ACLs) are a more specific implementation of the discretionary access control (DAC) model, where permissions are granted or denied based on a list of rules associated with each resource.

In an ACL, each resource (e.g., files, directories, network devices) has an associated list that specifies the users or groups and their access level to that particular resource. The access levels typically include permissions such as read, write, execute, and delete.

Using ACLs to Control Access to MQTT Messaging

In the context of MQTT (Message Queue Telemetry Transport), Access Control Lists (ACLs) are used to control access to various topics and actions within an MQTT broker. MQTT is a lightweight messaging protocol commonly used for Internet of Things (IoT) applications and other messaging scenarios. MQTT uses the publish/subscribe model, which is different from the traditional client/server model. It separates the client (publisher) that sends the message from the client (subscriber) that receives the message, and there is no need to build a direct connection between the publisher and the subscriber.

MQTT Publish-subscribe Architecture

Example of publisher/subscriber model with an EMQX MQTT broker

MQTT brokers use ACLs to enforce security and restrict access to specific topics based on defined rules. The ACLs define which clients are allowed to publish messages to particular topics and which clients are permitted to subscribe to specific topics. By configuring ACLs, MQTT brokers can ensure that only authorized devices or clients can publish and subscribe to certain topics, providing a controlled communication environment.

Typical Components of MQTT ACLs

  1. Topic Patterns: MQTT topic names are organized hierarchically, and ACLs can utilize wildcards to define patterns for matching multiple topics. The two primary wildcards used in MQTT are the plus sign (+) and the hash sign (#). The plus sign represents a single level in the hierarchy, while the hash sign represents any number of levels (including zero levels).
  2. Client Identifiers: MQTT clients connecting to the broker are identified by a unique client identifier. ACLs can use these identifiers to determine which clients can perform specific actions.
  3. Action Permissions: MQTT ACLs specify the allowed actions for each client on specific topics. The actions can include PUBLISH (sending messages) and SUBSCRIBE (receiving messages) permissions.

Example MQTT ACL Rules

  1. Allow a specific client with the identifier "sensor001" to publish messages on the topic "sensors/temperature":

    allow client sensor001 to publish to sensors/temperature
  2. Permit all clients to subscribe to any topic under the "sensors" hierarchy:

    allow all clients to subscribe to sensors/#
  3. Deny a specific client with the identifier "guest123" from subscribing to any topics:

    deny client guest123 to subscribe to #


Access control lists (ACLs) are critical to IoT systems. They provide a way to control resource access and ensure that only authenticated and authorized users can access restricted data. It's important to note that MQTT brokers may have different implementations of ACLs, and the exact syntax and capabilities of the ACL configuration can vary between different MQTT broker software.

Properly configuring MQTT ACLs is crucial to maintaining security and ensuring that MQTT communication remains secure and limited to authorized participants.

Continue exploring how EMQX can help secure your IoT Infrastructure:

For more about EMQX, please check our documentation, GitHub, Slack channel, and forum!

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