Access Control Guide

All Interaction Model operations (read attribute, write attribute, invoke command, read event) are governed by access control, and will be denied (status 0x7E Access Denied) if sufficient privilege for the operation is not obtained.

Overview

The initial Administer privilege is obtained by the commissioner implicitly over the PASE commissioning channel. This implicit Administer privilege is used to invoke commands on the server node during commissioning.

As part of commissioning, the commissioner invokes the AddNOC command, providing a CaseAdminNode argument. As well as commissioning the server node onto the fabric, that command automatically installs an ACL on the server node for Administer privilege using the provided CaseAdminNode argument as CASE subject. The subject may be a single node, or multiple nodes (by providing a CAT). These nodes become fabric administrators on that server node.

During and after commissioning, administrators manage ACLs for the fabric on the server node by reading and writing the fabric-scoped ACL attribute of the Access Control Cluster, which is always present on endpoint 0 on the server node. It is these ACLs that govern which Interaction Model operations are allowed or denied on that server node, for subjects on the fabric, via CASE and group messaging.

ACLs

ACLs are fabric-scoped data structures with the following fields:

Privilege
AuthMode
Subjects
Targets

Privilege

Privileges are:

View
Operate
Manage
Administer

An additional ProxyView privilege is not yet supported in the Matter SDK.

By default, the View privilege is required to read attributes or events, and the Operate privilege is required to write attributes or invoke commands.

However, clusters may require stricter privileges for certain operations on certain endpoints. For example, the Access Control Cluster requires the Administer privilege for all its operations.

If applicable, the ACL grants the privilege, and all less strict privileges subsumed by it. Therefore an ACL for Manage privilege will work for operations which require Operate or View privilege (but not Administer privilege).

AuthMode

Authentication modes are:

CASE
Group

The ACL applies only to subjects using that authentication mode.

Subjects

Subjects is a list containing zero, one, or more subject identifiers, which are:

Node ID for CASE auth mode
Group ID for Group auth mode

A CASE subject may be a CAT, which has its own tag and version mechanism.

The ACL applies only to the listed subjects; if no subjects are listed, the ACL applies to any subjects using the authentication mode.

Targets

Targets is a list containing zero, one, or more structured entries with fields:

Cluster
Endpoint
DeviceType

All fields are nullable, but at least one must be present, and the endpoint and device type fields are mutually exclusive (only one of those two may be present).

If cluster is present, the ACL is targeted to just that cluster.

If endpoint is present, the ACL is targeted to just that endpoint.

If device type is present, the ACL is targeted to just endpoints which contain that device type (as reported by the Descriptor Cluster).

Specifying device type in targets is not yet supported in the Matter SDK.

The ACL applies only to the listed targets; if no targets are listed, the ACL applies to any targets on the server node.

Limitations and Restrictions

The Matter Specification states that a Matter implementation must support at least:

3 ACLs per fabric
4 subjects per ACL
3 targets per ACL

There is no guarantee that an implementation supports any more than those minimums.

In the Matter SDK, using the example access control implementation, these values can be configured globally in CHIPConfig.h or per-app in CHIPProjectAppConfig.h by setting:

CHIP_CONFIG_EXAMPLE_ACCESS_CONTROL_MAX_ENTRIES_PER_FABRIC
CHIP_CONFIG_EXAMPLE_ACCESS_CONTROL_MAX_SUBJECTS_PER_ENTRY
CHIP_CONFIG_EXAMPLE_ACCESS_CONTROL_MAX_TARGETS_PER_ENTRY

Case Studies

Single Administrator

A single controller commissions a server node, providing its own CASE node ID for the automatically installed ACL:

Privilege: Administer
AuthMode: CASE
Subjects: 112233
Targets: (all)

The controller will be able to perform all Interaction Model operations on the server node, since it is the administrator.

Multiple Administrators

The commissioner provides a CAT for the automatically installed ACL:

Privilege: Administer
AuthMode: CASE
Subjects: 0xFFFFFFFD00010001
Targets: (all)

All controllers which have the CAT as part of their CASE credentials will be administrators for the server node.

Client Controllers

The commissioner installs an ACL for non-administrative controllers:

Privilege: View
AuthMode: CASE
Subjects: 4444, 5555, 6666
Targets: (all)

The non-administrative controllers are granted View privilege for the entire server node. This doesn't mean they can view the entire server node, as some clusters may require stricter privileges for reading attributes or events.

Group Messaging

The commissioner installs an ACL for group messaging:

Privilege: Operate
AuthMode: Group
Subjects: 123, 456
Targets: {cluster:onoff}, {endpoint:1}, {cluster:levelcontrol,endpoint:2}

Members of groups 123 and 456 are granted Operate privilege for the on/off cluster on any endpoint, any cluster on endpoint 1, and the level control cluster on endpoint 2.

