Any user with view access to the XWiki space can change the authenticator
A user who can access pages located in the XWiki space (by default, anyone) can access the page XWiki.Authentication.Administration and (unless an authenticator is set in xwiki.cfg) switch to another installed authenticator. Note that, by default, there is only one authenticator available (Standard XWiki Authenticator). So, if no authenticator extension was installed, it's not really possible to do anything for an attacker. Also, in most cases, if you have installed …