Clerk Security

Clerk Authentication Security Audit

Audit a Clerk-powered application for misconfigurations and integration vulnerabilities. Clerk handles the heavy auth lifting; the security surface is mostly integration.

When this skill applies

• Reviewing Clerk middleware and route protection
• Auditing Clerk webhook handlers
• Reviewing publishable vs secret key handling
• Checking organization / role configuration
• Auditing custom session claims and JWT templates

Workflow

Follow ../_shared/audit-workflow.md.

Phase 1: Stack detection

grep -E '"@clerk/' package.json
# Look for SDK
grep -E '"(@clerk/nextjs|@clerk/clerk-sdk-node|@clerk/backend|@clerk/clerk-react|@clerk/clerk-expo)":' package.json

Phase 2: Inventory

# Middleware (Next.js)
find . -name 'middleware.ts' -o -name 'middleware.js' | xargs grep -l 'clerk' 2>/dev/null

# Server-side calls
grep -rn 'clerkClient\|auth()\|currentUser()\|getAuth(' src/ app/ | head

# Webhook handlers
grep -rn 'svix\|verifyWebhook\|WebhookEvent' src/ app/ | head

# Env vars
grep -rn 'CLERK_' .env* 2>/dev/null

Phase 3: Detection — the checks

Key handling

Clerk has multiple keys:

• NEXT_PUBLIC_CLERK_PUBLISHABLE_KEY (or equivalent) — public, ships to client. Safe.
• CLERK_SECRET_KEY — server-only. Never in NEXT_PUBLIC_ / VITE_ / similar prefix.
• CLERK_WEBHOOK_SECRET (svix-style) — server-only. Used to verify webhooks.

• CLK-KEY-1 Secret key NOT prefixed with NEXT_PUBLIC_ / VITE_. Build inspection clean.
• CLK-KEY-2 Secret key sourced from secrets manager / env, not committed.
• CLK-KEY-3 Test keys and production keys not interchanged across environments (pk_test_ vs pk_live_).

Middleware / route protection

For Next.js with @clerk/nextjs:

• CLK-MW-1 clerkMiddleware() (v5+) or authMiddleware() (older) configured in middleware.ts.
• CLK-MW-2 Public routes explicitly listed; everything else protected.

```ts import { clerkMiddleware, createRouteMatcher } from '@clerk/nextjs/server';

const isPublicRoute = createRouteMatcher([ '/', '/sign-in(.)', '/sign-up(.)', '/api/webhooks/(.*)', ]);

export default clerkMiddleware((auth, req) => { if (!isPublicRoute(req)) auth().protect(); }); ```

• CLK-MW-3 Webhook routes in public matcher (Clerk's middleware shouldn't gate them; they have their own auth via signature).
• CLK-MW-4 Server Actions / Route Handlers still check auth() themselves — middleware is not the only line of defense.

Server-side auth checks

• CLK-SS-1 auth() called in server-side code (Server Component, Route Handler, Server Action) for ANY operation needing user identity.
• CLK-SS-2 userId from auth() used to scope DB queries — never trust IDs from request body.
• CLK-SS-3 currentUser() calls minimized (it fetches from Clerk API); use auth() for ID-only needs.

// app/api/posts/route.ts
import { auth } from '@clerk/nextjs/server';

export async function POST(req: Request) {
  const { userId } = auth();
  if (!userId) return new Response('Unauthorized', { status: 401 });
  
  const data = await req.json();
  // Use userId from auth(), NOT from data
  return db.posts.create({ ...data, authorId: userId });
}

Webhook signature verification

Clerk sends webhooks via Svix. Verification is mandatory.

• CLK-WH-1 Webhook handler verifies the svix-id, svix-timestamp, svix-signature headers using CLERK_WEBHOOK_SECRET.

```ts import { Webhook } from 'svix';

const wh = new Webhook(process.env.CLERKWEBHOOKSECRET!); const evt = wh.verify(payload, headers) as WebhookEvent; // throws on invalid ```

• CLK-WH-2 Raw body used for verification (not parsed JSON). In Next.js App Router, await req.text(); in Express, use express.raw() middleware for the webhook route.
• CLK-WH-3 Webhook handler is idempotent — replays don't double-process.
• CLK-WH-4 Webhook secret rotation procedure documented.

Organizations / roles

• CLK-ORG-1 If using Organizations: role checks via has({ role: 'org:admin' }) not via string comparison on claims.

``ts const { has } = auth(); if (!has({ role: 'org:admin' })) return Response('Forbidden', { status: 403 }); ``

• CLK-ORG-2 Custom roles defined in Clerk Dashboard, not invented client-side.
• CLK-ORG-3 Per-resource authz still done — role check alone doesn't verify "this user owns this resource".

Custom session claims / JWT templates

Clerk allows JWT templates that add claims. Used when handing tokens to other services.

• CLK-JWT-1 Tokens issued for external services have appropriate aud (audience) and limited TTL.
• CLK-JWT-2 External service verifies tokens against Clerk's JWKS endpoint, validates iss (issuer) and aud.
• CLK-JWT-3 Custom claims don't expose internal/sensitive data — they ship to whatever service consumes the token.
• CLK-JWT-4 Networkless verification with authorizedParties set on @clerk/backend verifyToken.

Allowed origins / redirect URLs

In Clerk Dashboard:

• CLK-CFG-1 Allowed origins listed are production + staging; no * or test domains in production.
• CLK-CFG-2 OAuth redirect URLs specific.
• CLK-CFG-3 Sign-in / sign-up redirect URLs validated — no open-redirect via redirect_url query param exploitation.

Client-side useUser / useAuth

• CLK-CL-1 Client-side checks are UX, not security. Backend still enforces.
• CLK-CL-2 Sensitive UI hidden from non-admins via useUser() but server enforces too.
• CLK-CL-3 Tokens from getToken() not stored in localStorage; pass directly to API calls.

Multi-session and impersonation

• CLK-MS-1 If Clerk's "impersonation" feature is used (admin signs in as user), audit logs capture the impersonator.
• CLK-MS-2 Multi-session enabled only if needed; if not, restrict to single session.

Password and 2FA policy

• CLK-PW-1 Password policy in Clerk Dashboard set to reasonable minimums.
• CLK-PW-2 MFA available; required for admin roles.
• CLK-PW-3 Magic link / SMS / OAuth providers reviewed; enable only what's needed.

Session lifetime

• CLK-SES-1 Session lifetime configured in Clerk Dashboard for the sensitivity of the app.
• CLK-SES-2 Idle timeout configured.

Logging

• CLK-LOG-1 Don't log full JWT tokens or session tokens server-side.
• CLK-LOG-2 userId logged in app logs for traceability (not PII unless user emails are PII-relevant in your context).

Dependencies

• CLK-DEP-1 Clerk SDK versions current; v5+ for Next.js App Router; SDK majors aligned across packages.

Phase 4: Triage

Critical: CLERK_SECRET_KEY exposed in client bundle; webhook handler skipping signature verification; auth().protect() missing from sensitive routes; allowed origins includes *.

Phase 5: Report

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

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.