Implement new_session functionality (#282)

aiohttp_session/__init__.py

@@ -120,6 +120,22 @@ async def get_session(request):
     return session
 
 
+async def new_session(request):
+    storage = request.get(STORAGE_KEY)
+    if storage is None:
+        raise RuntimeError(
+            "Install aiohttp_session middleware "
+            "in your aiohttp.web.Application")
+    else:
+        session = await storage.new_session()
+        if not isinstance(session, Session):
+            raise RuntimeError(
+                "Installed {!r} storage should return session instance "
+                "on .load_session() call, got {!r}.".format(storage, session))
+        request[SESSION_KEY] = session
+    return session
+
+
 def session_middleware(storage):
 
     if not isinstance(storage, AbstractStorage):
@@ -198,6 +214,9 @@ def _get_session_data(self, session):
             data = {}
         return data
 
+    async def new_session(self):
+        return Session(None, data=None, new=True, max_age=self.max_age)
+
     @abc.abstractmethod
     async def load_session(self, request):
         pass

demo/login_required_example.py

@@ -3,7 +3,7 @@
 
 from cryptography import fernet
 from aiohttp import web
-from aiohttp_session import setup, get_session
+from aiohttp_session import setup, get_session, new_session
 from aiohttp_session.cookie_storage import EncryptedCookieStorage
 
 
@@ -61,7 +61,9 @@ async def login(request):
     # actually implement business logic to check user credentials
     try:
         user_id = DATABASE.index(user_signature)
-        session = await get_session(request)
+        # Always use `new_session` during login to guard against
+        # Session Fixation. See aiohttp-session#281
+        session = await new_session(request)
         session['user_id'] = user_id
         return web.HTTPFound(router['restricted'].url_for())
     except ValueError:

docs/reference.rst

@@ -21,6 +21,24 @@ Public functions
    See example below in :ref:`Session<aiohttp-session-session>`
    section for :func:`get_session` usage.
 
+.. function:: new_session(request)
+
+   A :ref:`coroutine<coroutine>` for getting a new session regardless
+   of whether a cookie exists.
+
+   .. warning::
+
+      Always use :func:`new_session` instead of :func:`get_session`
+      in your login views to guard against Session Fixation attacks!
+
+   Example usage::
+
+      from aiohttp_session import new_session
+
+      async def handler(request):
+          session = await new_session(request)
+          session.new == True # This will always be True
+
 .. function:: session_middleware(storage)
 
    Session middleware factory.
@@ -214,6 +232,13 @@ implement both :meth:`~AbstractStorage.load_session` and
 
       .. versionadded:: 2.3
 
+   .. method:: new_session()
+
+      A :ref:`coroutine<coroutine>` for getting a new session regardless
+      of whether a cookie exists.
+
+      Return :class:`Session` instance.
+
    .. method:: load_session(request)
 
       An *abstract* :ref:`coroutine<coroutine>`, called by internal

tests/test_get_session.py

@@ -2,7 +2,8 @@
 
 from aiohttp.test_utils import make_mocked_request
 
-from aiohttp_session import Session, get_session, SESSION_KEY, STORAGE_KEY
+from aiohttp_session import (Session, get_session, SESSION_KEY, STORAGE_KEY,
+                             new_session, AbstractStorage)
 
 
 async def test_get_stored_session():
@@ -32,3 +33,49 @@ async def load_session(self, request):
 
     with pytest.raises(RuntimeError):
         await get_session(req)
+
+
+async def test_get_new_session():
+    req = make_mocked_request('GET', '/')
+    session = Session('identity', data=None, new=False)
+
+    class Storage(AbstractStorage):
+        async def load_session(self, request):
+            pass
+
+        async def save_session(self, request, response, session):
+            pass
+
+    req[SESSION_KEY] = session
+    req[STORAGE_KEY] = Storage()
+
+    ret = await new_session(req)
+    assert ret is not session
+
+
+async def test_get_new_session_no_storage():
+    req = make_mocked_request('GET', '/')
+    session = Session('identity', data=None, new=False)
+    req[SESSION_KEY] = session
+
+    with pytest.raises(RuntimeError):
+        await new_session(req)
+
+
+async def test_get_new_session_bad_return():
+    req = make_mocked_request('GET', '/')
+
+    class Storage(AbstractStorage):
+        async def new_session(self):
+            return ''
+
+        async def load_session(self, request):
+            pass
+
+        async def save_session(self, request, response, session):
+            pass
+
+    req[STORAGE_KEY] = Storage()
+
+    with pytest.raises(RuntimeError):
+        await new_session(req)