Can DBMS_SQL Not Be Used to Alter Users?

I'm trying to put together a package that will allow me to expire user passwords at predefined intervals. Part of the package is supposed to change the users password for him using the DBMS_SQL package routines. However, I get "insufficient privileges" errors when I run it, even though the owner of the procedure has full system privileges and can issue the command successfully from an interactive prompt. Is DBMS_SQL simply not capable of handling this SQL comnmand?

For the record, I'm running Oracle 7.1.5 on Alpha/OpenVMS. The relevant portion of the stored procedure follows.

    vSQL := 'alter user ' || vUserName || ' identified by '

            || upper(NewPassword);
    dbms_output.put_line('vSQL = "' || vSQL || '"');     sqlcursor := dbms_sql.open_cursor;
    dbms_sql.parse(sqlcursor, vSQL, dbms_sql.NATIVE);     sqlreturn := dbms_sql.execute(sqlcursor);

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