bpo-34866: Adding max_num_fields to cgi.FieldStorage (GH-9660)

Adding `max_num_fields` to `cgi.FieldStorage` to make DOS attacks harder by
limiting the number of `MiniFieldStorage` objects created by `FieldStorage`.

Lib/cgi.py

@@ -311,7 +311,8 @@ class FieldStorage:
     """
     def __init__(self, fp=None, headers=None, outerboundary=b'',
                  environ=os.environ, keep_blank_values=0, strict_parsing=0,
-                 limit=None, encoding='utf-8', errors='replace'):
+                 limit=None, encoding='utf-8', errors='replace',
+                 max_num_fields=None):
         """Constructor.  Read multipart/* until last part.
 
         Arguments, all optional:
@@ -351,10 +352,14 @@ class FieldStorage:
             for the page sending the form (content-type : meta http-equiv or
             header)
 
+        max_num_fields: int. If set, then __init__ throws a ValueError
+            if there are more than n fields read by parse_qsl().
+
         """
         method = 'GET'
         self.keep_blank_values = keep_blank_values
         self.strict_parsing = strict_parsing
+        self.max_num_fields = max_num_fields
         if 'REQUEST_METHOD' in environ:
             method = environ['REQUEST_METHOD'].upper()
         self.qs_on_post = None
@@ -578,12 +583,11 @@ class FieldStorage:
         qs = qs.decode(self.encoding, self.errors)
         if self.qs_on_post:
             qs += '&' + self.qs_on_post
-        self.list = []
         query = urllib.parse.parse_qsl(
             qs, self.keep_blank_values, self.strict_parsing,
-            encoding=self.encoding, errors=self.errors)
+            encoding=self.encoding, errors=self.errors,
-        for key, value in query:
+            max_num_fields=self.max_num_fields)
-            self.list.append(MiniFieldStorage(key, value))
+        self.list = [MiniFieldStorage(key, value) for key, value in query]
         self.skip_lines()
 
     FieldStorageClass = None
@@ -597,9 +601,9 @@ class FieldStorage:
         if self.qs_on_post:
             query = urllib.parse.parse_qsl(
                 self.qs_on_post, self.keep_blank_values, self.strict_parsing,
-                encoding=self.encoding, errors=self.errors)
+                encoding=self.encoding, errors=self.errors,
-            for key, value in query:
+                max_num_fields=self.max_num_fields)
-                self.list.append(MiniFieldStorage(key, value))
+            self.list.extend(MiniFieldStorage(key, value) for key, value in query)
 
         klass = self.FieldStorageClass or self.__class__
         first_line = self.fp.readline() # bytes
@@ -633,11 +637,23 @@ class FieldStorage:
             if 'content-length' in headers:
                 del headers['content-length']
 
+            # Propagate max_num_fields into the sub class appropriately
+            sub_max_num_fields = self.max_num_fields
+            if sub_max_num_fields is not None:
+                sub_max_num_fields -= len(self.list)
+
             part = klass(self.fp, headers, ib, environ, keep_blank_values,
                          strict_parsing,self.limit-self.bytes_read,
-                         self.encoding, self.errors)
+                         self.encoding, self.errors, sub_max_num_fields)
+
+            max_num_fields = self.max_num_fields
+            if max_num_fields is not None and part.list:
+                max_num_fields -= len(part.list)
+
             self.bytes_read += part.bytes_read
             self.list.append(part)
+            if max_num_fields is not None and max_num_fields < len(self.list):
+                raise ValueError('Max number of fields exceeded')
             if part.done or self.bytes_read >= self.length > 0:
                 break
         self.skip_lines()

Lib/test/test_cgi.py

@@ -381,6 +381,55 @@ Larry
         v = gen_result(data, environ)
         self.assertEqual(self._qs_result, v)
 
