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OpenAPI To Rego Code Generator

OpenAPI To Rego Code Generator allows for generation of Rego code given an OpenAPI 3.0 Specification.

Building

Build openapi-to-rego by running make build

Running

The examples directory contains samples of OpenAPI 3.0 specification in YAML and JSON.

$ ./openapi-to-rego examples/petstore.yaml

This will generate the Rego code for the OAS defined in examples/petstore.yaml. The generated Rego code will be written to a file policy.rego.

To specify a different file to output the Rego code, use the --output-filename flag.

Default package name for the Rego policy is httpapi.authz. To change this use the --package-name flag.

Run ./openapi-to-rego --help for more details.

Working

Generating Boolean Rules

openapi-to-rego leverages the Extensions Object in the OpenAPI 3.0 specification to generate Rego rules that return boolean values. The x-security-rego-boolean-filter extension serves this purpose.

Tha value for the x-security-rego-boolean-filter is a list of rules with conditions which are then included in the body of the resulting Rego rule.

In the example below, the value returned by the generated Rego rule, depends on the conditions specified in the rules field.

Each item in the rules object, specifies a new rule in the Rego policy while each operation specified in operations determines the expressions to be included in the rule body.

The following operations are supported:

paths:
  /pets/{petId}:
    get:
      summary: Info for a specific pet
      operationId: showPetById
      tags:
        - pets
      x-security-rego-boolean-filter:
      - rules:
        - operations:
          - eq:
            - $petId
            - token.payload.pets[_].petId
          - eq:
            - input.owner
            - token.payload.pets.owners[_]
        - operations:
          - eq:
            - $petId
            - token.payload.pets[_].petIdSmall
          - eq:
            - input.owner
            - token.payload.pets.owners_small[_]

The generated Rego for the above OAS would look like below:

package example
default allow = false

token = {"payload": payload} { io.jwt.decode(input.token, [_, payload, _]) }


allow = true {
  input.path = ["pets", petId]
  input.method = "GET"
  petId = token.payload.pets[_].petId
  input.owner = token.payload.pets.owners[_]
}

allow = true {
  input.path = ["pets", petId]
  input.method = "GET"
  petId = token.payload.pets[_].petIdSmall
  input.owner = token.payload.pets.owners_small[_]
}

In the allow rules, corresponding to the /pets/{petId} path object, the petId is a variable in the expression input.path = ["pets", petId] whose value will be bound to a value in the input that is provided to the policy. Variables in the path object, can be used in the rule conditions by prefixing them with $.

To see this example, run:

$ ./openapi-to-rego examples/petstore-rego-boolean-filter.yaml -p example

Generating Field Filter Rules

An extension object named x-security-rego-field-filter, can be used to generate Rego rules that return collections of values. The fields that need to be filtered in the client response can be specified using this extension.

The value for the x-security-rego-field-filter field is a list of objects, keyed on the Security Scheme Name as declared in the Security Requirement Object. The value for each security scheme is a list of fields that need to be filtered.

In the example below, x-security-rego-field-filter specifies two security schemes petstore_auth and api_key which are declared in the security section. openapi-to-rego will use the list of scope names for these security schemes to generate the Rego policy.

Each security scheme specified in x-security-rego-field-filter MUST be declared in the security section for an operation.

paths:
  /pets:
    post:
      summary: Create a pet
      operationId: createPets
      tags:
        - pets
      security:
      - petstore_auth:
        - write:pets
        - read:pets
      - api_key:
        - type:apiKey
        - name:api_key
        - in:header
      x-security-rego-field-filter:
      - petstore_auth:
        - name
        - ssn
      - api_key:
        - birthdate
        - ssn

The generated Rego for the above OAS would look like below:

package example
default allow = false

token = {"payload": payload} { io.jwt.decode(input.token, [_, payload, _]) }

filter = ["name","ssn"] {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.scopes["write:pets"]
  token.payload.scopes["read:pets"]
}

allow = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.scopes["write:pets"]
  token.payload.scopes["read:pets"]
}

filter = ["birthdate","ssn"] {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.scopes["type:apiKey"]
  token.payload.scopes["name:api_key"]
  token.payload.scopes["in:header"]
}

allow = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.scopes["type:apiKey"]
  token.payload.scopes["name:api_key"]
  token.payload.scopes["in:header"]
}

The filter rules specify the fields to filter in the client response. Notice that the filter rules contain the list of scopes corresponding to the security requirement defined for the post operation.

Also two allow rules for the post operation on /pets are created.

The policy expects a token to be provided in the input to verify the security requirements.

To see this example, run:

$ ./openapi-to-rego examples/petstore-rego-field-filter.yaml -p example

Generating List Filter Rules

In some scenarios it may be required to filter certain objects in the response that is returned to the client. The decision about whether or not to include an object in the response may depend on certain conditions that may be specified in the OAS. These conditions could be based on the values in the object itself or in the token provided to OPA etc.

The openapi-to-rego uses the x-security-rego-list-filter extension object to specify list of objects to be filtered and the conditions that determine whether or not an object should be included in the response.

In the example below, the x-security-rego-list-filter field is used to determine list of objects to be filtered and their filtering conditions.

The source field specifies the list of objects which need to be filtered. In the OAS below, source: list will be translated to input.list[x] in the Rego policy. This means OPA expects the objects to be filtered be provided as input using the list key.

The operations field specifies the list of operations to be performed on EACH object in input.list.

