OpenNDS, as used in Sierra Wireless ALEOS before 4.17.0.12 and other products, allows remote attackers to cause a denial of service (NULL pointer dereference, daemon crash, and Captive Portal outage) via a GET request to /opennds_auth/ that lacks a custom query string parameter and client-token.
https://www.forescout.com/resources/sierra21-vulnerabilities
https://openwrt.org/docs/guide-user/services/captive-portal/opennds