start-core-auth-server-primitives

name	start-core/auth-server-primitives
description	Server-side authentication primitives for TanStack Start: session cookies (HttpOnly, Secure, SameSite, __Host- prefix), session read/issue/destroy via createServerFn and middleware, OAuth authorization-code flow with state and PKCE, password-reset enumeration defense, CSRF for non-GET RPCs, rate limiting auth endpoints, session rotation on privilege change. Pairs with router-core/auth-and-guards for the routing side.
type	sub-skill
library	tanstack-start
library_version	1.166.2
requires	["start-core","start-core/server-functions","start-core/middleware"]
sources	["TanStack/router:docs/start/framework/react/guide/authentication-overview.md","TanStack/router:docs/start/framework/react/guide/authentication-server-primitives.md"]

Auth Server Primitives

This skill covers the server half of authentication: session storage, cookie issuance, OAuth flow, password-reset hardening, CSRF, rate limiting. For the routing half (_authenticated layout, beforeLoad redirects, RBAC checks), see router-core/auth-and-guards.

CRITICAL: A route guard does NOT protect a createServerFn on that route. Server functions are RPC endpoints reachable by direct POST regardless of which route renders them. Auth must be enforced inside the handler (or via middleware), not on the calling route. CRITICAL: Validating the shape of a client-supplied identifier (z.string().uuid().parse(...)) is not authorization. A parsed UUID is still some tenant — re-check membership against the session principal before using it. CRITICAL: Read session/cookies inside .handler() or middleware .server(), not at module scope. Module-level reads run before requests exist (and are also undefined on Cloudflare Workers).

Session Cookies

The recommended session storage is an HTTP-only cookie holding either an opaque session ID (with server-side lookup) or a signed/encrypted token. The cookie flags matter — set them all.

// src/server/session.ts
import {
  getRequestHeader,
  setResponseHeader,
} from '@tanstack/react-start/server'

const SESSION_COOKIE = '__Host-session' // __Host- prefix binds to the exact origin + path '/'
const ONE_DAY = 60 * 60 * 24

export function setSessionCookie(token: string) {
  setResponseHeader(
    'Set-Cookie',
    [
      `${SESSION_COOKIE}=${token}`,
      `HttpOnly`, // not readable from JS — defeats XSS exfiltration
      `Secure`, // HTTPS only (required for __Host- prefix)
      `SameSite=Lax`, // sent on top-level navigations, blocks most CSRF
      `Path=/`, // required for __Host- prefix
      `Max-Age=${ONE_DAY}`,
    ].join('; '),
  )
}

export function clearSessionCookie() {
  setResponseHeader(
    'Set-Cookie',
    `${SESSION_COOKIE}=; HttpOnly; Secure; SameSite=Lax; Path=/; Max-Age=0`,
  )
}

export function readSessionToken(): string | null {
  const header = getRequestHeader('cookie')
  if (!header) return null
  for (const part of header.split(/;\s*/)) {
    // Split only on the FIRST '=' — signed/base64 values often contain '='.
    const eq = part.indexOf('=')
    if (eq === -1) continue
    if (part.slice(0, eq) === SESSION_COOKIE) return part.slice(eq + 1)
  }
  return null
}

Flag rationale:

• HttpOnly — JavaScript can't read the cookie, so an XSS bug can't steal the session.
• Secure — HTTPS only. Required when using __Host- prefix.
• SameSite=Lax — blocks CSRF on most cross-origin POST/PUT/DELETE. Use Strict for highest-security flows where loss of cross-site GET navigation is acceptable.
• __Host- prefix — binds the cookie to the exact origin (no Domain attribute, Path must be /, Secure must be set). Prevents subdomain takeover from forging a session cookie.
• Path=/ — required by __Host-.
• Max-Age — finite lifetime so a stolen cookie isn't useful forever. Pair with server-side session rotation.

Session Lookup as Middleware

Use middleware to centralize session loading so every protected handler sees a typed session:

// src/server/auth-middleware.ts
import { createMiddleware } from '@tanstack/react-start'
import { readSessionToken } from './session'

export const authMiddleware = createMiddleware({ type: 'function' }).server(
  async ({ next }) => {
    const token = readSessionToken()
    const session = token ? await db.sessions.findValid(token) : null
    if (!session) throw new Error('Unauthorized')
    return next({ context: { session } })
  },
)

Attach it to every server function that needs a logged-in user:

import { createServerFn } from '@tanstack/react-start'
import { authMiddleware } from '~/server/auth-middleware'

export const getMyOrders = createServerFn({ method: 'GET' })
  .middleware([authMiddleware])
  .handler(async ({ context }) => {
    return db.orders.findMany({ where: { userId: context.session.userId } })
  })

Route guards do not cover this. A createFileRoute('/_authenticated/orders') with a beforeLoad redirect does NOT protect getMyOrders — the RPC is reachable via direct POST whether or not the user ever hits the route. Apply authMiddleware (or re-check inside .handler()) on every server function that needs auth.

