bpo-29613: Added support for SameSite cookies (GH-6413)

* bpo-29613: Added support for SameSite cookies

Implemented as per draft
https://tools.ietf.org/html/draft-west-first-party-cookies-07

* Documented SameSite

And suggestions by members.

* Missing space :(

* Updated News and contributors

* Added version changed details.

* Fix in documentation

* fix in documentation

* Clubbed test cases for same attribute into single.

* Updates

* Style nits + expand tests

* review feedback

5 files changed, 25 insertions, 0 deletions

diff --git a/Doc/library/http.cookies.rst b/Doc/library/http.cookies.rst
index fb8317a..f3457a0 100644
--- a/Doc/library/http.cookies.rst
+++ b/Doc/library/http.cookies.rst
@@ -137,11 +137,16 @@ Morsel Objects
    * ``secure``
    * ``version``
    * ``httponly``
+   * ``samesite``
 
    The attribute :attr:`httponly` specifies that the cookie is only transferred
    in HTTP requests, and is not accessible through JavaScript. This is intended
    to mitigate some forms of cross-site scripting.
 
+   The attribute :attr:`samesite` specifies that the browser is not allowed to
+   send the cookie along with cross-site requests. This helps to mitigate CSRF
+   attacks. Valid values for this attribute are "Strict" and "Lax".
+
    The keys are case-insensitive and their default value is ``''``.
 
    .. versionchanged:: 3.5
@@ -153,6 +158,9 @@ Morsel Objects
       :attr:`~Morsel.coded_value` are read-only.  Use :meth:`~Morsel.set` for
       setting them.
 
+   .. versionchanged:: 3.8
+      Added support for the :attr:`samesite` attribute.
+
 
 .. attribute:: Morsel.value
 
diff --git a/Lib/http/cookies.py b/Lib/http/cookies.py
index 7e0259e..4a44db8 100644
--- a/Lib/http/cookies.py
+++ b/Lib/http/cookies.py
@@ -281,6 +281,7 @@ class Morsel(dict):
         "secure"   : "Secure",
         "httponly" : "HttpOnly",
         "version"  : "Version",
+        "samesite" : "SameSite",
     }
 
     _flags = {'secure', 'httponly'}
diff --git a/Lib/test/test_http_cookies.py b/Lib/test/test_http_cookies.py
index 2ff6902..447f883 100644
--- a/Lib/test/test_http_cookies.py
+++ b/Lib/test/test_http_cookies.py
@@ -121,6 +121,19 @@ class CookieTests(unittest.TestCase):
         self.assertEqual(C.output(),
             'Set-Cookie: Customer="WILE_E_COYOTE"; HttpOnly; Secure')
 
+    def test_samesite_attrs(self):
+        samesite_values = ['Strict', 'Lax', 'strict', 'lax']
+        for val in samesite_values:
+            with self.subTest(val=val):
+                C = cookies.SimpleCookie('Customer="WILE_E_COYOTE"')
+                C['Customer']['samesite'] = val
+                self.assertEqual(C.output(),
+                    'Set-Cookie: Customer="WILE_E_COYOTE"; SameSite=%s' % val)
+
+                C = cookies.SimpleCookie()
+                C.load('Customer="WILL_E_COYOTE"; SameSite=%s' % val)
+                self.assertEqual(C['Customer']['samesite'], val)
+
     def test_secure_httponly_false_if_not_present(self):
         C = cookies.SimpleCookie()
         C.load('eggs=scrambled; Path=/bacon')
diff --git a/Misc/ACKS b/Misc/ACKS
index b951446..8b2931f 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1461,6 +1461,7 @@ Varun Sharma
 Daniel Shaulov
 Vlad Shcherbina
 Justin Sheehy
+Akash Shende
 Charlie Shepherd
 Bruce Sherwood
 Alexander Shigin
diff --git a/Misc/NEWS.d/next/Library/2018-04-07-13-49-39.bpo-29613.r6FDnB.rst b/Misc/NEWS.d/next/Library/2018-04-07-13-49-39.bpo-29613.r6FDnB.rst
new file mode 100644
index 0000000..a679cd9
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-04-07-13-49-39.bpo-29613.r6FDnB.rst
@@ -0,0 +1,2 @@
+Added support for the ``SameSite`` cookie flag to the ``http.cookies``
+module.