source: https://www.securityfocus.com/bid/4744/info

Quake II is a multiplayer game released by id Software. The source code has been made publically available, and versions are available for Windows and Linux. A vulnerability has been reported in some versions of the Quake II server.

While variable expansion is normally performed on the client side, a modified client may pass unexpanded variables such as $rcon_password to the server. The server will expand these variables within it's local context, potentially leaking sensitive information to the remote attacker.

you must modify your q2 client, that the client will not replace the $... variables in says
quick hack:
in qcommon/cmd.c
change the line
Cmd_TokenizeString (text, true);
to
Cmd_TokenizeString( text, false);