An overview of captive portals from Linksys: https://www.linksys.com/us/r/resource-center/captive-portal/
Secplicity – Security Simplified. 2016-08-26. Provides a good overview of the techniques used by attackers on networks secured by captive portals: https://www.secplicity.org/2016/08/26/lessons-defcon-2016-bypassing-captive-portals/
Electronic Frontier Foundation. 2017-08-07. The article discusses some of the security concerns of captive portals and best practices for securing them: https://www.eff.org/deeplinks/2017/08/how-captive-portals-interfere-wireless-security-and-privacy