An issue was discovered in PHP through 7.2.2. The php-fpm master process restarts a child process in an endless loop when using program execution functions (e.g., passthru, exec, shell_exec, or system) with a non-blocking STDIN stream, causing this master process to consume 100% of the CPU, and consume disk space with a large volume of error logs, as demonstrated by an attack by a customer of a shared-hosting facility.

References:
https://bugs.php.net/bug.php?id=70185
https://bugs.php.net/bug.php?id=75968
https://www.futureweb.at/Futureweb-OG-php-fpm-master-process-restarts-child-process-in-a_pid,54177,type,firmeninfo.html
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-9253