When a cookie is configured with the HttpOnly attribute set to true, the browser guaranties that no client-side script will be able to read it. In most cases, when a cookie is created, the default value of HttpOnly is false and it’s up to the developer to decide whether or not the content of the cookie can be read by the client-side script. As a majority of Cross-Site Scripting (XSS) attacks target the theft of session-cookies, the HttpOnly attribute can help to reduce their impact as it won’t be possible to exploit the XSS vulnerability to steal session-cookies.

Ask Yourself Whether

There is a risk if you answered yes to any of those questions.

Recommended Secure Coding Practices

Sensitive Code Example

Using Flask:

from flask import Response

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

Using FastAPI:

from fastapi import FastAPI, Response

app = FastAPI()

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

Compliant Solution

Using Flask:

from flask import Response

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

Using FastAPI:

from fastapi import FastAPI, Response

app = FastAPI()

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

See