Authorization in the App Layer: Using Casbin Flask Middleware
If your deployment environment doesn't support a service mesh like Envoy or an external authorization server (common in simpler, monolithic, or traditional hosting setups), you can certainly move the Casbin authorization check into your Flask application using middleware or decorators.
This approach centralizes authorization logic within the Python code, relying on dedicated extensions like flask-authz or Flask-Casbin.
1 The Middleware Approach: Trade-offs
Moving the authorization logic from the proxy layer (Envoy) into the application layer (Flask) has key trade-offs:
| Aspect | External Auth (Envoy) | Internal Middleware (Flask) |
|---|---|---|
| Performance | High. Request is blocked early before hitting the application server. | Lower. Request is processed by Flask, consuming Python resources before being blocked. |
| Decoupling | Complete. Security logic is a separate service. | Partial. Security logic is tightly coupled within the application's framework. |
| Implementation | Requires managing a separate authorization microservice. | Simple. Requires only a pip install and minimal configuration within app.py. |
| Policy Source | Uses a dedicated Adapter (e.g., Firestore, SQL). | Uses the same Adapter, but contention can be higher if many Flask instances reload the policy simultaneously. |
2 Implementation with flask-authz
The official flask-authz package, maintained by the PyCasbin team, provides a straightforward way to enforce policies either globally (as middleware) or per route (as a decorator) [2].
A. Setup and Initialization
First, install the library and initialize the Casbin Enforcer with your model (.conf file) and policy storage (.csv file or a database Adapter).
# Installation
# pip install flask-authz
from flask import Flask, jsonify
from flask_authz import CasbinEnforcer
from casbin.persist.adapters import FileAdapter # Replace with FirestoreAdapter for production
app = Flask(__name__)
# 1. Configuration: Tells the extension where to find the Casbin model
app.config['CASBIN_MODEL'] = 'casbin_model.conf'
# 2. Setup the Casbin Adapter (e.g., reading from a CSV file)
adapter = FileAdapter('rbac_policy.csv')
casbin_enforcer = CasbinEnforcer(app, adapter)
B. Route-Level Enforcement (Recommended)
The most precise way is to use the @casbin_enforcer.enforcer decorator on specific route functions. This allows you to protect only the necessary endpoints.
The decorator automatically extracts the subject (user), object (route path), and action (HTTP method) from the request and performs the enforcer.enforce(sub, obj, act) check. If the check fails, it automatically returns a 403 Forbidden response.
# Example Policy Rule in rbac_policy.csv:
# p, admin, /data/v1/*, *
# Example Route Implementation
@app.route('/api/v1/data/report', methods=['GET'])
@casbin_enforcer.enforcer
def get_report():
# This code only runs if Casbin authorizes the request
return jsonify({"data": "Confidential Report"})
# Example of a public route without the decorator
@app.route('/login', methods=['POST'])
def login():
return jsonify({"status": "authenticated"})
*Annotation: The decorator intercepts the request, runs the Casbin check against the loaded policy, and only executes the route handler if access is granted. *
C. Policy Management
Unlike the external service where policy updates are handled by a watcher, using the Flask middleware allows direct access to the Enforcer object to make changes during runtime.
@app.route('/manager', methods=['POST'])
@casbin_enforcer.enforcer
@casbin_enforcer.manager
def make_casbin_change(manager):
# 'manager' is the casbin.Enforcer object
# Example: Grant a user a new role
manager.add_grouping_policy("new_user_id", "editor")
return jsonify({'message': 'Role updated successfully'})
This uses the @casbin_enforcer.manager decorator to pass the live Enforcer instance (manager) into the function, ensuring administrative changes to the policy are performed against the same, current instance used for authorization checks.