The Django project just released an important security update that affects all Review Board 1.7.x servers, particularly public ones. It allows an attacker to perform a Denial-of-Service attack on the server through the authentication mechanism.
We recommend that everybody running a Review Board 1.7.x release immediately updates to Django 1.4.8. We will be putting out new releases of Review Board today, as well.
Please see the Django security announcement for more information.