It was possible to fake U.S.G.N. login IDs (spoofing) for a short period of time (until the masterserver finished the login check).
This is a security risk - especially if the server runs Lua scripts which check the U.S.G.N. ID right after the join in order to grant special admin rights.

The patched version of the dedicated server now checks the U.S.G.N. login ID before the player joins. This can delay the join process a bit but makes faking impossible.

You can disable this "pre join check" with a new setting: sv_checkusgnlogin

Servers with the option "U.S.G.N. only" ( sv_usgnonly 1) do not have that problem because the check is always performed before the join in this case.