bpo-35647: Fix path check in cookiejar (#11436)

* Refactor cookie path check as per RFC 6265

* Add tests for prefix match of path

* Add news entry

* Fix set_ok_path and refactor tests

* Use slice for last letter

Lib/http/cookiejar.py

@@ -993,7 +993,7 @@ class DefaultCookiePolicy(CookiePolicy):
             req_path = request_path(request)
             if ((cookie.version > 0 or
                  (cookie.version == 0 and self.strict_ns_set_path)) and
-                not req_path.startswith(cookie.path)):
+                not self.path_return_ok(cookie.path, request)):
                 _debug("   path attribute %s is not a prefix of request "
                        "path %s", cookie.path, req_path)
                 return False
@@ -1200,11 +1200,15 @@ class DefaultCookiePolicy(CookiePolicy):
     def path_return_ok(self, path, request):
         _debug("- checking cookie path=%s", path)
         req_path = request_path(request)
-        if not req_path.startswith(path):
-            _debug("  %s does not path-match %s", req_path, path)
-            return False
-        return True
+        pathlen = len(path)
+        if req_path == path:
+            return True
+        elif (req_path.startswith(path) and
+              (path.endswith("/") or req_path[pathlen:pathlen+1] == "/")):
+            return True
 
+        _debug("  %s does not path-match %s", req_path, path)
+        return False
 
 def vals_sorted_by_key(adict):
     keys = sorted(adict.keys())

Lib/test/test_http_cookiejar.py

@@ -720,6 +720,30 @@ class CookieTests(unittest.TestCase):
         req = urllib.request.Request("http://www.example.com")
         self.assertEqual(request_path(req), "/")
 
+    def test_path_prefix_match(self):
+        pol = DefaultCookiePolicy()
+        strict_ns_path_pol = DefaultCookiePolicy(strict_ns_set_path=True)
+
+        c = CookieJar(pol)
+        base_url = "http://bar.com"
+        interact_netscape(c, base_url, 'spam=eggs; Path=/foo')
+        cookie = c._cookies['bar.com']['/foo']['spam']
+
+        for path, ok in [('/foo', True),
+                         ('/foo/', True),
+                         ('/foo/bar', True),
+                         ('/', False),
+                         ('/foobad/foo', False)]:
+            url = f'{base_url}{path}'
+            req = urllib.request.Request(url)
+            h = interact_netscape(c, url)
+            if ok:
+                self.assertIn('spam=eggs', h, f"cookie not set for {path}")
+                self.assertTrue(strict_ns_path_pol.set_ok_path(cookie, req))
+            else:
+                self.assertNotIn('spam=eggs', h, f"cookie set for {path}")
+                self.assertFalse(strict_ns_path_pol.set_ok_path(cookie, req))
+
     def test_request_port(self):
         req = urllib.request.Request("http://www.acme.com:1234/",
                                      headers={"Host": "www.acme.com:4321"})

Misc/NEWS.d/next/Library/2018-12-30-14-35-19.bpo-35121.oWmiGU.rst

+Don't set cookie for a request when the request path is a prefix match of
+the cookie's path attribute but doesn't end with "/". Patch by Karthikeyan
+Singaravelan.