I just found out how it works: to login from a MBean (it's very weird I know), you have to use "client-login" as domain. Don't ask me why this thing only works like this, in my opinion could be a
JBoss bug
// e.g.
LoginContext loginContext = new LoginContext("client-login", new Subject(), new SecurityAssociationHandler());
loginContext.login();
Thanks anyway,
Tiago