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`.
(cherry picked from commit 209144831b0a19715bda3bd72b14a3e6192d9cc1)

Co-authored-by: matthewbelisle-wf <matthew.belisle@workiva.com>

1 files changed, 25 insertions, 9 deletions

diff --git a/Lib/cgi.py b/Lib/cgi.py
index f82cc6c..da4a346 100755
--- a/Lib/cgi.py
+++ b/Lib/cgi.py
@@ -328,7 +328,8 @@ 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:
@@ -368,10 +369,14 @@ 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
@@ -595,12 +600,11 @@ 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)
-        for key, value in query:
-            self.list.append(MiniFieldStorage(key, value))
+            encoding=self.encoding, errors=self.errors,
+            max_num_fields=self.max_num_fields)
+        self.list = [MiniFieldStorage(key, value) for key, value in query]
         self.skip_lines()
 
     FieldStorageClass = None
@@ -614,9 +618,9 @@ 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)
-            for key, value in query:
-                self.list.append(MiniFieldStorage(key, value))
+                encoding=self.encoding, errors=self.errors,
+                max_num_fields=self.max_num_fields)
+            self.list.extend(MiniFieldStorage(key, value) for key, value in query)
 
         klass = self.FieldStorageClass or self.__class__
         first_line = self.fp.readline() # bytes
@@ -650,11 +654,23 @@ 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()