Django API Gateway Authentication

This is a library which contains:

• an authentication provider for Django, allowing the headers passed from the API Gateway to be interpreted and used within a Django app,
• a set of permissions classes allowing the authentication information provided by the API Gateway to be used in authorization decisions throughout an API-based app,

Library developer quick start

This library is packages using poetry and uses our common Python CI pipeline. Make sue that poetry is installed and bootstrap your local environment via:

$ poetry install
$ poetry run pre-commit install

Required settings

The following Django settings are required to allow this library to be used:

• PERMISSIONS_SPECIFICATION_URL, if you plan to enforce authorization using a permissions spec this should contain the url (either in the form of a file://, gs:// or https:// url) which contains the permissions spec yaml document,
• UCAMLOOKUP_USERNAME and UCAMLOOKUP_PASSWORD, these are required if your permissions spec contains references to Lookup groups, which this library will need to query Lookup to determine membership of these groups.

Optional settings

The following Django settings can be used to customise how the library authenticates incoming requests as having been proxied by the API Gateway.

• API_GATEWAY_ENFORCE_ID_TOKEN_VERIFICATION - if set to False, then a warning is printed to the logs if verification failed but the request is allowed to process. If set to True then id token verification is enforced. For reasons of backwards compatibility, this setting defaults to False but it is strongly recommended that it be enabled.
• API_GATEWAY_JWT_TRUSTED_ISSUERS - a list of trusted JWT issuers. The default set correspond to Google's OAuth2 id token issuers.
• API_GATEWAY_JWT_ISSUER_CERTS_URL - a URL which is used to fetch a JSON document of the form {"[keyid]": "[PEM encoded cert]"} used to verify id token signatures. The default is to use Google's public certificate endpoint.
• API_GATEWAY_JWT_EXPECTED_AUDIENCE - the expected audience of the id token. The default is to use the base URL of the Django web application as determined by request.build_absolute_uri().
• API_GATEWAY_JWT_EXPECTED_AUTHORISED_PARTIES - a list of expected authorised parties of the id token. The default is to use a well known set of API Gateway service account identities.

Use in local development

The defaults for the various API_GATEWAY_... settings should be suitable for deployment. When using applications locally, they may need setting. Try to always set API_GATEWAY_ENFORCE_ID_TOKEN_VERIFICATION to True if possible. For example, if one is using the API Gateway emulator, one can set API_GATEWAY_JWT_TRUSTED_ISSUERS to match the issuer configured in the emulator and point API_GATEWAY_JWT_ISSUER_CERTS_URL at the /certs endpoint exposed by the emulator.

Expected headers passed from the API Gateway

This library expects the API Gateway to provide the following headers and uses them to determine the identity of the principal querying an API:

• x-api-org-name: the API Gateway organisation name,
  • This library ensures that this is provided by does not make any decisions based on its value,
• x-api-developer-app-class: should be the static string confidential or public,
  • This library ensures that this is provided by does not make any decisions based on its value,
• x-api_developer-app-id: the identifier for the developer app created within the API Gateway,
  • This library ensures that this is provided by does not make any decisions based on its value,
• x-api-oauth2-client-id: the identifier of the client credentials issued within the API Gateway,
  • This library ensures that this is provided by does not make any decisions based on its value,
• x-api-oauth2-scope: a space separated list of oauth2 scopes
  • This is used to determine access to resources within this library
• x-api-oauth2-user: the identifier of the user who is making use of the API - this can either be a crsid identifier identifying the user who is interacting with the API or an application identifier related to the application which is accessing the API.

Trusting the headers provided by the API Gateway

In order for this library to interpret the headers provided by the API Gateway, add the following to your restframework settings:

REST_FRAMEWORK = {
    "DEFAULT_AUTHENTICATION_CLASSES": [
        # rely on the API Gateway to provide authentication details
        "apigatewayauth.authentication.APIGatewayAuthentication",
    ],
    "DEFAULT_PERMISSION_CLASSES": [
        # make sure that the user is authenticated
        "rest_framework.permissions.IsAuthenticated",
        # ... other permissions classes ...
    ],
}

YOU MUST INCLUDE THE IsAuthenticated PERMISSION IF YOU WANT TO DENY UNAUTHENTICATED ACCESS TO THE API BACKEND.

