[1.4.x] Fixed second security issue in image uploading. Disclosure and release forthcoming.

Backport of b1d4634 from master.

Author: apollo13

django/forms/fields.py

@@ -570,20 +570,10 @@ def to_python(self, data):
  file = StringIO(data['content'])
 
  try:
- # load() is the only method that can spot a truncated JPEG,
- # but it cannot be called sanely after verify()
- trial_image = Image.open(file)
- trial_image.load()
-
- # Since we're about to use the file again we have to reset the
- # file object if possible.
- if hasattr(file, 'reset'):
- file.reset()
-
- # verify() is the only method that can spot a corrupt PNG,
- # but it must be called immediately after the constructor
- trial_image = Image.open(file)
- trial_image.verify()
+ # load() could spot a truncated JPEG, but it loads the entire
+ # image in memory, which is a DoS vector. See #3848 and #18520.
+ # verify() must be called immediately after the constructor.
+ Image.open(file).verify()
  except ImportError:
  # Under PyPy, it is possible to import PIL. However, the underlying
  # _imaging C module isn't available, so an ImportError will be