I've just rolled out version 1.800 of Webmin to all repositories. This includes a security fix for Authentic Theme, so we recommend updating immediately. If you cannot update Webmin at this time, switch the Webmin theme to Virtualmin Framed Theme (you can do that in Webmin->Webmin->Webmin Configuration->Webmin Themes; doing it in the per-user theme configuration will not mitigate this issue).
There are other changes in 1.800, but changelog will follow later. Security updates kinda take precedence over everything else.