Copied from email to security@python.org:

I have been doing memory profiling on a few python web frameworks and I noticed this issue in the cgi.FieldStorage class.

$ python example.py
Memory used: 523935744 bytes

The problem is there is no easy way to limit the number of MiniFieldStorage objects created by FieldStorage, so it goes unchecked in many frameworks like pyramid, pylons, webapp2, and flask. The end result is that on these frameworks, a 9MB request body (gzipped down to 9KB) can chew up ~500MB of memory on the server which is enough to effectively DOS it. The obvious way to prevent this currently is to check the content-length header and fail if it exceeds some value. But that solution has a major shortcoming because many frameworks want to allow large payloads, sometimes up to 10MB, as long as they contain a reasonable number of fields.

After talking with the security@python.org
 team and pylons dev team about it, we think the best solution is to add a max_num_fields param to the FieldStorage class, defaulting to None, which throws an error if max_num_fields is exceeded.

Sorry, looks like I forgot to attach example.py. Attaching now.

New changeset 209144831b0a19715bda3bd72b14a3e6192d9cc1 by Miss Islington (bot) (matthewbelisle-wf) in branch 'master':
bpo-34866: Adding max_num_fields to cgi.FieldStorage (GH-9660)
https://github.com/python/cpython/commit/209144831b0a19715bda3bd72b14a3e6192d9cc1

New changeset a66f279a1381dd5c1c27232ccf9f210d575e1dcc by Miss Islington (bot) in branch '3.7':
bpo-34866: Adding max_num_fields to cgi.FieldStorage (GH-9660)
https://github.com/python/cpython/commit/a66f279a1381dd5c1c27232ccf9f210d575e1dcc

New changeset 322a914965368ffd7e4f97ede50b351fdf48d870 by Miss Islington (bot) in branch '3.6':
bpo-34866: Adding max_num_fields to cgi.FieldStorage (GH-9660)
https://github.com/python/cpython/commit/322a914965368ffd7e4f97ede50b351fdf48d870

> https://github.com/python/cpython/commit/209144831b0a19715bda3bd72b14a3e6192d9cc1

This commit adds a new max_num_fields=None parameter to FieldStorage, parse_qs() and parse_qsl(): you must update the documentation in Doc/library/ as well.

For 3.7 an 3.6 changes, you have to specify the minor Python version (3.7.x and 3.6.x) in which the change has been introduce. Same comment for Python 2.7.

New changeset bc6f74a520112d25ef40324e3de4e8187ff2835d by Victor Stinner (matthewbelisle-wf) in branch '2.7':
bpo-34866: Add max_num_fields to cgi.FieldStorage (GH-9660) (GH-9969)
https://github.com/python/cpython/commit/bc6f74a520112d25ef40324e3de4e8187ff2835d

I suggest to not add the new parameter to 3.4 and 3.5 branches, even if it's a security fix. The fix requires to *use* the parameter, and I don't expect applications on Python 3.4 and 3.5 to be modified to use it.

That makes sense Victor, I agree. Thanks for merging those PRs.

Thanks Matthew Belisle for the nice security counter-measure!