An issue was discovered in OpenNDS before 10.1.2. It allows users to skip the splash page sequence (and directly authenticate) when it is using the default FAS key and OpenNDS is configured as FAS. Affected OpenNDS Captive Portal before version 10.1.2 fixed in OpenWrt master, OpenWrt 23.05 and OpenWrt 22.03 on 28. August 2023 by updating OpenNDS to version 10.1.3.
https://www.forescout.com/resources/sierra21-vulnerabilities
https://openwrt.org/docs/guide-user/services/captive-portal/opennds
https://github.com/openwrt/routing/commit/0b19771fb2dd81e7c428759610aed583171eed80
https://github.com/openNDS/openNDS/releases/tag/v10.1.2