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

You are at risk if you answered yes to any of those questions.

Recommended Secure Coding Practices

Sensitive Code Examples

Flask:

from flask import Response

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

Compliant Solution

Flask:

from flask import Response

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

See