Managing ACLs Using Chip-Tool

Usage

Entire List

The Access Control Cluster's ACL attribute is a list.

Currently, list operations for single entries (append, update, delete) are not yet supported in the Matter SDK, so the entire list must be written to the attribute to change any ACL.

The write operation may employ multiple messages, making it unreliable. In any case, ACLs are updated as they are processed, and take effect immediately.

The implication of this is that the administrator must ensure the first ACL in the list it is writing to the ACL attribute is one granting itself Administer privilege. Otherwise, the administrator may lose its administrative access during the write operation.

Null Fields

The tool requires all fields to be provided, even if null.

Incorrect: {"cluster": 6}

Correct: {"cluster": 6, "endpoint": null, "deviceType": null}

Fabric Index

The ACL attribute is fabric-scoped, so each ACL has a fabric index.

The tool requires this field to be provided, but ignores it when performing the actual write.

When reading ACLs, the proper fabric index is shown.

Enums Identifiers

The tool requires numerical values for enums and identifiers.

The privileges are:

View: 1
Operate: 3
Manage: 4
Administer: 5

The authentication modes are:

CASE: 2
Group: 3

Some typical clusters:

On/Off: 6
Level Control: 8
Descriptor: 29
Binding: 30
Access Control: 31
Basic: 40

Examples

Automatically Installed ACL

After commissioning with chip-tool, assuming CaseAdminNode is 112233, the automatically installed ACL is:

out/debug/standalone/chip-tool accesscontrol read acl 1 0

Endpoint: 0 Cluster: 0x0000_001F Attribute 0x0000_0000 DataVersion: 2578401031
  ACL: 1 entries
    [1]: {
      FabricIndex: 1
      Privilege: 5
      AuthMode: 2
      Subjects: 1 entries
        [1]: 112233
      Targets: null
    }

Installing a CASE ACL

out/debug/standalone/chip-tool accesscontrol write acl '[{"fabricIndex": 0, "privilege": 5, "authMode": 2, "subjects": [112233], "targets": null}, {"fabricIndex": 0, "privilege": 1, "authMode": 2, "subjects": [4444, 5555, 6666], "targets": null}]' 1 0

Endpoint: 0 Cluster: 0x0000_001F Attribute 0x0000_0000 DataVersion: 2578401034
  ACL: 2 entries
    [1]: {
      FabricIndex: 1
      Privilege: 5
      AuthMode: 2
      Subjects: 1 entries
        [1]: 112233
      Targets: null
     }
    [2]: {
      FabricIndex: 1
      Privilege: 1
      AuthMode: 2
      Subjects: 3 entries
        [1]: 4444
        [2]: 5555
        [3]: 6666
      Targets: null
     }

Installing a Group ACL

out/debug/standalone/chip-tool accesscontrol write acl '[{"fabricIndex": 0, "privilege": 5, "authMode": 2, "subjects": [112233], "targets": null}, {"fabricIndex": 0, "privilege": 1, "authMode": 2, "subjects": [4444, 5555, 6666], "targets": null}, {"fabricIndex": 0, "privilege": 3, "authMode": 3, "subjects": [123, 456], "targets": [{"cluster": 6, "endpoint": null, "deviceType": null}, {"cluster": null, "endpoint": 1, "deviceType": null}, {"cluster": 8, "endpoint": 2, "deviceType": null}]}]' 1 0

Endpoint: 0 Cluster: 0x0000_001F Attribute 0x0000_0000DataVersion: 2578401041
  ACL: 3 entries
    [1]: {
      FabricIndex: 1
      Privilege: 5
      AuthMode: 2
      Subjects: 1 entries
        [1]: 112233
      Targets: null
     }
    [2]: {
      FabricIndex: 1
      Privilege: 1
      AuthMode: 2
      Subjects: 3 entries
        [1]: 4444
        [2]: 5555
        [3]: 6666
      Targets: null
     }
    [3]: {
      FabricIndex: 1
      Privilege: 3
      AuthMode: 3
      Subjects: 2 entries
        [1]: 123
        [2]: 456
      Targets: 3 entries
        [1]: {
          Cluster: 6
          Endpoint: null
          DeviceType: null
         }
        [2]: {
          Cluster: null
          Endpoint: 1
          DeviceType: null
         }
        [3]: {
          Cluster: 8
          Endpoint: 2
          DeviceType: null
         }
     }

Managing ACLs Using Chip-Repl

Usage

Entire List

See the important notes in the chip-tool section, as they also apply to chip-repl.

Null Fields

Null fields may be omitted.

OK: Target(cluster=6, endpoint=Null, deviceType=Null)

Also OK: Target(cluster=6)

The above assumes Target and Null are defined at global scope, which is not normally the case.