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ACL Permissions

ACL (Access Control List) permissions are the rules that decide which MQTT topics a client can access and what it can do on them. There is no separate ACL Permissions page in the broker. ACL permissions live inside Groups: each group holds a list of permission rows, and a client inherits them through the groups it is assigned to by Group Rules.

What an ACL permission is​

Each permission is a single rule with three parts:

  • Permission: Allow or Deny the matching access.
  • Action: Subscribe or Publish.
  • Topic: the topic the rule applies to, with MQTT wildcards supported.

How ACL permissions work​

When a device tries to publish to a topic or subscribe to one, the broker checks the permissions in the device's groups. It matches the topic against the rules, checks the action (subscribe or publish), and grants or denies access based on the matching rule.

Topic patterns and wildcards​

ACL permissions support MQTT topic wildcards:

  • Single-level wildcard (+) matches any single topic level. For example, sensors/+/temperature matches sensors/device1/temperature and sensors/device2/temperature.
  • Multi-level wildcard (#) matches multiple topic levels. For example, sensors/# matches all topics under the sensors hierarchy.

Use cases​

Use ACL permissions to:

  • Keep devices to the topics that belong to their function.
  • Block access to sensitive data topics.
  • Draw boundaries between device types or applications.
  • Meet security standards that require access control.