Tutorial: Writing policies for a simple photo-sharing service

This documentation is for an as-yet unreleased version of Cerbos. Choose 0.40.0 from the version picker at the top right or navigate to https://docs.cerbos.dev for the latest version.

Getting started

We will use Docker to run the server and the compiler.

docker pull ghcr.io/cerbos/cerbos:0.41.0-prerelease

Create a file named .cerbos.yaml with the following contents:

---
server:
  httpListenAddr: ":3592"

storage:
  driver: "disk"
  disk:
    directory: /photo-share/policies

Create a directory named policies to hold the policies.

You can find all the policies and tests used in this tutorial at https://github.com/cerbos/photo-share-tutorial.

The Apatr application

Apatr is a simple photo-sharing service that allows users to upload their photos and optionally share them with the rest of the world. Users sign-up to the service either by creating their own user account on the website or by signing-in with an identity provider (IDP) like Google or Facebook. Apatr uses a third-party identity management tool to manage these accounts and authenticating users to the site. Once they are logged-in, users can do the following:

Create albums to organize their photos
Upload photos to albums
Share albums or individual photos with other Apatr users
Share albums or individual photos with the internet

Apatr also employs a team of moderators to investigate complaints and remove any illegal or offensive items from the site. To respect users privacy, moderators are only allowed to view photos or albums that are public or those that have been flagged as inappropriate by another user.

Apatr’s identity provider allows defining roles for users. The roles currently defined in this system are:

moderator: Member of the moderator team
user: Authenticated users

Resources and actions

In the Apatr application, the most obvious resource hierarchy is the following:

Album
- Photo
  - Caption
  - Comment
- Description
User Profile

Album permissions matrix
Resource	Action	Allowed role	Condition
album:object	`create`	`user`
	`delete`	`user`	If user owns the album
	`delete`	`moderator`	If the album is flagged as inappropriate
	`share`	`user`	If user owns the album
	`unshare`	`user`	If user owns the album
	`view`	`user`	If user owns the album If the album is public
	`view`	`moderator`	If the album is flagged as inappropriate If the album is public
	`flag`	`user`	If the album is public

Derived roles

There are some recurring themes in the above permissions matrix.

People who have the user role can be either owners or viewers depending on the resource they are trying to access
Moderators get extra capabilities when the content is flagged as inappropriate

These capabilities are determined based on contextual information. Let’s codify them so that they can be reused.

---
apiVersion: "api.cerbos.dev/v1"
description: |-
  Common dynamic roles used within the Apatr app
derivedRoles:
  name: apatr_common_roles (1)
  definitions:
    - name: owner (2)
      parentRoles: ["user"] (3)
      condition:
        match:
          expr: request.resource.attr.owner == request.principal.id (4)

    - name: abuse_moderator
      parentRoles: ["moderator"]
      condition:
        match:
          expr: request.resource.attr.flagged == true

1	Name that we will use to import this set of roles
2	Descriptive name for this derived role
3	The static roles (from the identity provider) to which this derived role applies to
4	An expression that is applied to the request to determine when this role becomes activated

Save the above definition as apatr_common_roles.yaml in the policies directory.

Run the compiler to make sure that the contents of the file are valid.

docker run -it -v $(pwd):/photo-share ghcr.io/cerbos/cerbos:0.41.0-prerelease \
    compile /photo-share/policies

Resource policies

Let’s write a resource policy for the album:object resource.

---
apiVersion: api.cerbos.dev/v1
resourcePolicy:
  version: "default" (1)
  importDerivedRoles:
    - apatr_common_roles (2)
  resource: "album:object"
  rules:
    - actions: ['*']
      effect: EFFECT_ALLOW
      derivedRoles:
        - owner

    - actions: ['view', 'flag']
      effect: EFFECT_ALLOW
      roles:
        - user
      condition:
        match:
          expr: request.resource.attr.public == true

    - actions: ['view', 'delete']
      effect: EFFECT_ALLOW
      derivedRoles:
        - abuse_moderator

1	You can have multiple policy versions for the same resource (e.g. production vs. staging). If the request does not explicitly specify the version, the `default` policy takes effect.
2	Import the roles we defined earlier

Save the above policy definition as resource_album_object.yaml inside the policies directory.

