Threat Modeling Agent Skill

Bitwarden's Engagement Model

Bitwarden follows a 4-phase engagement model for security work. This skill primarily supports Phase 1 (engineering-owned) and assists with Phase 2-4 artifacts.

Phase 1: Initial Security Assessment (Engineering Team)

• Check Bitwarden's Architecture Decision Records for existing decisions covering the system or components under review (see ${CLAUDE_PLUGIN_ROOT}/references/adr-alignment.md) — a threat model should align with already-accepted architecture, not silently re-derive or contradict it
• Create data flow diagrams (Mermaid)
• Define security requirements separate from product requirements
• Propose security definitions (threat model + security goals)
• Identify initial threats using STRIDE (see references/stride-framework.md)

Phase 2: AppSec Team Review (AppSec + Engineering)

• Share data flow diagrams and security definitions in advance
• Walk through system architecture collaboratively
• Validate or refine proposed security definitions
• Identify additional threats, assess risk
• Avoid assuming external mitigations exist

Phase 3: Implementation (Engineering Team)

• Implement necessary security mitigations
• Create Jira follow-up work for threats without existing protections
• Include security considerations in sprint planning

Phase 4: Testing & Validation (Engineering + AppSec)

• Verify mitigations work as intended
• Adopt adversarial mindset during code review
• Test hypotheses (e.g., "Can I bypass SSO?") by working backwards
• Update security definitions as the system evolves

Security Definitions

Security Definitions (SDs) are Bitwarden's formal construct for communicating the security posture of a system. Each definition has three components: a threat model (attacker capabilities), security goals (what the system guarantees), and an accepted goal status (honest assessment of whether the goal is currently met).

Use Bitwarden's standard vocabulary when writing definitions — see references/bitwarden-vocabulary.md for the full glossary. Align security goals with Bitwarden's security principles (P01-P06) — see references/security-principles.md.

Threat Model Component

Describe attacker capabilities AND limitations — what they can and cannot do. Always state both sides to scope the definition precisely:

• "Attacker can run a user space process after the user's client has logged out" + "Attacker does not have access to secure storage mechanisms"
• "Attacker has database access and can read and write to the Send table" + "Attacker does not have access to the ASP.NET Core Data Protection encryption keys"

Include concrete examples where helpful (e.g., "An example for this is a stolen device"). Don't assume external mitigations are in place — even if obtaining an auth token is difficult, still explore what happens if an attacker has one.

Apply these rules when scoping the threat model:

• Prune dominated threats. If the attacker capability you're describing is strictly weaker than one already accepted as out-of-scope, delete the SD — its residual-risk statement collapses to a tautology like "equivalent to full user-account compromise". See references/writing-quality-sds.md for the dominated-threat anti-pattern and the term Dominated Threat in references/bitwarden-vocabulary.md.
• Include passive observers, not just adversaries. For any secret or protected data that crosses into an external service (LLM provider, log aggregator, analytics pipeline, training-data collector), write at least one SD whose attacker is honest-but-curious. Confidentiality harms often arise from _visibility_, not malice — an adversarial framing alone misses the baseline concern. See the Passive Observer vocabulary entry.
• Verify "attacker does not have X" against the target platforms. Every limitation must be factually true on every OS/runtime in scope. Common pitfall: assuming kernel-level privileges are required for a capability that is actually unprivileged on Linux and Windows (e.g., reading another process's environment). If the limit isn't true, the SD is mis-scoped.

Security Goals Component

State concise, testable guarantees about what cannot happen given the threat model. Reference specific assets (tokens, keys, vault data):

• "Valid tokens cannot be accessed by attacker after the user's client has logged out"
• "Attacker cannot retrieve any decrypted MasterKeys that do not belong to them"
• "Attacker can perform reads on encrypted email addresses lists only"

Every goal carries a Rationale — three pieces, one line each:

• Principle — which Bitwarden principle (P01–P06) the goal enforces. See references/security-principles.md.
• Asset — the specific data, key, or token being protected.
• Harm — the user-visible consequence if the goal is violated (e.g., "master password exposed to third-party LLM provider and potentially their training pipeline").

A goal without a rationale is a claim, not a requirement. Rationales let reviewers judge whether the goal is load-bearing or can be cut.

Two additional rules on goal framing:

• Reality-check goals against runtime. Goals that claim a secret is "cleared from memory", "zeroized", or "not retained" are unenforceable in garbage-collected, string-interned runtimes (JavaScript, .NET, JVM, Python). If the runtime cannot uphold the goal, restate it in terms of what the runtime _can_ guarantee (scope minimization, short-lived references, process isolation), or mark Accepted Goal Status as explicitly not met and link the systemic limitation. Do not write goals the language cannot back.
• Prefer stdin or file-descriptor handoff over env/argv for secrets. If the goal forbids secret exposure to process.env or argv, the implementation MUST use stdin or an inherited file descriptor. An SD whose goal forbids env exposure but whose implementation passes the secret through env is internally inconsistent — fix the design, or fix the goal, but do not ship both.

