Added support for cookie SameSite attribute

docs/config.rst

@@ -208,6 +208,16 @@ The following configuration values are used internally by Flask:
 
     Default: ``False``
 
+.. py:data:: SESSION_COOKIE_SAMESITE
+
+    Restrict how cookies are sent with requests from external sites. Can
+    be set to ``'Lax'`` (recommended) or ``'Strict'``.
+    See :ref:`security-cookie`.
+
+    Default: ``None``
+
+    .. versionadded:: 1.0
+
 .. py:data:: PERMANENT_SESSION_LIFETIME
 
     If ``session.permanent`` is true, the cookie's expiration will be set this
@@ -361,13 +371,15 @@ The following configuration values are used internally by Flask:
    ``LOGGER_HANDLER_POLICY``, ``EXPLAIN_TEMPLATE_LOADING``
 
 .. versionchanged:: 1.0
-
     ``LOGGER_NAME`` and ``LOGGER_HANDLER_POLICY`` were removed. See
     :ref:`logging` for information about configuration.
 
     Added :data:`ENV` to reflect the :envvar:`FLASK_ENV` environment
     variable.
 
+    Added :data:`SESSION_COOKIE_SAMESITE` to control the session
+    cookie's ``SameSite`` option.
+
 
 Configuring from Files
 ----------------------
@@ -635,4 +647,3 @@ Example usage for both::
     # or via open_instance_resource:
     with app.open_instance_resource('application.cfg') as f:
         config = f.read()
-

docs/security.rst

@@ -184,6 +184,9 @@ contains the same data. ::
 
 - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-XSS-Protection
 
+
+.. _security-cookie:
+
 Set-Cookie options
 ~~~~~~~~~~~~~~~~~~
 
@@ -194,17 +197,21 @@ They can be set on other cookies too.
 - ``Secure`` limits cookies to HTTPS traffic only.
 - ``HttpOnly`` protects the contents of cookies from being read with
   JavaScript.
-- ``SameSite`` ensures that cookies can only be requested from the same
-  domain that created them. It is not supported by Flask yet.
+- ``SameSite`` restricts how cookies are sent with requests from
+  external sites. Can be set to ``'Lax'`` (recommended) or ``'Strict'``.
+  ``Lax`` prevents sending cookies with CSRF-prone requests from
+  external sites, such as submitting a form. ``Strict`` prevents sending
+  cookies with all external requests, including following regular links.
 
 ::
 
     app.config.update(
         SESSION_COOKIE_SECURE=True,
         SESSION_COOKIE_HTTPONLY=True,
+        SESSION_COOKIE_SAMESITE='Lax',
     )
 
-   response.set_cookie('username', 'flask', secure=True, httponly=True)
+    response.set_cookie('username', 'flask', secure=True, httponly=True, samesite='Lax')
 
 Specifying ``Expires`` or ``Max-Age`` options, will remove the cookie after
 the given time, or the current time plus the age, respectively. If neither
@@ -237,6 +244,9 @@ values (or any values that need secure signatures).
 - https://developer.mozilla.org/en-US/docs/Web/HTTP/Cookies
 - https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie
 
+.. _samesite_support: https://caniuse.com/#feat=same-site-cookie-attribute
+
+
 HTTP Public Key Pinning (HPKP)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

flask/app.py

@@ -284,6 +284,7 @@ class Flask(_PackageBoundObject):
         'SESSION_COOKIE_PATH':                  None,
         'SESSION_COOKIE_HTTPONLY':              True,
         'SESSION_COOKIE_SECURE':                False,
+        'SESSION_COOKIE_SAMESITE':              None,
         'SESSION_REFRESH_EACH_REQUEST':         True,
         'MAX_CONTENT_LENGTH':                   None,
         'SEND_FILE_MAX_AGE_DEFAULT':            timedelta(hours=12),

flask/sessions.py

@@ -249,6 +249,13 @@ def get_cookie_secure(self, app):
         """
         return app.config['SESSION_COOKIE_SECURE']
 
+    def get_cookie_samesite(self, app):
+        """Return ``'Strict'`` or ``'Lax'`` if the cookie should use the
+        ``SameSite`` attribute. This currently just returns the value of
+        the :data:`SESSION_COOKIE_SAMESITE` setting.
+        """
+        return app.config['SESSION_COOKIE_SAMESITE']
+
     def get_expiration_time(self, app, session):
         """A helper method that returns an expiration date for the session
         or ``None`` if the session is linked to the browser session.  The
@@ -362,6 +369,7 @@ def save_session(self, app, session, response):
 
         httponly = self.get_cookie_httponly(app)
         secure = self.get_cookie_secure(app)
+        samesite = self.get_cookie_samesite(app)
         expires = self.get_expiration_time(app, session)
         val = self.get_signing_serializer(app).dumps(dict(session))
         response.set_cookie(
@@ -371,5 +379,6 @@ def save_session(self, app, session, response):
             httponly=httponly,
             domain=domain,
             path=path,
-            secure=secure
+            secure=secure,
+            samesite=samesite
         )

tests/test_basic.py

@@ -319,6 +319,7 @@ def test_session_using_session_settings(app, client):
         SESSION_COOKIE_DOMAIN='.example.com',
         SESSION_COOKIE_HTTPONLY=False,
         SESSION_COOKIE_SECURE=True,
+        SESSION_COOKIE_SAMESITE='Lax',
         SESSION_COOKIE_PATH='/'
     )
 
@@ -333,6 +334,34 @@ def index():
     assert 'path=/' in cookie
     assert 'secure' in cookie
     assert 'httponly' not in cookie
+    assert 'samesite' in cookie
+
+
+def test_session_using_samesite_attribute(app, client):
+    @app.route('/')
+    def index():
+        flask.session['testing'] = 42
+        return 'Hello World'
+
+    app.config.update(SESSION_COOKIE_SAMESITE='invalid')
+
+    with pytest.raises(ValueError):
+        client.get('/')
+
+    app.config.update(SESSION_COOKIE_SAMESITE=None)
+    rv = client.get('/')
+    cookie = rv.headers['set-cookie'].lower()
+    assert 'samesite' not in cookie
+
+    app.config.update(SESSION_COOKIE_SAMESITE='Strict')
+    rv = client.get('/')
+    cookie = rv.headers['set-cookie'].lower()
+    assert 'samesite=strict' in cookie
+
+    app.config.update(SESSION_COOKIE_SAMESITE='Lax')
+    rv = client.get('/')
+    cookie = rv.headers['set-cookie'].lower()
+    assert 'samesite=lax' in cookie
 
 
 def test_session_localhost_warning(recwarn, app, client):

tox.ini

@@ -16,7 +16,7 @@ deps =
     blinker
     python-dotenv
 
-    lowest: Werkzeug==0.9
+    lowest: Werkzeug==0.14
     lowest: Jinja2==2.4
     lowest: itsdangerous==0.21
     lowest: Click==4.0