Run the compiler to make sure that the contents of the policies directory are valid.

docker run -it -v $(pwd):/photo-share ghcr.io/cerbos/cerbos:0.41.0-prerelease compile /photo-share/policies

Let’s start the server and try out a request.

docker run -it -v $(pwd):/photo-share -p 3592:3592 ghcr.io/cerbos/cerbos:0.41.0-prerelease \
    server --config=/photo-share/.cerbos.yaml

If you like to use Postman, Insomnia or any other software that supports OpenAPI, the Cerbos OpenAPI definitions can be downloaded by accessing http://localhost:3592/schema/swagger.json.

Alicia trying to view her own album

cat <<EOF | curl --silent "http://localhost:3592/api/check/resources?pretty" -d @-
{
  "requestId": "test01",
  "includeMeta": true,
  "principal": {
    "id": "alicia",
    "policyVersion": "default",
    "roles": [
      "user"
    ]
  },
  "resources": [
    {
      "actions": [
        "view"
      ],
      "resource": {
        "id": "XX125",
        "policyVersion": "default",
        "kind": "album:object",
        "attr": {
          "owner": "alicia",
          "public": false,
          "flagged": false
        }
      }
    }
  ]
}
EOF

{
  "requestId": "test01",
  "results": [
    {
      "resource": {
        "id": "XX125",
        "kind": "album:object",
        "policyVersion": "default"
      },
      "actions": {
        "view": "EFFECT_ALLOW"
      },
      "meta": {
        "actions": {
          "view": {
            "matchedPolicy": "resource.album_object.vdefault"
          }
        },
        "effectiveDerivedRoles": [
          "owner"
        ]
      }
    }
  ]
}

Writing tests to verify behaviour

It’s not practical to start the server and manually make requests every time a policy is updated. So let’s write some tests instead.

Create a new directory named tests and create a file named album_object_test.yaml with the following contents.

---
name: AlbumObjectTestSuite
description: Tests for verifying the album:object resource policy
resources:
  alicia_private_album:
    id: "XX125"
    kind: "album:object"
    attr:
      owner: "alicia"
      public: false
      flagged: false

  alicia_public_album:
    id: "XX525"
    kind: "album:object"
    attr:
      owner: "alicia"
      public: true
      flagged: false

  alicia_flagged_album:
    id: "XX666"
    kind: "album:object"
    attr:
      owner: "alicia"
      public: true
      flagged: true

principals:
  alicia:
    id: "alicia"
    roles: ["user"]

  bradley:
    id: "bradley"
    roles: ["user"]

  maria:
    id: "maria"
    roles: ["moderator", "user"]

tests:
  - name: View album
    input: &testInput
      principals:
        - alicia
        - bradley
        - maria
      actions:
        - view
      resources:
        - alicia_private_album
        - alicia_public_album
        - alicia_flagged_album
    expected:
      - &viewExp
        principal: alicia
        resource: alicia_private_album
        actions:
          view: EFFECT_ALLOW

      - <<: *viewExp
        resource: alicia_public_album

      - <<: *viewExp
        resource: alicia_flagged_album

      - <<: *viewExp
        principal: bradley
        resource: alicia_public_album

      - <<: *viewExp
        principal: bradley
        resource: alicia_flagged_album

      - <<: *viewExp
        principal: maria
        resource: alicia_public_album

      - <<: *viewExp
        principal: maria
        resource: alicia_flagged_album

  - name: Delete album
    input:
      <<: *testInput
      actions:
        - delete
    expected:
      - &deleteExp
        principal: alicia
        resource: alicia_private_album
        actions:
          delete: EFFECT_ALLOW

