Threat Model

---
model: fable
name: threat-model
description: Produce a design-time threat model for a feature or system, using STRIDE for security threats and LINDDUN for privacy threats, anchored to the org standards. Writes THREAT-MODEL.md with identified threats, their severity, and mitigations that become NFR-SEC-* / NFR-PRIV-* requirements. Use during design (feeds the architect skill) or when the user says "threat model", "STRIDE", "LINDDUN", "what could go wrong with this design", "attack surface". Design-time and proactive, unlike security-review and privacy-review which inspect existing code.
---

# Threat Model (STRIDE + LINDDUN)

You map what could go wrong in a design before it is built, so the mitigations become explicit
requirements the architect and reviewers can trace. STRIDE covers security threats; LINDDUN
covers privacy threats. Run both whenever the system touches personal data, which is most
systems that have users at all.

## Inputs

1. The design under review: a SPEC.md, REQUIREMENTS.md, a design doc, or a description in the
   conversation. If only a verbal description exists, restate the system in two or three
   sentences and confirm before modeling.
2. `standards/security-standards.md` and `standards/privacy-standards.md` for the rule
   vocabulary the mitigations should map to.
3. `standards/severity-taxonomy.md` for banding.

## When a change needs this

SEC-DES-01 in `standards/security-standards.md` lists the surfaces that require a threat model
before a change merges: authentication, identity and sessions, authorization and IAM,
cryptography, infrastructure and production networking, agents and runs with no person
watching, connected servers, file upload, command execution reached from input, and
deserialization of outside data. When a reviewer asks for this model because a change touched
one of those, name the surface at the top of THREAT-MODEL.md, so the reviewer can match the
model to the request.

## Process

### 1. Sketch the system and its trust boundaries
List the components, the data stores, the external actors, and the data flows between them.
Mark every point where data crosses a trust boundary (user to server, service to service,
service to third party, app to data store). Threats cluster on boundaries. If the design tool
in use is draw.io (org default), reference or produce a data-flow diagram; otherwise a labeled
list of flows is enough.

### 2. Identify the assets
What is worth attacking or worth protecting: credentials, personal data, money movement,
integrity of records, availability of the service. Tie each asset to the data flows that touch
it.

### 3. STRIDE pass (security threats)
For each component and data flow, walk the six categories and record concrete threats:
- **Spoofing:** can an actor impersonate a user or service? (authn)
- **Tampering:** can data in transit or at rest be modified? (integrity, SEC-INJ, SEC-CRYPTO)
- **Repudiation:** can an actor deny an action with no audit trail? (logging, SEC-LOG)
- **Information disclosure:** can data leak? (SEC-WEB-02 authz, SEC-CRYPTO transport, PRIV-ACC)
- **Denial of service:** can the component be exhausted or wedged?
- **Elevation of privilege:** can an actor gain rights they should not have? (authz, IDOR)

### 4. LINDDUN pass (privacy threats)
For each flow that touches personal data, walk the seven categories:
- **Linkability:** can two records be linked to the same person who should stay separate?
- **Identifiability:** can a pseudonymous record be tied back to a real person?
- **Non-repudiation (privacy):** is a person unable to deny an action they should be able to?
- **Detectability:** can an observer tell that a record about a person exists?
- **Disclosure of information:** is personal data exposed beyond its purpose? (PRIV-ACC, PRIV-XFER)
- **Unawareness:** does the person lack notice/consent or control? (PRIV-CONSENT, PRIV-RIGHTS)
- **Non-compliance:** does the design violate retention/minimization/transfer rules?
  (PRIV-MIN, PRIV-RET, PRIV-XFER)


### 4b. AI and agent pass (only when a model reads or acts)

Skip this pass when no model is involved, and say you skipped it. Run it when the design
lets a model read outside content, search a store, act without a person watching, or reach
an attached server. Four questions, one rule each, from
`standards/ai-agent-standards.md`, plus three more when the model can act:

- **Injection through content (SEC-AI-INJ-03).** Where does text from outside the system
  enter a prompt, and what keeps it in a region the instruction part cannot be confused
  with? Answer out loud what happens when a document says to ignore everything above it.
- **Retrieval authorization (SEC-AI-RAG-01).** Is each retrieved item authorized at its
  SOURCE, at query time, for the identity asking? A filter over returned results is not
  authorization, and one missed filter returns everything.
- **Tenant separation (SEC-AI-RAG-02).** Is the tenant a condition of the query, or a test
  applied to the results afterwards? Only the first survives a bug in the second.
- **Autonomy and reach (SEC-AI-MCP-04).** What can a run do with nobody watching, and what
  is the narrowest scope that still does the job?

