XSS in next redirection parameter (CVE-2021-3346)

Reported by Niklas Volcz in RT#1212321

I have found an XSS (cross-site scripting) vulnerability in foris (version: 100.5.1) login. The impact of the vulnerability is so far limited to phishing and by redirecting the login credentials to an attacker but I am sure there is a lot of evil stuff you can do with an XSS like this. I read somewhere that foris will be replaced in the near future but my router still uses it.

The vulnerability: The next parameter in the login page does not escape/sanitize HTML.

Proof of concept: http://192.168.1.1/foris/config/?next=CANARY%22%3E%3Cscript%3Ealert(%22BOOM%22)%3C/script%3E