In MODX Revolution before 2.5.7, a user with resource edit permissions can inject an XSS payload into the title of any post via the pagetitle parameter to connectors/index.php.
In MODX Revolution before 2.5.7, an attacker might be able to trigger XSS by injecting a payload into the HTTP Host header of a request. This is exploitable only in conjunction with other issues such as Cache Poisoning.