Accepted Goal Status Component

Provide an honest assessment of the current state:

• Goal is met — Explain how (e.g., "User state clearing includes removal of the stored token from disk")
• Goal is partially met — Break down what works and what doesn't, using separate indicators for each aspect
• Goal is not met — Explain the gap and why it is accepted
• Best Effort — For goals dependent on platform capabilities (e.g., "This goal is not upheld for clients that do not have access to secure storage such as web and browser")

When a goal is known to be broken, link to the relevant tracking issue. Note scoping caveats (e.g., "These definitions do not apply in the case of a Vault Timeout set to Never").

Two additional rules:

• Quantify "brief" or "short-lived" rationales. If acceptance of residual risk rests on "the exposure is short", state the bound. For example: "Secret resides in the child process env for the duration of bw unlock, which scales with KDF iterations and vault size — observed between 1 and 8 seconds on representative hardware." _Brief without a number is not an accepted status — it is a hope._ See the Exposure Window vocabulary entry.
• Enforce internal consistency. The Threat Model, Security Goal, and Accepted Goal Status must agree. If the threat model puts capability X in-scope, the goal must defend against X, and the status must say whether that defense holds. If the goal forbids env exposure but the implementation uses env, the SD is wrong — pick which of the three to change and change it. Inconsistency is not a style issue; it is the SD failing to describe the system.

Writing Security Definitions

• It's OK to be wrong — the purpose is to start the conversation and see if these can be broken
• Start with what the system SHOULD guarantee, then validate through threat analysis
• Separate macro-level definitions (e.g., end-to-end encryption) from micro-level definitions specific to the feature
• Number definitions sequentially (SD1, SD2, SD3) — each is a self-contained unit
• Include a glossary of feature-specific terms when the feature introduces domain-specific vocabulary
• Prioritize by impact, not by enumeration. A short document listing the 3–5 threats that actually shape the design is more useful than a 15-SD document that buries the important ones in noise. Before adding an SD, ask: _"If this threat didn't exist, would the design change?"_ If the answer is no, it is likely code-quality commentary, not a security definition.
• Tag each SD with a Criticality level (Critical / High / Medium / Low) and order the document by Criticality descending, so reviewers see the load-bearing SDs first. See references/writing-quality-sds.md for the prioritization heuristic.
• Verbosity is a failure mode. The same anti-pattern that plagued early LLM code review — long lists with low signal — also plagues generated security definitions. Cut SDs that describe implementation-detail concerns (e.g., a future maintainer editing a constant to contain shell metacharacters) unless they are load-bearing to the design.

Artifact Generation

Use the templates in examples/ when generating artifacts:

• examples/security-definition-document.md — Full SD document template with glossary, numbered definitions, Criticality tagging, goal rationale, and accepted goal status
• examples/data-flow-diagram.md — Mermaid DFD template with trust boundaries
• examples/threat-catalog.md — Threat catalog table and mitigation tracking templates

Consult these references when writing or reviewing SDs:

• references/writing-quality-sds.md — Anti-patterns (dominated threats, adversarial-only attackers, unenforceable goals, aspirational limitations, shell-quoting SDs, the "brief exposure" trap) and the self-consistency checklist
• references/bitwarden-vocabulary.md — Standard terms, including Passive Observer, Dominated Threat, and Exposure Window
• references/security-principles.md — P01–P06, referenced by every goal's Rationale line
• references/stride-framework.md — STRIDE categories for structured threat identification
• ${CLAUDE_PLUGIN_ROOT}/references/adr-alignment.md — Architecture Decision Record alignment checks shared across security-engineer skills

When to Engage AppSec

Teams should initiate a full engagement with the AppSec team (#team-eng-appsec) when:

• Greenfield projects or new services
• Data sharing modifications (organization memberships, Send, sharing features)
• New IPC channels between components
• Cross-domain or cross-origin functionality
• Uncertain about security implications — perform an Initial Security Assessment first and post findings to #team-eng-appsec with a note indicating uncertainty about whether a full engagement is needed

Quick questions (e.g., concerns about a third-party library or coding practice) don't need a full engagement — post those directly to #team-eng-appsec.

Critical Rules

• Separate product requirements from security requirements in tech breakdowns. They serve different purposes and have different stakeholders.
• Security definitions are living documents. Revisit them when features change, new threats emerge, or security issues are discovered.
• Complexity increases vulnerability risk. Flag overly complex security-critical code as tech debt. Complex code with numerous dependencies and intricate logic is exceptionally challenging to secure.
• Threat modeling will never identify all vulnerabilities. It's one tool among many. Balance it with code analysis, security testing, and adversarial review.
• Don't assume external mitigations. When defining the threat model, explore what happens if an attacker bypasses external controls.
• Dominated or implementation-trivial threats are noise. Cut SDs whose residual-risk text reduces to "equivalent to full user-account compromise" or whose only mitigation is "reviewers notice a constant being edited". They degrade signal-to-noise and hide the threats that actually matter.
• Every security goal carries a rationale. Tie each goal to a Bitwarden principle (P01–P06), the protected asset, and the user-visible harm. Goals without rationales cannot be prioritized or evaluated for necessity, and tend to survive review by inertia rather than merit.

Before finalizing a set of SDs, apply the self-consistency checklist in references/writing-quality-sds.md.