samesite-compat-check

This is a Python port of the Chromium project's browser compatibility check for SameSite=None-cookies.

samesite-compat-check exposes a single function should_send_same_site_none which takes a User-Agent string and returns True if SameSite=None is supported by the browser with the given User-Agent and False if not.

If User-Agent is None then should_send_same_site_none returns True.
This is the only addition on top of the pseudocode from the Chromium project

The reason for this added behavior is to handle the case where the User-Agent-header is not sent. The logic is that only browsers that specifically mishandle SameSite=None should have the attribute omitted. You can safely send SameSite=None-attribute to a browser that doesn't recognize the attribute (e.g. an older browser). As in that case SameSite will be ignored, and you'll automatically get the behavior you intended, as cookies before the SameSite-attibute were introduced worked as similar to SameSite=None.

SameSite cookies

You probably already know what SameSite-cookies are -- if not, I recommend you read https://web.dev/samesite-cookies-explained/ at the very least.

SameSite=None-cookies are mostly relevant to people who needs cookies on CORS requests or need cookies inside an iframe. So if you don't use cookies when doing CORS or in iframes (or use SameSite=None for some other reason) you can safely ignore this package.

You may say -- who uses iframes nowadays? Well, plenty of things do! Youtube Embeds, the Intercom widget, that new little "Log in with Google" popover on sites like Medium.

Well, if you're one those people who need SameSite=None, you'll find that a good portion of your users have browsers that are incompatible with SameSite cookies that have the value None.

That's where this library comes in. It checks the User-Agent string, and returns whether it is from a browser that is incompatible.

Recommendation is to just not use SameSite for those browsers. Since the browsers are "old" and don't enforce the SameSite-rules for cookies where the flag is not present, they will have the right behavior when you don't send SameSite=None.

You'll need to pass in the value of the User-Agent-header from the library you're using (Django, Flask...) and then either set or not set SameSite=None on the cookie.

Usage

General use:

from samesite_compat_check import should_send_same_site_none

if should_send_same_site_none(user_agent):
    # Set cookie with `SameSite=None`
else:
    # Set cookie without any `SameSite` attribute

Django

if should_send_same_site_none(request.META['HTTP_USER_AGENT']):
    response.cookies['my_precious_cookie']['samesite'] = 'None'
else:
    pass

Setting response.set_cookie(..., samesite='none') will be allowed from Django 3.1 and onwards. Currently only lax and strict are allowed in response.set_cookie(). Django only allows lowercase values for samesite. Accoring to the spec lowercase/uppercase should not matter, however I have not tested that particular detail in a browser.

Flask

from flask import request

kwargs = {}
if should_send_same_site_none(request.headers.get('User-Agent')):
    kwargs['samesite'] = 'None'

# Requires Werkzeug>=1.0.0 (a Flask dependency) in order to use `samesite` 
response.set_cookie(
    'my_precious_cookie', value='123abc', secure=True, **kwargs
)

Requirements

samesite-compat-check has no dependencies and is tested on Python 2.7, 3.4, 3.5, 3.6, 3.7, 3.8, and PyPy 3.

Installation

pip install samesite-compat-check

Useful references

• https://github.com/jotes/django-cookies-samesite
  This package serves as a good example of implementing the SameSite cookie in Django.
  However, only checks for Chrome/Chromium version 51-66, not iOS, macOS, and UC Browser. However
• https://github.com/GoogleChromeLabs/samesite-examples
  This repo has examples for setting a cookie with the SameSite-attribute in Python and in Flask, but does not implement any of the browser incompatibility checks.
• https://github.com/linsight/should-send-same-site-none
  A Javascript package similar to this one.
• https://itnext.io/user-agent-sniffing-only-way-to-deal-with-upcoming-samesite-cookie-changes-6f79a18e541
  An alternative implementation in Javascript that isn't based off the Chromium project pseudocode.

Authors

samesite-compat-check was written by Malthe Jørgensen <malthe.jorgensen@gmail.com> at Peergrade Inc., and is a port of Chromium's pseudocode for checking browsers incompatible with SameSite=None cookies which can be found here: https://www.chromium.org/updates/same-site/incompatible-clients

That pseudocode is Copyright 2019 Google LLC. and released under the Apache 2.0 license.

The tests use a sample of User-Agent strings from https://developers.whatismybrowser.com/useragents/.