connoppp.cgi on ZTE ZXDSL 831CII devices does not require HTTP Basic Authentication, which allows remote attackers to modify the PPPoE configuration or set up a malicious configuration via a GET request.
https://www.exploit-db.com/exploits/43188/
https://euvd.enisa.europa.eu/vulnerability/EUVD-2017-8122
http://support.zte.com.cn/support/news/LoopholeInfoDetail.aspx?newsId=1008762