During installation, certain parameters (admin_username and admin_password) are not escaped in the setup form.
This issue is resolved in 3.1.14 stable, although existing users are advised to remove this file prior to deploying to a production server.
github.com/FriendsOfPHP/security-advisories/blob/master/silverstripe/framework/SS-2015-016-1.yaml
github.com/silverstripe/silverstripe-framework
github.com/silverstripe/silverstripe-framework/commit/4c73721bab0d543eee6137e3c00aa8ec727e95d1
www.silverstripe.org/software/download/security-releases/ss-2015-016