---
title: "Threat Modelling: Assets, Adversaries and Trust Boundaries"
description: "Security work starts by naming what is worth protecting, who might want it, and where data crosses from a place you control into one you do not. Without that, defences are chosen by habit, and the eff"
canonical: https://lightmysky.com/learn/computing/threat-modelling-assets-adversaries-and-trust-boundaries-mt_VRXTkrNFvx
source: https://lightmysky.com/learn/computing/threat-modelling-assets-adversaries-and-trust-boundaries-mt_VRXTkrNFvx.md
retrieved: 2026-11-12
---

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# Threat Modelling: Assets, Adversaries and Trust Boundaries

Security work starts by naming what is worth protecting, who might want it, and where data crosses from a place you control into one you do not. Without that, defences are chosen by habit, and the effort lands away from where the risk is.

Subject: Computing · Area: Networks & Security · Ages 21 to 22
Page: https://lightmysky.com/learn/computing/threat-modelling-assets-adversaries-and-trust-boundaries-mt_VRXTkrNFvx

## Ready when they can

- Draw a system's trust boundaries and mark every crossing
- List assets and rank them by what their loss would cost
- Match a defence to a named adversary rather than to a general worry

## Lesson: Name it before you defend it

Security starts by naming three things: what is worth protecting, who might want it, and where trust or control changes. Without that list, defences get picked by habit and land where the risk is not.

**Example.** A school mail system names logins and pupil records as assets; losing pupil records causes the most harm. Phishers are the named adversary, sending fake login pages. The defence is checking the sender and, for a suspect message, opening the known app instead of its link.

Draw the system and mark every trust boundary, each place trust or control changes, even inside your own system. A hop onto the Internet is only one example. Rank each asset by what its loss would cost, from a leaked lunch menu up to stolen identities. Then match each defence to one named threat, not to a vague worry. Encryption without an authenticated peer key does not stop an active attacker in the middle.

Test your model with one question per defence: which adversary does this stop? If a defence answers no named threat, move that effort to a crossing that needs it.

*(drawing: A boundary sits where trust or control changes, inside one system or on the way out.)*

Phishers copy a real login page and ask for your password there. The clue is the copied page asking for secrets, not the device around it.

For a suspect login message, check the sender but do not sign in from its link. Open the known app or type the known address yourself, then sign in there.

**Recap.** Name assets, name adversaries, mark every crossing, and tie each defence to the threat it answers.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [TLS: Putting the Pieces Together](https://lightmysky.com/learn/computing/tls-putting-the-pieces-together-mt_JzIwi-BYG0)

## Optional connections

- [Cyber Threats and How Systems Defend](https://lightmysky.com/learn/computing/cyber-threats-and-how-systems-defend-mt_zMJzhIdEWd)

## Opens up

- [Authentication, Authorisation and Secure Defaults](https://lightmysky.com/learn/computing/authentication-authorisation-and-secure-defaults-mt_ofIiD4Bbpk)