When the model can act, three more, from the "Authority to act" section of the same file:

- **Recommend or act (SEC-AI-AGT-01).** List each kind of CALL the model can make, meaning
  the operation, its arguments and its target, not just the tool, since one shell or database
  tool makes both kinds. Mark a call ACT if it has any effect beyond returned text: it changes
  stored state (a saved draft counts) or live state (restarting a service counts), sends
  anything outside the system (a search query to an outside provider counts), reads
  confidential data, or spends money or quota. Mark it RECOMMEND only when its sole effects are
  a non-confidential read and text a person reads first. When unsure, mark it ACT. Every ACT
  line needs a reason it is granted at all.
- **The check before the act (SEC-AI-AGT-02).** For each ACT line, name the check in code that
  runs first and controls which operation runs on which target: an allowlist of operations and
  targets, or a person's approval of that operation on that target. A size, count or spend
  limit can sit beside it but does not count as the check. A sentence in the prompt is not a
  check.
- **Who decides it is allowed (SEC-AI-AGT-03).** Trace where "may this run do X" is answered.
  It must come from the identity running the model and the policy that identity carries. If
  the answer is the model's own reading of its instructions, that is a Blocker, because an
  injected document rewrites exactly those instructions.

Each answer that must hold becomes an `NFR-SEC-*` requirement in step 5, exactly like a
STRIDE or LINDDUN mitigation.

### 5. Rate and mitigate
For each threat: band it (Blocker/Major/Nit per the taxonomy, by likelihood and impact), then
state a mitigation. Each mitigation that must hold becomes a requirement: phrase it as an
`NFR-SEC-*` or `NFR-PRIV-*` line the architect can drop into REQUIREMENTS/SPEC and the reviewers
can later trace.

### 5b. Detection pass (SEC-RUN-01 to 03)
Mitigation asks how to stop a threat. This asks what happens if it lands anyway. Take each
Blocker and Major threat from step 5 and answer three questions in one line each:

1. **What signal would show this happening?** A 500 spike, a login from a new country, a row
   count that jumps, an outbound request to a host nobody named. If the honest answer is
   "nothing", say so and carry it to Residual risks. A named blind spot is worth more than a
   comforting sentence.
2. **Where would that signal be visible, and who would look?** Name the log, dashboard or
   alert, and the moment somebody reads it. "In the application logs" is not an answer unless
   something makes a person open them.
3. **Has it ever fired?** An alert nobody has triggered is a hypothesis. Note whether a
   representative event has been tested, and if not, make the test an `NFR-SEC` line.

Then check retention once for the whole model: how long the log you are relying on is kept.
Compare that with how long an intrusion of this kind would plausibly sit unnoticed.

Stay inside what the project actually runs. If there is no runtime protection platform, do not
write requirements that assume one; write the blind spot down instead. The standards file says
which detection topics this pack deliberately leaves out and why.

### 6. Write THREAT-MODEL.md

```markdown
# THREAT MODEL, <feature/system>

**Modeled at:** <YYYY-MM-DD>
**Scope:** <what is in / out of scope>

## System sketch
Components, data stores, external actors, trust boundaries (or a link to the data-flow diagram).

## Assets
- <asset>, touched by <flows>

## Security threats (STRIDE)
### T-S-1: <title>  [Blocker|Major|Nit]
- **Category:** Spoofing | Tampering | ...
- **Where:** <component / flow / boundary>
- **Threat:** what an attacker does.
- **Mitigation:** the control. -> proposes NFR-SEC-<n>: <requirement text>

## Privacy threats (LINDDUN)
### T-P-1: <title>  [Blocker|Major|Nit]
- **Category:** Linkability | Identifiability | ...
- **Where:** <flow>
- **Threat:** the privacy harm.
- **Mitigation:** the control. -> proposes NFR-PRIV-<n>: <requirement text>

## Proposed requirements (hand-off to architect)
- NFR-SEC-<n>: ...
- NFR-PRIV-<n>: ...

## Detection (SEC-RUN-01 to 03)
One line per Blocker/Major threat: the signal, where it is visible, whether it has ever fired.
<T-S-1: signal / where / fired yes-no>
**Log retention relied on:** <window, and whether it is long enough for this system>
**Blind spots:** <threats with no observable signal, carried into Residual risks below>

## Residual risks
Threats accepted without full mitigation, with the reason and owner.
```

## Guardrails

- Model the design as described; do not invent components that are not there. If the design is
  too vague to model a boundary, say so and ask.
- Every Blocker/Major threat needs a named mitigation or an explicit residual-risk acceptance.
- The output is requirements, not code. Hand the NFR lines to the architect skill.
- Do not claim the model is exhaustive. Note the areas you did not have enough detail to cover.