Merge pull request #19044 from edx/douglashall/csrf-protection-decision

Add decisions record related to frontend apps and CSRF protection.

openedx/core/djangoapps/oauth_dispatch/docs/decisions/0010-csrf-protection.rst

+10. Dealing with Django CSRF Protection in Frontend Apps
+--------------------------------------------------------
+
+Status
+------
+
+Accepted
+
+Context
+-------
+
+For background, please see:
+
+* `Transport JWT in HTTP Cookies`_, where we decided the mechanism that frontend apps will use to authenticate with
+  backend API services.
+
+* `Django CSRF Protection`_, which contains a description of Django's CSRF protection.
+
+Frontend apps need a way to obtain a CSRF token when making POST, PUT, and DELETE requests to backend API services
+implemented as Django applications with CSRF protection enabled. Each backend service is generally deployed with a CSRF
+token cookie scoped to the domain of the given service (ecommerce.edx.org) and named accordingly (ecommerce_csrftoken).
+Each backend service uses a unique SECRET_KEY to produce the CSRF token, so a CSRF token created by one service will not
+be valid for another service. Therefore, frontend apps served by different subdomains will not have access to these CSRF
+token cookies.
+
+.. _Transport JWT in HTTP Cookies: https://github.com/edx/edx-platform/blob/master/openedx/core/djangoapps/oauth_dispatch/docs/decisions/0009-jwt-in-session-cookie.rst
+.. _Django CSRF Protection: https://docs.djangoproject.com/en/2.1/ref/csrf/
+
+Decisions
+---------
+
+#. **CSRF Token API Endpoint** We will implement an API view that constructs a valid CSRF token and returns
+   it in a JSON response. This API view will be added to the `edx-drf-extensions`_ python library under a
+   new "csrf" Django app which will be added to INSTALLED_APPS in each backend service as we encounter
+   frontend apps that need to make POST, PUT, and DELETE API requests to a given backend service. The CSRF token API
+   endpoint will be protected from cross-domain XHR requests using our standard CORS protections (only requests
+   originating from pages loaded from one of the domains listed in the CORS_ORIGIN_WHITELIST setting will be
+   allowed).
+
+#. **Shared HTTP Client CSRF Token Management Code** We will add code to the `@edx/frontend-auth`_ npm package
+   that applies a request interceptor to the Axios HTTP client which will ensure that any POST, PUT, and DELETE
+   requests are made with the appropriate CSRF token header. This code will make use of the the CSRF token API
+   Endpoint described above to obtain a valid CSRF token for the given backend service for which a request is
+   being made.
+
+.. _edx-drf-extensions: https://github.com/edx/edx-drf-extensions
+.. _@edx/frontend-auth: https://github.com/edx/frontend-auth
+
+Rejected Alternatives
+---------------------
+
+Shared SECRET_KEY Across Backend Services
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+One alternative to the solution outlined above would be to share the configured SECRET_KEY setting value
+across all backend services and change the CSRF_COOKIE_DOMAIN value to a wildcard off of the second-level
+domain of the Open edX installation (e.g. .edx.org). This would allow frontend apps to read the CSRF token
+from the cookie that is set when a user logs via the authentication service (currently LMS). Since the
+SECRET_KEY value would be shared across all backend services, the frontend would be able to use the same
+CSRF token to make POST, PUT, and DELETE API requests to each of these services.
+
+We rejected this alternative due to the fact that it seems more secure to limit the exposure of the SECRET_KEY
+value to a single service.
+
+References
+----------
+
+* https://fractalideas.com/blog/making-react-and-django-play-well-together-single-page-app-model/