+    def test_max_num_fields(self):
+        # For application/x-www-form-urlencoded
+        data = '&'.join(['a=a']*11)
+        environ = {
+            'CONTENT_LENGTH': str(len(data)),
+            'CONTENT_TYPE': 'application/x-www-form-urlencoded',
+            'REQUEST_METHOD': 'POST',
+        }
+
+        with self.assertRaises(ValueError):
+            cgi.FieldStorage(
+                fp=BytesIO(data.encode()),
+                environ=environ,
+                max_num_fields=10,
+            )
+
+        # For multipart/form-data
+        data = """---123
+Content-Disposition: form-data; name="a"
+
+a
+---123
+Content-Type: application/x-www-form-urlencoded
+
+a=a&a=a
+---123--
+"""
+        environ = {
+            'CONTENT_LENGTH':   str(len(data)),
+            'CONTENT_TYPE':     'multipart/form-data; boundary=-123',
+            'QUERY_STRING':     'a=a&a=a',
+            'REQUEST_METHOD':   'POST',
+        }
+
+        # 2 GET entities
+        # 2 top level POST entities
+        # 2 entities within the second POST entity
+        with self.assertRaises(ValueError):
+            cgi.FieldStorage(
+                fp=BytesIO(data.encode()),
+                environ=environ,
+                max_num_fields=5,
+            )
+        cgi.FieldStorage(
+            fp=BytesIO(data.encode()),
+            environ=environ,
+            max_num_fields=6,
+        )
+
     def testQSAndFormData(self):
         data = """---123
 Content-Disposition: form-data; name="key2"

Lib/test/test_urlparse.py

@@ -880,6 +880,13 @@ class UrlParseTestCase(unittest.TestCase):
                                                           errors="ignore")
         self.assertEqual(result, [('key', '\u0141-')])
 
+    def test_parse_qsl_max_num_fields(self):
+        with self.assertRaises(ValueError):
+            urllib.parse.parse_qs('&'.join(['a=a']*11), max_num_fields=10)
+        with self.assertRaises(ValueError):
+            urllib.parse.parse_qs(';'.join(['a=a']*11), max_num_fields=10)
+        urllib.parse.parse_qs('&'.join(['a=a']*10), max_num_fields=10)
+
     def test_urlencode_sequences(self):
         # Other tests incidentally urlencode things; test non-covered cases:
         # Sequence and object values.

Lib/urllib/parse.py

@@ -628,7 +628,7 @@ def unquote(string, encoding='utf-8', errors='replace'):
 
 
 def parse_qs(qs, keep_blank_values=False, strict_parsing=False,
-             encoding='utf-8', errors='replace'):
+             encoding='utf-8', errors='replace', max_num_fields=None):
     """Parse a query given as a string argument.
 
         Arguments:
@@ -649,11 +649,15 @@ def parse_qs(qs, keep_blank_values=False, strict_parsing=False,
         encoding and errors: specify how to decode percent-encoded sequences
             into Unicode characters, as accepted by the bytes.decode() method.
 
+        max_num_fields: int. If set, then throws a ValueError if there
+            are more than n fields read by parse_qsl().
+
         Returns a dictionary.
     """
     parsed_result = {}
     pairs = parse_qsl(qs, keep_blank_values, strict_parsing,
-                      encoding=encoding, errors=errors)
+                      encoding=encoding, errors=errors,
+                      max_num_fields=max_num_fields)
     for name, value in pairs:
         if name in parsed_result:
             parsed_result[name].append(value)
@@ -663,7 +667,7 @@ def parse_qs(qs, keep_blank_values=False, strict_parsing=False,
 
 
 def parse_qsl(qs, keep_blank_values=False, strict_parsing=False,
-              encoding='utf-8', errors='replace'):
+              encoding='utf-8', errors='replace', max_num_fields=None):
     """Parse a query given as a string argument.
 
         Arguments:
@@ -683,9 +687,21 @@ def parse_qsl(qs, keep_blank_values=False, strict_parsing=False,
         encoding and errors: specify how to decode percent-encoded sequences
             into Unicode characters, as accepted by the bytes.decode() method.
 
+        max_num_fields: int. If set, then throws a ValueError
+            if there are more than n fields read by parse_qsl().
+
         Returns a list, as G-d intended.
     """
     qs, _coerce_result = _coerce_args(qs)
+
+    # If max_num_fields is defined then check that the number of fields
+    # is less than max_num_fields. This prevents a memory exhaustion DOS
+    # attack via post bodies with many fields.
+    if max_num_fields is not None:
+        num_fields = 1 + qs.count('&') + qs.count(';')
+        if max_num_fields < num_fields:
+            raise ValueError('Max number of fields exceeded')
+
     pairs = [s2 for s1 in qs.split('&') for s2 in s1.split(';')]
     r = []
     for name_value in pairs:

Misc/NEWS.d/next/Library/2018-10-03-11-07-28.bpo-34866.ML6KpJ.rst

@@ -0,0 +1,2 @@
+Adding ``max_num_fields`` to ``cgi.FieldStorage`` to make DOS attacks harder by
+limiting the number of ``MiniFieldStorage`` objects created by ``FieldStorage``.