Lax cookie parsing in http.cookies could be a security issue when combined

pitrou · pitrou · commit 637e4544afda · 2014-09-17T00:25:57.000+02:00
with non-standard cookie handling in some Web browsers.

Reported by Sergey Bobrov.
diff --git a/Lib/http/cookies.py b/Lib/http/cookies.py
@@ -431,6 +431,7 @@ def OutputString(self, attrs=None):
 _LegalCharsPatt  = r"[\w\d!#%&'~_`><@,:/\$\*\+\-\.\^\|\)\(\?\}\{\=]"
 _CookiePattern = re.compile(r"""
     (?x)                           # This is a verbose pattern
+    \s*                            # Optional whitespace at start of cookie
     (?P<key>                       # Start of group 'key'
     """ + _LegalCharsPatt + r"""+?   # Any word of at least one letter
     )                              # End of group 'key'
@@ -534,7 +535,7 @@ def __parse_string(self, str, patt=_CookiePattern):
 
         while 0 <= i < n:
             # Start looking for a cookie
-            match = patt.search(str, i)
+            match = patt.match(str, i)
             if not match:
                 # No more cookies
                 break
diff --git a/Lib/test/test_http_cookies.py b/Lib/test/test_http_cookies.py
@@ -179,6 +179,15 @@ def test_quoted_meta(self):
         </script>
         """)
 
+    def test_invalid_cookies(self):
+        # Accepting these could be a security issue
+        C = cookies.SimpleCookie()
+        for s in (']foo=x', '[foo=x', 'blah]foo=x', 'blah[foo=x'):
+            C.load(s)
+            self.assertEqual(dict(C), {})
+            self.assertEqual(C.output(), '')
+
+
 class MorselTests(unittest.TestCase):
     """Tests for the Morsel object."""
 
diff --git a/Misc/ACKS b/Misc/ACKS
@@ -140,6 +140,7 @@ Martin Bless
 Pablo Bleyer
 Erik van Blokland
 Eric Blossom
+Sergey Bobrov
 Finn Bock
 Paul Boddie
 Matthew Boedicker
diff --git a/Misc/NEWS b/Misc/NEWS
@@ -32,6 +32,10 @@ Core and Builtins
 Library
 -------
 
+- Lax cookie parsing in http.cookies could be a security issue when combined
+  with non-standard cookie handling in some Web browsers.  Reported by
+  Sergey Bobrov.
+
 - Issue #22384: An exception in Tkinter callback no longer crashes the program
   when it is run with pythonw.exe.
 
