Latest IPB is usually the most secure BUT this is always dependant on the host or dedicated server & the protections / php functions that are in place / disabled.
here is a tip:
Disable the following functions in your php.ini you can make a php info page to find the location of your php.ini
how to make a phpinfo php file - http://kb.mediatemple.net/questions/764/How+can+I+create+a+phpinfo.php+page%3F#gs
Disabled PHP Functions escapeshellarg, escapeshellcmd, exec, ini_alter, parse_ini_file, passthru, pcntl_exec, popen, proc_close, proc_get_status, proc_nice, proc_open, proc_terminate, show_source, shell_exec, symlink, system, pcntl_alarm, pcntl_fork, pcntl_waitpid, pcntl_wait, pcntl_wifexited, pcntl_wifstopped, pcntl_wifsignaled, pcntl_wexitstatus, pcntl_wtermsig, pcntl_wstopsig, pcntl_signal, pcntl_signal_dispatch, pcntl_get_last_error, pcntl_strerror, pcntl_sigprocmask, pcntl_sigwaitinfo, pcntl_sigtimedwait, pcntl_exec, pcntl_getpriority, pcntl_setpriority,