HTTP Origin is only sent on cross-origin requests in Firefox

Therefore, only raise Forbidden if Origin (or Referer for MSIE) is sent
(which is a protected header and all modern browsers (except IE)).

Also add a basic unit test which asserts the failure for false origins.

isso/views/comments.py

@@ -33,8 +33,8 @@ class JSON(Response):
 
 
 def csrf(view):
-    """A decorator to check if HTTP_Origin matches configured host. If not,
-    return 401 Forbidden. See
+    """A decorator to check if Origin matches Host (may be empty if in the same
+    origin, except for IE of course). When MSIE, use Referer. See
 
        * https://www.owasp.org/index.php/Cross-Site_Request_Forgery_(CSRF)_Prevention_Cheat_Sheet#Checking_The_Origin_Header
        * http://tools.ietf.org/html/draft-abarth-origin-09
@@ -45,11 +45,13 @@ def csrf(view):
 
     def dec(self, environ, request, *args, **kwargs):
 
+        hosts = map(parse.host, self.conf.getiter("host"))
+
         if UserAgent(environ).browser == "msie":  # yup
-            origin = request.headers.get("Referer", "")
-        else:
-            origin = request.headers.get("Origin", "")
-        if parse.host(origin) not in map(parse.host, self.conf.getiter("host")):
+            if parse.host(request.headers.get("Referer", "")) not in hosts:
+                raise Forbidden("CSRF")
+        elif "Origin" in request.headers:
+            if parse.host(request.headers.get("Origin", "")) not in hosts:
                 raise Forbidden("CSRF")
 
         return view(self, environ, request, *args, **kwargs)

specs/fixtures.py

@@ -11,7 +11,6 @@ class FakeIP(object):
 
     def __call__(self, environ, start_response):
         environ['REMOTE_ADDR'] = self.ip
-        environ['HTTP_ORIGIN'] = "http://localhost:8080"
         return self.app(environ, start_response)
 
 

specs/test_comments.py

@@ -274,6 +274,23 @@ class TestComments(unittest.TestCase):
         self.client.delete('/id/1')
         assert '/' not in self.app.db.threads
 
+    def testCSRF(self):
+
+        payload = json.dumps({"text": "..."})
+
+        assert self.client.post('/new?uri=%2F', data=payload,
+                                headers={"Origin": "http://localhost:8080"}
+        ).status_code == 201
+
+        assert self.client.post('/new?uri=%2F', data=payload,
+                                headers={"Referer": "http://other.example/asdf",
+                                         "User-Agent": "msie"}
+        ).status_code == 403
+
+        assert self.client.post('/new?uri=%2F', data=payload,
+                                headers={"Origin": "http://other.example"}
+        ).status_code == 403
+
 
 class TestModeratedComments(unittest.TestCase):