The following operations are supported:

paths:
  /pets:
    post:
      summary: Create a pet
      operationId: createPets
      tags:
        - pets
      security:
      - petstore_auth:
        - write:pets
        - read:pets
      - api_key:
        - type:apiKey
        - name:api_key
        - in:header
      x-security-rego-list-filter:
      - source: list
        operations:
        - eq:
          - owner
          - token.payload.username
      - source: list
        operations:
        - membership:
          - owner
          - token.payload.members
        - lt:
          - age
          - 18
      - source: list
        operations:
        - membership:
          - owner
          - token.payload.members
        - gte:
          - age
          - 18
        - eq:
          - hasHouseKey
          - true

The generated Rego for the above OAS would look like below:

package example
default allow = false

token = {"payload": payload} { io.jwt.decode(input.token, [_, payload, _]) }

list_filter[x] {
  input.path = ["pets"]
  input.method = "POST"
  x := input.list[_]
  x.owner = token.payload.username
}

list_filter[x] {
  input.path = ["pets"]
  input.method = "POST"
  x := input.list[_]
  x.owner = token.payload.members[_]
  x.age < 18
}

list_filter[x] {
  input.path = ["pets"]
  input.method = "POST"
  x := input.list[_]
  x.owner = token.payload.members[_]
  x.age >= 18
  x.hasHouseKey = true
}

allow = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.scopes["write:pets"]
  token.payload.scopes["read:pets"]
}

allow = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.scopes["type:apiKey"]
  token.payload.scopes["name:api_key"]
  token.payload.scopes["in:header"]
}

For each item in x-security-rego-list-filter, a corresponding list_filter rule is created with conditions extracted from the operations field specified in an x-security-rego-list-filter item. These conditions are then applied on each item in input.list which is list of objects to filter and is provided to OPA as input.

openapi-to-rego assumes an operand not prefixed with token is a field in the object being evaluated.

Also two allow rules for the post operation on /pets are created.

To see this example, run:

$ ./openapi-to-rego examples/petstore-rego-list-filter.yaml -p example

Generating Overwrite Rules

openapi-to-rego defines the x-security-rego-overwrite-filter filter extension for scenarios where the value of a field in an object needs to be overwritten based on conditions that may be specified in the OAS. These conditions could be based on the values in the input object itself or in the token provided to OPA etc.

The field key in the x-security-rego-overwrite-filter filter extension specifies the name of a field in the input object whose value needs to be overwritten to the value specified in the value key of the extension based on conditions specified in the rules field.

The generated Rego policy returns an object with a key equal to the value of the field key in the extension and value equal to the value key of the extension or the existing value of the field key in the input object.

Each item in the rules object, specifies a new helper rule in the Rego policy while each operation specified in operations determines the expressions to be included in the rule body.

The following operations are supported:

The negate key controls how the results from the helper rules, apply towards asssignment of the final value for the field key in the result returned by OPA.

paths:
  /pets:
    post:
      summary: Create a pet
      operationId: createPets
      tags:
        - pets
      security:
      - petstore_auth:
        - write:pets
        - read:pets
      x-security-rego-overwrite-filter:
      - field: enrolleeClaimSummaryList
        value: null
        negated: true
        rules:
        - operations:
          - eq:
            - token.payload.enrollee_type
            - '"primary"'
          - lt:
            - enrolleeAge
            - 18
        - operations:
          - eq:
            - token.payload.enrollee_idd
            - enrolleeId
          - gte:
            - age
            - 18
          - eq:
            - enrolleeSignedWaiver
            - true
      - field: enrolleeList
        value: hello
        negated: false
        rules:
        - operations:
          - eq:
            - token.payload.enrollee_type
            - '"secondary"'
          - lt:
            - enrolleeAge
            - 18
        - operations:
          - membership:
            - owner
            - token.payload.dependents
          - gte:
            - age
            - 18
          - eq:
            - enrolleeSignedWaiver
            - true

The generated Rego for the above OAS would look like below:

package example
default allow = false

token = {"payload": payload} { io.jwt.decode(input.token, [_, payload, _]) }

response["enrolleeClaimSummaryList"] = null {
	not allow1
}

response["enrolleeClaimSummaryList"] = input.object.enrolleeClaimSummaryList {
	allow1
}

allow1 = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.enrollee_type = "primary"
  input.object.enrolleeAge < 18
}

allow1 = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.enrollee_idd = input.object.enrolleeId
  input.object.age >= 18
  input.object.enrolleeSignedWaiver = true
}

response["enrolleeList"] = hello {
	allow2
}

response["enrolleeList"] = input.object.enrolleeList {
	not allow2
}

allow2 = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.enrollee_type = "secondary"
  input.object.enrolleeAge < 18
}

allow2 = true {
  input.path = ["pets"]
  input.method = "POST"
  input.object.owner = token.payload.dependents[_]
  input.object.age >= 18
  input.object.enrolleeSignedWaiver = true
}

allow = true {
  input.path = ["pets"]
  input.method = "POST"
  token.payload.claims["write:pets"]
  token.payload.claims["read:pets"]
}

The response rule returns the final value for the field key specified in the extension. Notice how the value for enrolleeClaimSummaryList is set to null (ie. value key from the extension) when allow1 is NOT true while for enrolleeList it is set to hello (ie. value key from the extension) when allow2 is true. This behaviour is controlled by the negate field in the extension.

To see this example, run:

$ ./openapi-to-rego examples/petstore-rego-overwrite-filter.yaml -p example