XWiki is a generic wiki platform. In versions starting from 15.3-rc-1 to before 15.10.14, from 16.0.0-rc-1 to before 16.4.6, and from 16.5.0-rc-1 to before 16.10.0-rc-1, 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 an SSO authenticator is installed and utilized (like OIDC or LDAP for example), the worst an attacker can do is break authentication by switching back to the standard authenticator (that's because it's impossible to login to a user which does not have a stored password, and that's usually what SSO authenticator produce). This issue has been patched in versions 15.10.14, 16.4.6, and 16.10.0-rc-1.