Fastify Security

Fastify Security Audit

Audit Fastify HTTP servers. Fastify's schema-first design provides strong defaults if used correctly.

When this skill applies

• Reviewing Fastify route definitions and schemas
• Auditing plugin chain and encapsulation
• Reviewing hooks (onRequest, preHandler, preValidation, onResponse)
• Checking security plugin configuration

Workflow

Follow ../_shared/audit-workflow.md. Companion: nodejs-express-security for cross-cutting Node concerns.

Phase 1: Stack detection

grep -E '"fastify":|"@fastify/' package.json

Phase 2: Inventory

# Route definitions
grep -rn 'fastify\.\(get\|post\|put\|delete\|patch\|register\)' src/ | head -50

# Schemas
grep -rnE 'schema:\s*{' src/ | head -20

# Hooks
grep -rn '\.addHook\(\|preHandler:\|preValidation:\|onRequest:' src/

# Security plugins
grep -nE '@fastify/(helmet|cors|rate-limit|jwt|cookie|session|multipart|csrf-protection)' package.json

Phase 3: Detection — the checks

Schema validation

Fastify validates inputs against JSON Schema on every request — if you provide one.

• FST-SCH-1 Every route has a schema for body, params, querystring. Missing schema = no validation.
• FST-SCH-2 Schema uses strict types and ranges:

``ts fastify.post('/users', { schema: { body: { type: 'object', required: ['email', 'password'], additionalProperties: false, // ← strips/rejects extras properties: { email: { type: 'string', format: 'email', maxLength: 254 }, password: { type: 'string', minLength: 8, maxLength: 128 }, }, }, }, }, async (req, reply) => { ... }); ``

• FST-SCH-3 additionalProperties: false set globally OR on every schema. Without it, mass assignment is possible.
• FST-SCH-4 Response schemas defined — they filter the response to only declared fields (built-in defense against accidental data exposure):

``ts schema: { response: { 200: { type: 'object', properties: { id: { type: 'string' }, displayName: { type: 'string' }, // passwordHash explicitly absent }, }, }, } ``

• FST-SCH-5 AJV configured strictly (default in current Fastify); custom keywords reviewed for soundness.

Hooks

Fastify has multiple hook points; auth typically in onRequest or preHandler.

• FST-HK-1 Auth hook applied to protected routes via plugin scope or global hook with opt-out for public routes.
• FST-HK-2 Hooks run in registration order; auth before validation is fine, but verify the order matches intent.
• FST-HK-3 Hooks throwing errors propagate to error handler — don't swallow.

Plugin encapsulation

Fastify plugins create scopes. Auth applied in one plugin doesn't apply to sibling plugins unless registered up-stack:

// BAD — auth only applies to /api/v1 subtree
fastify.register(async (app) => {
  app.addHook('preHandler', authHook);
  app.register(userRoutes, { prefix: '/api/v1' });
});
// /api/v2 routes registered elsewhere have NO auth

// GOOD — auth at top level
fastify.addHook('preHandler', authHook);
fastify.register(userRoutes, { prefix: '/api/v1' });
fastify.register(adminRoutes, { prefix: '/api/v2' });

• FST-PLG-1 Auth/security hooks at the top level OR encapsulated explicitly per plugin scope.
• FST-PLG-2 fastify-plugin wrapper used when a plugin's effects (including hooks) should escape its scope. Conversely, plugins that should be scoped should NOT use fastify-plugin.

Security plugins

• FST-SP-1 @fastify/helmet registered. Same options as Express helmet.
• FST-SP-2 @fastify/cors with specific origin allowlist.
• FST-SP-3 @fastify/rate-limit registered globally (or per-route for fine-tuning).
• FST-SP-4 @fastify/csrf-protection if using cookie-based session.
• FST-SP-5 @fastify/multipart with size limits if file uploads.
• FST-SP-6 @fastify/cookie and @fastify/session configured securely (httpOnly, secure, sameSite — see saas-security-pack/saas-frontend-hardening/references/cookie-config.md).

Body parser limits

• FST-BP-1 bodyLimit set on FastifyInstance (default 1MB; lower if appropriate, never higher without specific reason).
• FST-BP-2 Per-route override for upload routes only.

JWT (@fastify/jwt)

• FST-JWT-1 Secret/key from env, not committed.
• FST-JWT-2 Algorithm specified (don't accept none).
• FST-JWT-3 See saas-security-pack/saas-code-security-review/references/jwt-validation.md.

Error handling

• FST-ERR-1 setErrorHandler configured to scrub internal details in production.
• FST-ERR-2 Validation errors return generic messages (don't echo full schema paths that reveal internal field names).

Logging

• FST-LOG-1 Fastify's pino logger configured to redact sensitive paths:

``ts const fastify = Fastify({ logger: { redact: ['req.headers.authorization', 'req.headers.cookie', 'req.body.password'], }, }); ``

• FST-LOG-2 No password / token in request bodies logged at info level.

Microservice / WS

If using @fastify/websocket:

• FST-WS-1 WebSocket connection auth via the initial HTTP upgrade — same auth as REST routes.
• FST-WS-2 Origin validation on upgrade.

Dependencies

• FST-DEP-1 Fastify v4 or v5 (current). Older versions deprecated.
• FST-DEP-2 @fastify/* packages match Fastify major version.

Phase 4: Triage

Critical: route without schema accepting body; encapsulation bug where auth hook missing from a route group; default bodyLimit with file upload routes.

Phase 5: Report

Use ../_shared/findings-schema.md. Prefix IDs with FST-.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: hlsitechio
• Source: hlsitechio/claude-skills-security
• License: MIT

Install and usage instructions live in the source repository linked above.