SECURITY DEFINER changes CURRENT_USER?

Hi all;

I found an unexpected behavior while trying to write a function to allow
users to change their own passwords. The function is as follows:

CREATE OR REPLACE FUNCTION change_password(VARCHAR)
RETURNS BOOL AS '
DECLARE
username VARCHAR;
CMD VARCHAR;
password ALIAS FOR $1;
BEGIN
SELECT INTO username CURRENT_USER;
CMD := ''ALTER USER '' || username || '' WITH PASSWORD '';
CMD := CMD || '''''''' || password || '''''''';
EXECUTE CMD;
RETURN TRUE;
end;
' LANGUAGE 'plpgsql' VOLATILE SECURITY DEFINER

I would expect this to change the password of the user currently logged
in but instead it changes MY password. Evidently when a function is
called which is set to SECURITY DEFINER, it changes the context of the
current user. The CURRENT_USER then returns the name of the definer
rather than the invoker of the function.

So this being said-- are there any workarounds that don't allow anyone
to change anyone else's password?

Best Wishes,
Chris Travers