Issuing a Session on Login

// src/server/login.functions.ts
import { createServerFn } from '@tanstack/react-start'
import { z } from 'zod'
import { setSessionCookie } from './session'

export const login = createServerFn({ method: 'POST' })
  .inputValidator(z.object({ email: z.string().email(), password: z.string() }))
  .handler(async ({ data }) => {
    const user = await db.users.findByEmail(data.email)
    // Always run verifyPasswordHash — even when the user doesn't exist —
    // so the user-not-found branch takes the same time as wrong-password.
    // DUMMY_PASSWORD_HASH is a hash of any throwaway password computed once
    // at startup with the same algorithm/cost as real password hashes.
    const hashToCheck = user?.passwordHash ?? DUMMY_PASSWORD_HASH
    const passwordMatches = await verifyPasswordHash(hashToCheck, data.password)
    const ok = user != null && passwordMatches
    if (!ok) throw new Error('Invalid email or password')

    // ROTATE on privilege change: destroy any existing session, then issue fresh.
    await db.sessions.revokeAllForUser(user.id)
    const token = await db.sessions.create({ userId: user.id })
    setSessionCookie(token)
    return { ok: true }
  })

Logout

import { createServerFn } from '@tanstack/react-start'
import { authMiddleware } from '~/server/auth-middleware'
import { clearSessionCookie } from '~/server/session'

export const logout = createServerFn({ method: 'POST' })
  .middleware([authMiddleware])
  .handler(async ({ context }) => {
    await db.sessions.revoke(context.session.id)
    clearSessionCookie()
    return { ok: true }
  })

OAuth: state + PKCE

For OAuth authorization-code flow, generate a one-time state (CSRF defense) and a PKCE verifier (defense against authorization-code interception). Store both in a short-lived signed cookie keyed to this exact login attempt.

// src/server/oauth.functions.ts
import { createServerFn } from '@tanstack/react-start'
import { redirect } from '@tanstack/react-router'
import {
  getRequestHeader,
  setResponseHeader,
} from '@tanstack/react-start/server'
import crypto from 'node:crypto'

const OAUTH_STATE_COOKIE = '__Host-oauth' // expires fast; one-shot

function base64url(buf: Buffer) {
  return buf
    .toString('base64')
    .replace(/=/g, '')
    .replace(/\+/g, '-')
    .replace(/\//g, '_')
}

export const startOAuth = createServerFn({ method: 'GET' }).handler(
  async () => {
    const state = base64url(crypto.randomBytes(32))
    const verifier = base64url(crypto.randomBytes(32))
    const challenge = base64url(
      crypto.createHash('sha256').update(verifier).digest(),
    )

    setResponseHeader(
      'Set-Cookie',
      `${OAUTH_STATE_COOKIE}=${signed({ state, verifier })}; HttpOnly; Secure; SameSite=Lax; Path=/; Max-Age=600`,
    )

    throw redirect({
      href:
        `https://provider.example/authorize` +
        `?response_type=code` +
        `&client_id=${process.env.OAUTH_CLIENT_ID}` +
        `&redirect_uri=${encodeURIComponent(process.env.OAUTH_REDIRECT_URI!)}` +
        `&state=${state}` +
        `&code_challenge=${challenge}` +
        `&code_challenge_method=S256`,
    })
  },
)

In the callback handler, verify the cookie state matches the returned state and exchange the code with the verifier. If state is missing or doesn't match, abort — the request did not originate from your startOAuth.

Password Reset: defeat user enumeration

When a user requests a reset, do not let the response shape or timing reveal whether the email is registered.

import { createServerFn } from '@tanstack/react-start'
import { z } from 'zod'

export const requestPasswordReset = createServerFn({ method: 'POST' })
  .inputValidator(z.object({ email: z.string().email() }))
  .handler(async ({ data }) => {
    const user = await db.users.findByEmail(data.email)
    if (user) {
      const token = await db.passwordResets.issue(user.id)
      await sendResetEmail(user.email, token)
    }
    // Always 200, always the same body, regardless of whether the user exists.
    // The user is told to check their inbox; no confirmation either way.
    return { ok: true }
  })

Do NOT:

• Return 200 if exists, 404 if not.
• Use a different message ("we sent you a link" vs "no account found").
• Skip the work when the user doesn't exist (timing leak — measurable from the wire).

CSRF for non-GET RPCs

SameSite=Lax on the session cookie blocks most cross-site CSRF for POST/PUT/DELETE. Two cases need extra defense:

1. Top-level GET navigation that mutates — never do this. Always use POST/PUT/DELETE for mutations.
2. POST from a page on a sibling subdomain — SameSite=Lax does NOT block this; verify the Origin header matches your app's origin in middleware.

import { createMiddleware } from '@tanstack/react-start'
import { getRequest } from '@tanstack/react-start/server'

export const csrfMiddleware = createMiddleware().server(async ({ next }) => {
  const request = getRequest()
  if (request.method !== 'GET' && request.method !== 'HEAD') {
    const origin = request.headers.get('origin')
    // Compare the FULL origin (scheme + host + port) — host alone lets
    // http://example.com pass a check meant for https://example.com.
    if (!origin || new URL(origin).origin !== process.env.APP_ORIGIN) {
      throw new Error('Origin check failed')
    }
  }
  return next()
})

Attach this to global request middleware in src/start.ts so it covers every non-GET request, including server routes and SSR.

