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Zero Trust: verify explicitly, use least privilege, assume breach

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Azure architecture and services

Zero Trust: verify explicitly, use least privilege, assume breach

Short Summary

In this lesson, you’ll learn what the Zero Trust security model is (and what it is not). You’ll learn the three guiding principles—verify explicitly, use least privilege, and assume breach—and how they change day-to-day access decisions. You’ll also see why “inside the corporate network” is not treated as automatic trust.

Learning Objectives

By the end of this lesson, you will be able to:

  • Define Zero Trust and explain why it’s a security model (not a single product).
  • Describe the three guiding principles: verify explicitly, use least privilege access, and assume breach.
  • Contrast Zero Trust thinking with perimeter-based (“castle-and-moat”) security.
  • Apply each principle to simple access decisions using identity and device/context signals.
  • Identify common misunderstandings that lead to “implicit trust” in modern environments.

Core Concepts

Zero Trust is a security model: a set of guiding ideas for how you make trust decisions. The core idea is straightforward:

  • No user, device, app, or network location is automatically trusted, even if it looks “internal.”

You’ll often hear Zero Trust summarized as “never trust, always verify.” In Microsoft’s guidance, the practical meaning is that you verify each access request using signals (evidence) instead of assumptions.

Verify explicitly

Verify explicitly means you make access decisions by authenticating and authorizing based on available signals:

  • Authentication: proving who you are (for example, signing in).
  • Authorization: deciding what you’re allowed to access (for example, “can you open this app/data?”).

Examples of signals you might use include identity, sign-in risk, device compliance/health, location, and the sensitivity of the resource. The key point: network location alone is not proof of trust—it’s just one signal.

Use least privilege access

Use least privilege access means you grant only what’s needed:

  • Minimum permissions (just enough access)
  • Smallest scope (only the specific resource(s) needed)
  • Shortest duration (just in time)

This reduces accidental damage and limits what an attacker can do if an account or device is compromised.

Assume breach

Assume breach means you design as if compromise can happen (or is already happening). Instead of relying on a single “perfect wall,” you focus on limiting impact:

  • Minimize blast radius (reduce how much damage a single compromise can cause)
  • Segment access (prevent easy lateral movement to other systems)
  • Detect and respond (monitoring, alerting, investigation)

This principle is about containment and resilience, not panic.

How this differs from “castle-and-moat”

In perimeter-based security (“castle-and-moat”), the network boundary is treated as the main line of defense, and internal traffic may be trusted by default. Zero Trust assumes that boundary isn’t enough, because people work from anywhere, cloud apps live outside the perimeter, and credentials/devices can be compromised.

Practical Understanding

Practical Situation 1: When you see “inside the network,” don’t treat it as automatic trust

A user is on the office network and tries to access a sensitive cloud application. Someone argues this should be allowed because the user is “internal.”

How to think about it: With Zero Trust, “internal network” is not a free pass. You still verify explicitly using identity and context signals before granting access.

Common misunderstanding: “Internal access is automatically safe.” In Zero Trust, internal and external requests both need verification.

Practical Situation 2: When you see “someone needs access,” start from least privilege

A contractor needs access to one resource to complete a task for two days. The quick shortcut is to grant broad permissions so they “won’t be blocked.”

How to think about it: Least privilege means you grant the minimum needed permissions, scoped to the specific resource, for a limited time.

Common misunderstanding: “Least privilege slows you down.” In practice, it reduces risk while still enabling work—because access is intentional and limited.

Practical Situation 3: When you see “what if an account is compromised,” think containment

A company strengthens sign-in requirements but has weak segmentation and limited monitoring. If an attacker gets one account, they can move around quietly.

How to think about it: Assume breach pushes you to limit blast radius and improve detection so a compromise is contained and discovered faster.

Common misunderstanding: “Assume breach just means stronger prevention.” It mainly emphasizes containment, visibility, and response.

Practical Situation 4: When you see “VPN gives access,” remember identity and context still matter

A company relies heavily on a Virtual Private Network (VPN) and firewall rules, assuming that once users are connected, they can access what they need safely.

How to think about it: Zero Trust doesn’t forbid perimeter controls, but it doesn’t treat “connected to VPN” as sufficient trust. Access decisions still depend on identity, context, and least privilege.

Common misunderstanding: “Zero Trust replaces VPNs and firewalls.” Zero Trust changes how trust decisions are made; perimeter tools may still exist, but they’re not the only gate.

Common Pitfalls

  • Mistake: Assuming Zero Trust is a single product you can buy. Correction: Zero Trust is a strategy and set of principles implemented through multiple controls and policies.

  • Mistake: Thinking Zero Trust means blocking everything or trusting nothing forever. Correction: You verify explicitly and then grant the right level of access based on signals and risk.

  • Mistake: Believing Zero Trust applies only to remote/external users. Correction: Zero Trust applies to internal and external access; network location alone doesn’t make a request trustworthy.

  • Mistake: Treating a strong perimeter as “good enough.” Correction: Perimeter controls help, but Zero Trust shifts trust decisions to identity and context for each access request.

  • Mistake: Ignoring “assume breach” and focusing only on prevention. Correction: Build for detection, containment, segmentation, and response to reduce impact when compromise happens.

Check Your Understanding

  1. Explain, in your own words, why “inside the corporate network” isn’t treated as automatic trust in Zero Trust.
  2. Write a short “verify explicitly” checklist: which signals would you want to evaluate before granting access?
  3. Describe a task where least privilege changes what access you grant compared to giving broad permissions.
  4. Imagine an attacker gains access to one account. What would “assume breach” push you to design or monitor to limit impact?
  5. Compare the perimeter (“castle-and-moat”) mindset and Zero Trust mindset using one example from cloud app access.

Further Reading