      - <<: *deleteExp
        resource: alicia_public_album

      - <<: *deleteExp
        resource: alicia_flagged_album

      - <<: *deleteExp
        principal: maria
        resource: alicia_flagged_album

Now run the compiler, pointing it to the tests directory.

docker run -it -v $(pwd):/photo-share ghcr.io/cerbos/cerbos:0.41.0-prerelease \
    compile --tests=/photo-share/tests /photo-share/policies

Test results
└──AlbumObjectTestSuite (album_object_test.yaml) [18 OK]

18 tests executed [18 OK]

See https://github.com/cerbos/photo-share-tutorial for an example of using Cerbos GitHub Actions in a CI workflow to compile and test policies.

Using schemas to enforce type safety [Optional]

The derived roles and resource policy rules we defined above rely on certain attributes being present in the attr sections of the incoming request. To ensure that API requests are strictly-typed and contain required attributes, we can define schemas for the principal and resource attributes sections.

Create a new directory named _schemas inside the policies directory.

mkdir policies/_schemas

Let’s add a JSON schema defining the data types and required fields for album:object resources. Create a file named album_object.json inside the policies/_schemas directory with the following contents:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "type": "object",
  "properties": {
    "owner": {
      "type": "string"
    },
    "public": {
      "type": "boolean"
    },
    "flagged": {
      "type": "boolean"
    }
  },
  "required": [
    "owner"
  ]
}

Now update policies/resource_album_object.yaml to add the reference to the schema:

---
apiVersion: api.cerbos.dev/v1
resourcePolicy:
  version: "default"
  importDerivedRoles:
    - apatr_common_roles
  resource: "album:object"
  rules:
    - actions: ['*']
      effect: EFFECT_ALLOW
      derivedRoles:
        - owner

    - actions: ['view', 'flag']
      effect: EFFECT_ALLOW
      roles:
        - user
      condition:
        match:
          expr: request.resource.attr.public == true

    - actions: ['view', 'delete']
      effect: EFFECT_ALLOW
      derivedRoles:
        - abuse_moderator

  schemas:
    resourceSchema:
      ref: cerbos:///album_object.json (1)

1	Defines the schema to use for validating resource attributes

Update .cerbos.yaml to enable schema enforcement.

---
server:
  httpListenAddr: ":3592"

storage:
  driver: "disk"
  disk:
    directory: /photo-share/policies

schema:
  enforcement: reject

Now start the server again and send a request that does not conform to the schema. The server response should contain a list of validation errors.

docker run -it -v $(pwd):/photo-share -p 3592:3592 ghcr.io/cerbos/cerbos:0.41.0-prerelease \
    server --config=/photo-share/.cerbos.yaml

Invalid request

cat <<EOF | curl --silent "http://localhost:3592/api/check/resources?pretty" -d @-
{
  "requestId": "test02",
  "includeMeta": true,
  "principal": {
    "id": "alicia",
    "policyVersion": "default",
    "roles": [
      "user"
    ]
  },
  "resources": [
    {
      "actions": [
        "view"
      ],
      "resource": {
        "id": "XX125",
        "policyVersion": "default",
        "kind": "album:object",
        "attr": {
          "public": "false",
          "flagged": "false"
        }
      }
    }
  ]
}
EOF

{
  "requestId": "test02",
  "results": [
    {
      "resource": {
        "id": "XX125",
        "kind": "album:object",
        "policyVersion": "default"
      },
      "actions": {
        "view": "EFFECT_DENY"
      },
      "validationErrors": [
        {
          "message": "missing properties: 'owner'",
          "source": "SOURCE_RESOURCE"
        },
        {
          "path": "/public",
          "message": "expected boolean, but got string",
          "source": "SOURCE_RESOURCE"
        },
        {
          "path": "/flagged",
          "message": "expected boolean, but got string",
          "source": "SOURCE_RESOURCE"
        }
      ],
      "meta": {
        "actions": {
          "view": {
            "matchedPolicy": "resource.album_object.vdefault"
          }
        }
      }
    }
  ]
}