Multiple +Message Apps (Softbank +Message App for Android prior to version 10.1.7, Softbank +Message App for iOS prior to version 1.1.23, NTT DOCOMO +Message App for Android prior to version 42.40.2800, NTT DOCOMO +Message App for iOS prior to version 1.1.23, KDDI +Message App for Android prior to version 1.0.6, and KDDI +Message App for iOS prior to version 1.1.23) do not verify X.509 certificates from SSL servers, which allows man-in-the-middle attackers to spoof servers and obtain sensitive information via a crafted certificate.
https://www.softbank.jp/mobile/info/personal/news/service/20180927a/
https://www.nttdocomo.co.jp/info/notice/page/180927_00.html
https://www.au.com/information/notice_mobile/service/2018-002/