An HTTP method is safe if it doesn't alter the state of the server i.e it leads to a read-only operation. Common safe HTTP methods: GET, HEAD, and OPTIONS. Whereas, POST, PUT, and DELETE are unsafe because they alter the server state.

The use of both safe and unsafe HTTP methods on a view makes the application vulnerable to Cross-Site Request Forgery (CSRF). CSRF protections are responsible for protecting operations performed by unsafe HTTP methods. They do not protect if safe HTTP methods used for a route that can change the state of an application.

It is recommended to use safe HTTP methods only when read-only operations need to be performed. Don't use safe and unsafe methods together.

Bad practice

For Django:

from django.views.decorators.http import require_http_methods

@require_http_methods(["GET", "POST"]) # Sensitive
def register(request):
    ...

For Flask:

import flask

from flask import Flask

app = Flask(__name__)

@app.route('/sensitive', methods=['GET', 'POST'])  # Sensitive
def register():
    ...

Recommended

For Django

from django.views.decorators.http import require_POST, require_GET

@require_POST
def register(request):
    ...

@require_GET
def post(request):
    ...

For Flask

import flask

from flask import Flask

app = Flask(__name__)

@app.route('/sensitive', methods=['POST'])  # Sensitive
def register():
    ...

@app.route('/sensitive', methods=['GET'])  # Sensitive
def hello_world():
    return "Hello World"

References:

• OWASP Top 10 2021 Category A04 - Insecure Design
• SANS 25
• CWE-352
• Cross Site Request Forgery