pixiv: XSS Reflected at https://sketch.pixiv.net/ Via `next_url`

Hi, I Found XSS Reflected at https://sketch.pixiv.net/ Via Success URL Follow Me : Steps : 1. Open the URL below: https://sketch.pixiv.net/resignrequest/success?nexturl=javascript%3Aalert%2F%2Fdocument.domain 2. Pop ups appear : Impact If an attacker can control a script that is executed in the...