Nextauth Security

NextAuth.js / Auth.js Security Audit

Audit applications using NextAuth.js (now Auth.js). Covers v4 (next-auth) and v5 (Auth.js).

When this skill applies

• Reviewing the auth configuration object (providers, callbacks, pages)
• Auditing JWT vs database session setup
• Reviewing callbacks for safety (jwt, session, signIn, redirect)
• Checking OAuth client credentials and provider config
• Auditing custom adapters

Workflow

Follow ../_shared/audit-workflow.md. Companion: nextjs-security for Next-specific concerns.

Phase 1: Stack detection

grep -E '"(next-auth|@auth/.+)":' package.json
# Find the auth config file
find . -path '*/api/auth/[*nextauth*].ts*' 2>/dev/null
find . -name 'auth.ts' -o -name 'auth.config.ts' 2>/dev/null | head

Detect: v4 (next-auth) vs v5 (@auth/* modular). API differs.

Phase 2: Inventory

# Auth config
cat src/auth.ts auth.config.ts app/api/auth/\[...nextauth\]/route.ts 2>/dev/null

# Callbacks
grep -rn 'callbacks:\|async jwt\|async session\|async signIn\|async redirect' . --include='*.ts' --include='*.js'

# Providers
grep -rn 'GoogleProvider\|GitHubProvider\|CredentialsProvider\|EmailProvider' .

# Env vars
grep -E '^NEXTAUTH_|^AUTH_' .env* 2>/dev/null

Phase 3: Detection — the checks

Environment / secrets

• NXA-SEC-1 NEXTAUTH_SECRET (v4) or AUTH_SECRET (v5) set — required for JWT signing and cookie encryption. Generate with openssl rand -base64 32.
• NXA-SEC-2 NEXTAUTH_URL (v4) set in production to actual URL — without it, callback redirects break or behave unexpectedly.
• NXA-SEC-3 OAuth client secrets (GITHUB_SECRET, GOOGLE_CLIENT_SECRET, etc.) in env, never committed.
• NXA-SEC-4 Production and dev clients separate (different OAuth apps with different secrets, different callback URLs).

Session strategy

• NXA-SS-1 Strategy explicitly set (session: { strategy: 'jwt' } or 'database'). Default behavior changes between versions.
• NXA-SS-2 JWT strategy: sessions are stateless; logout doesn't immediately invalidate tokens (token lifetime trade-off). Acceptable for most apps; not for high-sensitivity (banking, admin tools) where revocation matters.
• NXA-SS-3 Database strategy: session row deleted on signOut; revocation works immediately.
• NXA-SS-4 maxAge reasonable (default 30 days; reduce for sensitive apps).

Callbacks — jwt

callbacks: {
  async jwt({ token, user, account, profile }) {
    if (user) {
      token.id = user.id;
      token.role = user.role;
    }
    return token;
  },
}

• NXA-CB-JWT-1 Initial sign-in copies safe claims to token. Don't store secrets (refresh tokens may be stored but encrypted by NextAuth in v4 / handled in v5).
• NXA-CB-JWT-2 Token NOT used to store mutable user state — DB lookups in subsequent requests for role changes.
• NXA-CB-JWT-3 Returning data unaltered when no refresh needed.

Callbacks — session

async session({ session, token }) {
  session.user.id = token.id;
  session.user.role = token.role;
  return session;
}

• NXA-CB-SES-1 Session callback projects safe fields. Don't expose raw token to the client — session is what the browser sees via useSession().
• NXA-CB-SES-2 Sensitive fields (e.g., accessToken for downstream API calls) included only if the client truly needs them. Better: client uses cookie session, server makes downstream calls with stored token.

Callbacks — signIn

async signIn({ user, account, profile, credentials }) {
  // Run checks before allowing sign-in
  if (account.provider === 'github' && !profile.email_verified) return false;
  if (!isAllowedEmail(user.email)) return false;
  return true;
}

• NXA-CB-SIG-1 signIn callback used to reject specific accounts (banned users, unallowed email domains).
• NXA-CB-SIG-2 Don't return true unconditionally for security-sensitive apps that should restrict sign-ups.
• NXA-CB-SIG-3 Email-based signups verify profile.email_verified (some providers don't verify).

Callbacks — redirect

async redirect({ url, baseUrl }) {
  // Default behavior is to only allow relative URLs or same-origin
  if (url.startsWith('/')) return `${baseUrl}${url}`;
  if (new URL(url).origin === baseUrl) return url;
  return baseUrl;
}

• NXA-CB-RED-1 redirect callback returns to base URL or relative paths only. Without the callback, default allows same-origin only — fine. But custom callbacks must preserve the restriction; common bug returns url unchanged → open redirect.

Providers

• NXA-PR-CRD-1 CredentialsProvider (username/password) — authorize function uses constant-time password comparison (bcrypt.compare), not string equality.

• NXA-PR-CRD-2 CredentialsProvider returns null on failed auth — never throws with sensitive details that propagate to client.

• NXA-PR-CRD-3 Email enumeration: same response time for "user doesn't exist" vs "wrong password". Adding a sleep or always running bcrypt comparison helps.

• NXA-PR-OAUTH-1 OAuth providers configured with prompt: 'consent' for sensitive permissions (forces re-consent).

• NXA-PR-OAUTH-2 Scopes minimized — request only what's needed.

• NXA-PR-OAUTH-3 Provider-specific gotchas: GitHub requires explicit email scope; Google's prompt: 'consent' to refresh tokens.

• NXA-PR-EML-1 EmailProvider (magic link): SMTP credentials in env. Email content uses templates that don't reveal user data.

• NXA-PR-EML-2 Magic link tokens have short TTL (10-15 min); single-use enforced.

Pages

• NXA-PG-1 Custom sign-in page (if defined) preserves CSRF token and uses Auth.js's API correctly.
• NXA-PG-2 Custom error pages don't echo back arbitrary query parameters (XSS risk).

CSRF

• NXA-CSRF-1 NextAuth applies CSRF protection on credential sign-in (double-submit cookie). Verify not disabled.
• NXA-CSRF-2 Custom routes that act as sign-in pages include csrfToken from getCsrfToken() (v4) or equivalent (v5).

Database adapter

If using a database adapter (Prisma, Drizzle, etc.):

• NXA-ADP-1 Adapter's tables (User, Account, Session, VerificationToken) protected by RLS or app-level scoping. No raw user list endpoint shipping these tables.
• NXA-ADP-2 Adapter version matches NextAuth/Auth.js version.

Cookie configuration

• NXA-CK-1 Cookie config sets secure: true, httpOnly: true, sameSite: 'lax'. Defaults are usually right; verify not overridden.
• NXA-CK-2 Production deployment serves over HTTPS so the secure flag is effective.

Webhook / event handlers

If using events: { signIn, signOut, ... }:

• NXA-EV-1 Event handlers don't throw errors that interrupt the auth flow — wrap in try/catch.
• NXA-EV-2 Event handlers don't perform synchronous I/O that delays auth response.

Client-side patterns

• NXA-CL-1 useSession() calls in components don't leak session data to third parties.
• NXA-CL-2 Server-side getServerSession() used in Server Components / API routes; not relying on client session for security.

Dependencies

• NXA-DEP-1 NextAuth v4 (next-auth) or Auth.js v5 (@auth/* packages). v5 is current.
• NXA-DEP-2 Adapter version matches NextAuth major.

Phase 4: Triage

Critical: NEXTAUTH_SECRET empty / default; redirect callback returning unchecked url; CredentialsProvider with string equality on password; OAuth client secret committed.

Phase 5: Report

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