UEFI Security Terms Seed

What is Measured Boot?

Quick note explaining Measured Boot for BIOS/UEFI and embedded firmware readers.

October 18, 2023 2 min read

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Measured Boot records measurements of boot components into TPM PCRs and an event log.

Why it matters

Explains firmware trust and protection mechanisms.
Helps debug Secure Boot, measured boot, and variable-protection behavior.
Useful when reviewing boot security policy.

Practical example

Example: Secure Boot decides whether an image is allowed to run; Measured Boot records what actually ran.

Quick checklist

Which policy or key database is involved?
Is the image/variable signed or measured as expected?
Do logs report authentication, measurement, or access-denied errors?

Quick takeaway

Measured Boot is a small concept, but it often becomes important when reading logs or debugging real firmware.

How I usually read it

I try not to treat Measured Boot as a dictionary entry. I read it as part of a firmware path: who produces it, who consumes it, and what symptom appears when it is wrong. That habit makes the note useful during debugging, not only during study.

Where it shows up

For security and SMM topics, I read Measured Boot as a trust-boundary question. Who can call this path? Where does the buffer come from? When is the policy locked? What happens if the input is controlled by an attacker?

In a real debugging session

Treat Measured Boot as part of a boot chain, not as an isolated term: Boot Manager reads NVRAM → selects a boot option → parses the Device Path → opens the .efi file → transfers control to the loader. When a system boots the wrong target, the routing metadata is often guilty before the loader itself.

A practical check is to dump the boot variables, see which option the value points to, confirm that the option is active, and then inspect whether the embedded device path still matches the current disk and partition layout.

Public references

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