How can you ensure an operating system is secure throughout its lifecycle?

There have been many legacy and newer models which work on Design security or securely Concepts such as Layered defence Secure by design Zero Trust Provide a guiding light when designing or architecting something TOGAF , SABSE , zero Trust, NIST have great amount of information on it which is reliable.

One must ask the question, what does the operating system depend on. If this is not addressed as part of the overall security design, then even with the best efforts in building a trustworthy OS, the system cannot be secure. Look into work done by the Trusted Computing Group. Capabilities like Trusted Platform Module came from these forward thinkers. Addressing trust in hardware and firmware the OS depends on is critical and this is much more than complying with standards, it includes a complete trusted sourcing approach.

Everything that is used to create the OS needs to be vetted as thoroughly as the code that goes into the OS. This includes all components of the build process and any code inherited from other sources. Is your compiler secure. Is the path (supply chain) associated with your compiler secure. Is the code you use from a secure supply chain. Has is been vetted. Is it packaged securely (e.g. digitally signed).

There are 2 basic approaches to securely deploy an OS. The first depends fully hardening the base OS and associated installers. This approach is sure to break applications that companies depend on and will result in a lot of work after deployment. The second approach follows the 'infrastructure as code' approach seen in devsecops tools like Chef. In this approach the OS and associated applications are locked down immediately after OS installatiol. There are several competing solutions in the infrastruture as code area. Chef, puppet, ansible, are all multiplatform and have a strong global user base. When scaling to 100K to millions of endpoints, a true enterprise class solution is needed (e.g. Chef).

Signed code is critical with redundant methods recommended (e.g. signed patch and signed patch verification file using different cryptography and source credentials). We also need to address operating environments (cloud, virtual, physical, container) and a variety of operating systems (windows, linux, android, ios, ...) holistically as there are very few applications completely contained in a single server on a dedicated OS. The modern application environment is much more complex and tools from OS or Platform manufacturers/providers are not sufficiently robust. Again, we can look at the devsecops world to find solutions like Chef that can ensure desired state is maintained across the modern application environment.

Never forget HOW the overall solution works. Everything depends on DNS, so ensure your architecture and technology for DNS is secure. Same is true for authentication and certificate validation services. Allways monitor from both inside and outside the item being monitored. Netflow/IPFix collection and analysis solutions (e.g. Flowmon) can tell you when something inappropriate is happening in very close to real time. It is not possible to manage what you cannot see and the world has changed with affordable solutions now available to let you see all that is taking place on your networks.