Security

Security: Threat-Model & Harden

When to use

• Designing any feature that handles auth, authorization, personal/financial data, file upload, payments, or untrusted input.
• Reviewing a diff or PR for vulnerabilities before merge.
• Running the dedicated security audit phase before production readiness.
• Responding to a suspected vulnerability, leaked secret, or incident.

This applies to every project type: web, mobile, desktop, API, CLI, game, agent system, embedded, blockchain. The attack surface differs; the discipline does not.

Workflow

1. Define what you're protecting. List assets (user data, credentials, funds, model/IP, availability) and trust boundaries (who/what crosses from untrusted to trusted). For agent/LLM systems, treat prompts, tool calls, and retrieved content as untrusted input.
2. Threat-model with STRIDE. For each trust boundary, ask: Spoofing, Tampering, Repudiation, Information disclosure, Denial of service, Elevation of privilege. Write down the realistic threats and rank by impact × likelihood. Record assumptions explicitly.
3. Decide controls per threat. Map each ranked threat to a concrete control (authn, authz, validation, encryption, rate limit, logging, isolation). Prefer platform/framework-provided controls over hand-rolled crypto or auth.
4. Apply secure-by-default during build. Validate and encode all external input at the boundary; parameterize queries; authorize every request server-side; deny by default. Keep secrets out of code and logs. See Standards.
5. Review the diff. Run the security review pass against .claude/checklists/security.md: injection, broken access control, secrets, dependency CVEs, insecure deserialization, SSRF, weak crypto, misconfig. Use SAST/dependency scanning where available.
6. Test the controls. Add tests that assert authorization failures, input rejection, and rate limits actually trigger. Negative tests are the point. Coordinate with .claude/skills/testing/SKILL.md.
7. Audit before launch. Verify the full checklist, confirm secrets are managed (see .claude/skills/devops/SKILL.md), confirm logging/alerting on security events, and document residual risks and their owners.
8. Document & monitor. Record the threat model and decisions in an ADR (.claude/skills/documentation/SKILL.md), wire up alerts for auth failures/anomalies, and define the incident path in a runbook.

Standards

• Do authorize every action server-side on the resource, checking ownership/role — never trust client-supplied identity or hidden fields.
• Do validate input against an allowlist and encode output for its sink (HTML, SQL, shell, URL, JSON, log).
• Do keep secrets in a manager/vault or platform secret store; inject via env at runtime; rotate on exposure. Never commit .env, keys, or tokens.
• Do use parameterized queries / prepared statements / safe ORMs; never build queries by string concatenation.
• Do use vetted libraries for crypto, password hashing (argon2/bcrypt/scrypt), JWT/session handling; pin and patch dependencies.
• Do enforce TLS in transit and encryption at rest for sensitive data; set security headers / secure cookie flags where applicable.
• Do rate-limit and add abuse controls on auth, expensive, and write endpoints; fail closed.
• Do treat LLM/agent inputs and tool outputs as untrusted: constrain tool permissions, sanitize, and prevent prompt-injection from reaching privileged actions.
• Do-not roll your own crypto, auth, or session management.
• Do-not log secrets, tokens, full PANs, passwords, or full PII.
• Do-not disable TLS verification, use eval on input, or run untrusted code without isolation.
• Do-not perform destructive or irreversible production security actions (key deletion, prod data wipe, force-disabling auth) without explicit human approval — see project destructive-action rules.

Common mistakes to avoid

• Broken access control: checking a role in the UI but not on the server; IDOR via predictable IDs with no ownership check.
• Secrets in source, CI logs, error messages, or client bundles.
• Trusting client-side validation; trusting redirect/url/host parameters (SSRF/open redirect).
• Verbose errors leaking stack traces, internal hostnames, or query structure to users.
• Outdated dependencies with known CVEs and no scanning in CI.
• Prompt injection in agent systems escalating to data exfiltration or unauthorized tool use because tools were over-permissioned.
• Treating "internal" services as trusted and skipping authn/authz between them.

Output format

A threat model + hardening record: asset/trust-boundary list, ranked STRIDE threats, control per threat with status, and residual-risk register with owners. For reviews, produce a findings list with severity, location, and remediation. Reference .claude/templates/architecture.md for boundaries and log the model as an ADR via .claude/skills/documentation/SKILL.md.

Related checklists

• .claude/checklists/security.md
• .claude/checklists/production.md
• .claude/checklists/qa.md

Related agents

• .claude/agents/quality/security-auditor.md
• .claude/agents/core/orchestrator.md
• .claude/agents/engineering/backend-engineer.md