NRPE (the Nagios Remote Plug-In Executor) allows the passing of $() to plugins/scripts which, if run under bash, will execute that shell command under a subprocess and pass the output as a parameter to the called script. Using this, it is possible to get called scripts, such as check_http, to execute arbitrary commands under the uid that NRPE/nagios is running as (typically, 'nagios').

With this update NRPE will deny remote requests containing a bash command substitution.