# Defense in Depth & the Kubernetes Security Model

## Security is a posture, not a single setting

There is no single "make it secure" switch in Kubernetes. Real security comes from **defense in
depth** — multiple independent layers, each reducing what an attacker (or a bug) can do if an
earlier layer fails. If a container has a vulnerability that lets an attacker execute arbitrary
code, the question that actually matters is: what can that code DO once it's running? A
container running as root, with every Linux capability, on a writable filesystem, with no
restrictions at all, can do far more damage than one deliberately hardened to need almost
nothing.

## Least privilege: grant only what's actually needed

The organizing principle behind everything in this course is **least privilege**: a workload
should have exactly the permissions and capabilities it needs to do its job, and nothing more.
This sounds obvious stated plainly, but the DEFAULT behavior of a plain container is closer to
"maximum privilege unless told otherwise" — root user, all standard Linux capabilities, a
writable filesystem. Hardening a Pod means deliberately narrowing that down.

## SecurityContext: where you actually configure this

`securityContext` (settable at the Pod level and per-container) is where you declare these
restrictions:

```yaml
spec:
  securityContext:
    runAsNonRoot: true
    runAsUser: 1000
  containers:
  - name: app
    securityContext:
      readOnlyRootFilesystem: true
      capabilities:
        drop: ["ALL"]
```

- **`runAsNonRoot` / `runAsUser`** — the container process runs as a specific non-root user
  instead of root. If a container image was built to require root and you set this, it will
  fail to start — which is a good, loud failure, not a silent security gap. Hardening
  sometimes surfaces an image's OWN bad assumptions.
- **`capabilities: drop: ["ALL"]`** — Linux capabilities are fine-grained permissions
  (`NET_BIND_SERVICE`, `SYS_ADMIN`, dozens more) a process can hold even without full root.
  Dropping all of them, then adding back only the specific ones actually needed, is the
  least-privilege pattern applied to capabilities specifically.
- **`readOnlyRootFilesystem: true`** — the container's own filesystem cannot be written to at
  all (use an explicit `emptyDir` volume, ROOT 08, for anything that genuinely needs to write
  temporary files). This closes off a whole category of attack that relies on writing a
  malicious file to disk.

## Why this matters even when "nothing is currently wrong"

None of these settings fix a bug or a vulnerability — they limit the BLAST RADIUS if one exists
that you don't yet know about. A container hardened with non-root, dropped capabilities, and a
read-only filesystem gives a successful exploit dramatically less to work with than an
unhardened one, even against the exact same vulnerability. This is the entire point of defense
in depth: you don't rely on any single layer being perfect.

## Instructor Notes

**Approximate timing:** 15 minutes read; 10-12 minutes live with the demonstration.

**Demo plan:** Have the demonstration activity ready to run right after this lesson — seeing a
concrete before/after Pod definition lands the abstract "least privilege" principle far better
than the lesson text alone.

**Likely misconceptions to watch for:**
- Students sometimes think hardening settings are only relevant "if you expect to be attacked."
  Reframe: defense in depth assumes SOMETHING will eventually go wrong somewhere (a
  vulnerability, a bug, a misconfiguration) and prepares for that, rather than betting
  everything on nothing ever going wrong.
- `runAsNonRoot: true` failing a Pod's startup is sometimes read as "the security setting is
  broken." It's the opposite — it's correctly refusing to let a root-requiring image start,
  which is valuable, actionable information about the image itself.

**Discussion question:** "If dropping all capabilities breaks a container that turns out to
genuinely need one specific capability, what's the right fix — removing the drop, or something
more targeted?" (Answer: add back only that ONE specific capability rather than removing the
blanket drop — least privilege means "exactly what's needed," not "give up and grant
everything again.")

**Optional extension:** ask students to identify one setting from this lesson that would have
made zero difference against a vulnerability that lets an attacker read environment variables
(a good check that they understand these are layered, not universal, protections — the answer
is that none of SecurityContext's settings here directly protect secrets stored in env vars;
that's a different concern, covered later in this course).