Flask check: Secure set cookie

This check follows Internet cookie best practices to help you keep your Flask application’s cookies safe by ensuring they are only sent over HTTPS, unreadable by scripts, and by not blindly sending cookies to anyone who asks. This check alerts when the secure, httponly, and samesite keyword arguments are not supplied to response.set_cookie.


Modern web browsers support some great features for keeping web cookies secure. Lots of things can go wrong with cookies: they can be sniffed in transit, scanned by a rogue script, or slurped up by mischevious domains.

Three of these settings recommended by Flask are:

  • secure - only allows cookies to be transmitted over HTTPS
  • httponly - prevents scripts from reading cookies
  • samesite - blocks cookies from being sent with cross-site requests

Under the hood, web servers send HTTP headers to the browser instructing it how to handle cookies. Flask supports setting these headers conveniently through the keyword arguments secure, httponly, and samesite on response.set_cookie.

This check encourages using these security features by alerting on calls to response.set_cookie without all three keyword arguments. However, the check does not enforce the settings to be "on." In doing so, the check forces a developer to be explicit about their security settings while allotting for situations where the settings need to be disabled.

This check will alert on these cases, for example:

from flask import make_response
def none_set():
    response = make_response()
    response.set_cookie("cookie_name", "cookie_value")
    return response

def some_set():
    r = make_response()
    # some values are set but not others
    r.set_cookie("cookie1", "cookie_value", secure=True)
    r.set_cookie("cookie2", "cookie_value", httponly=True)
    r.set_cookie("cookie3", "cookie_value", samesite='Lax')
    r.set_cookie("cookie4", "cookie_value", secure=True, httponly=True)
    r.set_cookie("cookie5", "cookie_value", httponly=True, samesite='Lax')
    return r

This check will not alert on these cases:

import flask
def index():
    response = flask.make_response()
    response.set_cookie("cookie1", "cookie_value", secure=True, httponly=True, samesite='Lax')
    response.set_cookie("cookie2", "cookie_value", secure=True, httponly=True, samesite='Strict')
    response.set_cookie("cookie3", "cookie_value", secure=False, httponly=False, samesite=None)
    return response