When a cookie is protected with the secure attribute set to true it will not be send by the browser over an unencrypted HTTP request and thus cannot be observed by an unauthorized person during a man-in-the-middle attack.

Why is this an issue?

When a cookie is created without the secure attribute set to true, browsers will transmit it over unencrypted HTTP connections as well as HTTPS. An attacker who can observe or intercept network traffic—for example on a public Wi-Fi network—can read the cookie value in cleartext.

What is the potential impact?

Session hijacking

If a session cookie is transmitted over an unencrypted HTTP connection, an attacker who can intercept the traffic can steal it. With a valid session cookie, the attacker can impersonate the victim and gain full access to their account without knowing their password. Even on sites that primarily use HTTPS, a single HTTP request containing the session cookie is enough to expose it.

How to fix it in Flask

Pass secure=True to set_cookie to prevent the cookie from being transmitted over unencrypted HTTP connections.

Code examples

Noncompliant code example

from flask import Response

@app.route('/')
def index():
    response = Response()
    response.set_cookie('key', 'value')  # Noncompliant
    return response

Compliant solution

from flask import Response

@app.route('/')
def index():
    response = Response()
    response.set_cookie('key', 'value', secure=True)
    return response

How to fix it in FastAPI

Pass secure=True to set_cookie to prevent the cookie from being transmitted over unencrypted HTTP connections.

Code examples

Noncompliant code example

from fastapi import FastAPI, Response

app = FastAPI()

@app.get('/')
async def index(response: Response):
    response.set_cookie('key', 'value')  # Noncompliant
    return {"message": "Hello world!"}

Compliant solution

from fastapi import FastAPI, Response

app = FastAPI()

@app.get('/')
async def index(response: Response):
    response.set_cookie('key', 'value', secure=True)
    return {"message": "Hello world!"}

Resources

Standards