Django released a new set of security releases that protect against swamping a server when passing certain strings to a few different functions used for building URLs and truncating content. See their announcement for the details on the fixes.
We maintain security-hardened builds of Django 1.6.x, the version series we use for Review Board 2.0 through 3.0. We've put out a new Django 126.96.36.199 release that contains these fixes.
To upgrade to this release, run:
$ pip install -U https://downloads.reviewboard.org/releases/Django/1.6/Django-188.8.131.52.tar.gz
$ easy_install -U http://downloads.reviewboard.org/releases/Django/1.6/Django-184.108.40.206.tar.gz