What Linux does NASA use?

I'm curious to know, what specific version or distribution of Linux does NASA utilize for their operations and research? Given the complexity and critical nature of their work, I'm sure they have a tailored setup that meets their unique needs. Is it a custom-built version, or do they rely on a widely-used distribution with specific modifications? And how does NASA ensure the security and stability of their Linux systems, given the sensitive nature of their data and projects?