This will enforce that any requests to your API have provided the expected headers which the API Gateway provides to give details about the identity of the principal.

NOTE this library expects that all requests are proxied via the API Gateway, but does not authenticate the API Gateway directly. You must ensure that only the API Gateway can contact your backend, without this protection the headers which this library uses for authentication can be trivially spoofed.

For local development the above headers should be provided in order to allow the information provided by the API gateway to be mocked.

Authorization helpers

This library provides a set of permissions classes which allow authentication decisions to be made based on the information provided from the API Gateway

Enforcing scopes

The HasAnyScope permissions class can be used to enforce that a client has a token granted by the API Gateway with any of the given scopes, e.g.:

from rest_framework import viewsets
from apigatewayauth.permissions import HasAnyScope

class ExampleViewSet(viewsets.ReadOnlyModelViewSet):

  permission_classes = [
    # ensure that a client has either the examples.readonly or examples scopes:
    HasAnyScope(
      'https://api.apps.cam.ac.uk/example/examples.readonly',
      'https://api.apps.cam.ac.uk/example/examples'
    )
  ]

Enforcing specified permissions

This library includes modules to allow a 'permissions specification' to be read to determine if a principal has a given permission.

A permissions spec is a yaml document which should contain permission names as keys and the principals or groups which are granted this permission, e.g.:

READ_EXAMPLES:
  principals:
    - 1234@application.api.apps.cam.ac.uk
    - wgd23@v1.person.identifiers.cam.ac.uk
  groups:
    - 105217@groups.lookup.cam.ac.uk

WRITE_EXAMPLES:
  principals:
    - abcd@application.api.apps.cam.ac.uk

This library provides helpers to allow an app to determine whether a client has been granted a given permission in a permission spec, e.g.:

from rest_framework import viewsets
from apigatewayauth.permissions import SpecifiedPermission

IsExampleReader = SpecifiedPermission("READ_EXAMPLES")
IsExampleWriter = SpecifiedPermission("WRITE_EXAMPLES")

class ExampleViewSet(viewsets.ModelViewSet):

  def get_permissions(self):
    if self.action in ('retrieve', 'list'):
      self.permission_classes = [IsExampleReader | IsExampleWriter]
    elif self.action in ('create', 'update', 'destroy')
      self.permission_classes = [IsExampleWriter]
    return super().get_permissions()

Limiting actions to 'resource owners'

This library contains a helper permission which allows actions to be limited to resource owners. This requires integration at the model and queryset level, to allow querysets to be filtered based on the identity of the principal using the API. This defers decisions about who 'owns' a given model to the model class. For example:

from django.db import models
from rest_framework import viewsets
from identitylib.identifiers import Identifier

from apigatewayauth.permissions import IsResourceOwningPrincipal



class Example(models.Model):

  name = models.TextField('Name')
  # this field contains the identifier of a principal - e.g. 'wgd23@v1.person.identifiers.cam.ac.uk'
  owner = models.TextField('Owner')

  @staticmethod
  def get_queryset_for_principal(principal_identifier: Identifier):
    """
    This method should return a queryset which limits results to those owned by the principal
    identified by the given identifier. This is used to limit results returned from list endpoints.

    """

    return Example.objects.filter(owner__iexact=str(principal_identifier))

  def is_owned_by(self, principal_identifier: Identifier):
    """
    This method returns whether an instance of this model is owned by the principal identified
    by the given identifier. This is used in read, update or delete operations to determine if
    the given principal can act on this object.

    """

    return self.owner.lower() == str(principal_identifier).lower()


class ExampleViewSet(viewsets.ModelViewSet):
  """
  A viewset that exposes our example model.

  """

  # use our resource owning principal permission class
  permission_classes = [IsResourceOwningPrincipal]


  def get_queryset(self):
    """
    Ensure that the queryset we use is limited based on the conditions we've defined on our model.

    """

    return IsResourceOwningPrincipal.get_queryset_for_principal(Example, self.request)