Rate Limiting Auth Endpoints

A login endpoint without rate limiting is a credential-stuffing target. Limit per-IP (and ideally per-account) with a sliding window.

import { createMiddleware } from '@tanstack/react-start'
import { getRequest } from '@tanstack/react-start/server'

function rateLimitMiddleware(opts: {
  key: string
  max: number
  windowMs: number
}) {
  return createMiddleware().server(async ({ next }) => {
    const request = getRequest()
    const ip =
      request.headers.get('cf-connecting-ip') ??
      request.headers.get('x-forwarded-for')?.split(',')[0] ??
      'unknown'
    const bucketKey = `rl:${opts.key}:${ip}`
    const allowed = await rateLimiter.consume(
      bucketKey,
      opts.max,
      opts.windowMs,
    )
    if (!allowed) throw new Error('Too many requests')
    return next()
  })
}

// On the login server function:
export const login = createServerFn({ method: 'POST' }).middleware([
  rateLimitMiddleware({ key: 'login', max: 5, windowMs: 60_000 }),
])
// ...

Session Rotation on Privilege Change

Whenever the user's privileges change — login, logout, role change, password change — destroy the old session and issue a new one. This neutralizes session-fixation attacks where an attacker plants their own session ID in the victim's browser before login.

// In the login handler (already shown above): destroy any pre-login session, then create a fresh one.
await db.sessions.revokeAllForUser(user.id)
const token = await db.sessions.create({ userId: user.id })
setSessionCookie(token)

// On password change / role grant:
await db.sessions.revokeAllForUser(user.id) // destroy existing
const token = await db.sessions.create({ userId: user.id }) // issue fresh
setSessionCookie(token)

Common Mistakes

CRITICAL: Trusting the route guard for server-function auth

// WRONG — the RPC is callable directly via POST regardless of the route
export const Route = createFileRoute('/_authenticated/orders')({
  beforeLoad: ({ context }) => {
    if (!context.auth.isAuthenticated) throw redirect({ to: '/login' })
  },
})
const getMyOrders = createServerFn({ method: 'GET' }).handler(async () => {
  return db.orders.findMany() // ← anyone can hit the RPC and get all orders
})

// CORRECT — auth enforced on the handler itself
const getMyOrders = createServerFn({ method: 'GET' })
  .middleware([authMiddleware])
  .handler(async ({ context }) => {
    return db.orders.findMany({ where: { userId: context.session.userId } })
  })

CRITICAL: Treating shape validation as authorization

A parsed UUID is some workspace, not an authorized workspace.

// WRONG — UUID is well-formed but the user may not be a member
const getWorkspaceData = createServerFn({ method: 'GET' })
  .middleware([authMiddleware])
  .inputValidator(z.object({ workspaceId: z.string().uuid() }))
  .handler(async ({ context, data }) => {
    return db.workspaces.findById(data.workspaceId) // missing membership check!
  })

// CORRECT — verify the session principal has access to that workspace
const getWorkspaceData = createServerFn({ method: 'GET' })
  .middleware([authMiddleware])
  .inputValidator(z.object({ workspaceId: z.string().uuid() }))
  .handler(async ({ context, data }) => {
    const member = await db.memberships.find({
      userId: context.session.userId,
      workspaceId: data.workspaceId,
    })
    if (!member) throw new Error('Not a member of this workspace')
    return db.workspaces.findById(data.workspaceId)
  })

HIGH: Returning different responses based on email existence

Already covered above — requestPasswordReset must return the same body regardless of whether the email matches a user.

HIGH: Reading cookies/env at module scope

// WRONG — module-load time, before any request exists
const SESSION_SECRET = process.env.SESSION_SECRET
export function signSession(payload) {
  return sign(payload, SESSION_SECRET)
}

// CORRECT — read inside per-request callback
export function signSession(payload) {
  return sign(payload, process.env.SESSION_SECRET)
}

On Cloudflare Workers and other edge runtimes, the module-level read evaluates to undefined even on the server because env is injected per-request. See start-core/execution-model.

MEDIUM: Long-lived sessions with no rotation

A session token that never rotates is functionally a long-lived credential. Rotate on login, logout, password change, and role/permission change.

Cross-References

• router-core/auth-and-guards — the routing side: _authenticated layout, beforeLoad, redirect, RBAC checks.
• start-core/server-functions — how to expose RPCs (and how the route guard does NOT cover them).
• start-core/middleware — composing authMiddleware and others.
• start-core/execution-model — why module-level env/secret reads are wrong.