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`.
diff --git a/Lib/cgi.py b/Lib/cgi.py
index b655a05..adf4dcb 100755
--- a/Lib/cgi.py
+++ b/Lib/cgi.py
@@ -311,7 +311,8 @@
"""
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 @@
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 @@
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
@@ -597,9 +601,9 @@
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
@@ -633,11 +637,23 @@
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()