The Trusted Computing Base (TCB) is the total set of hardware, firmware, and software components that are critical to enforcing a system’s security policy. If any part of the TCB fails or is subverted, the security of the entire system can no longer be guaranteed.
The guiding principle is to keep the TCB as small and well verified as possible, because everything inside must be trusted absolutely and everything outside it is untrusted by default. A smaller TCB means less code to audit, fewer places for flaws to hide, and a more defensible boundary. The reference monitor concept lives here: the part of the TCB that mediates every access request between subjects and objects must be tamperproof, always invoked, and small enough to be thoroughly tested. A frequent misunderstanding is thinking the TCB is only the kernel. In practice it can include firmware, security-relevant drivers, and hardware, and drawing that boundary accurately is what lets designers reason about what they trust.
How is the Trusted Computing Base tested on the CISSP?
The TCB is core to Domain 3 (Security Architecture and Engineering), which carries 13% of the exam. Expect the reference monitor, the security kernel, and the idea of minimizing trusted code. Study Domain 3 Security Architecture and Engineering to tie these together.