Nagios Core before 4.3.3 creates a nagios.lock PID file after dropping privileges to a non-root account, which might allow local users to kill arbitrary processes by leveraging access to this non-root account for nagios.lock modification before a root script executes a "kill `cat /pathname/nagios.lock`" command.
check_by_ssh in Nagios nagios-plugins 2.4.5 allows arbitrary command execution via ProxyCommand, LocalCommand, and PermitLocalCommand with \${IFS}. This has been categorized both as fixed in e8810de, and as intended behavior.