FastCGI is a protocol that lets a web server hand requests to separate application processes efficiently instead of starting a new process for every request. It was designed to improve performance and scalability for dynamic web applications such as PHP sites running through PHP-FPM.
In cybersecurity, FastCGI matters because it sits at a boundary between the web server and the application runtime. If request parsing, memory handling, or process isolation is flawed, an attacker can sometimes abuse that boundary to crash a service or influence how application data is handled. Misconfigurations and vulnerable implementations have been part of real remote code execution paths, especially when network-facing software exposes FastCGI-related components. Defenders reduce risk by patching the web stack, limiting access to application endpoints, and ensuring the FastCGI interface is not exposed beyond the intended local or trusted network path.