[Thirdparty] Stored XSS in chat module - nextcloud server 9.0.51 installed in ubuntu 14.0.4 LTS

How it was found

I found stored XSS vulnerability in nextcloud server's chat module

Nextcloud Server version - 9.0.51 OS - Ubuntu 14.0.4 Browser - Internet Explorer 11

Steps:

1. Login as non-admin user(attacker) and change full name containing XSS payload - elamaran'>"><script>alert(document.domain)</script>
2. Login as admin/non-admin(victim) and go to chat module
3. Click "Show information" of the attacker
4. Then the stored XSS payload in attacker's name will get execute in nextcloud domain

POC Video URL - https://youtu.be/UU60IthJWxI