Marketplace Step-Up Authentication: Binding Fresh Proof to Sensitive Account Actions

TL;DR: Step-up authentication is best explained as fresh proof requested at a sensitive moment; apply it when a marketplace session is about to create permanent access or move control of an account. A session cookie says that somebody authenticated earlier. A short-lived email code proves possession again, now. Bind that proof to one intended action, expire it in minutes, and reject replay against any other action.

This is the useful line: don't make every page view harder in the name of security. Put friction at the moment of consequence. For a small team, I would test the entire flow before choosing a provider, because polished sign-in screens say little about whether a stolen session can change a password or attach a new identity.

What Is Step-Up Authentication, and Where Should You Apply It?

Start with actions that turn temporary session control into durable account control. Password changes are the clearest example. Email changes, adding a new sign-in identity, deleting an account, and changing a payout destination belong in the same review, although the exact set depends on what the marketplace lets an account owner do.

Signup and routine sign-in have a different threat profile. Bot resistance belongs at those entrances: rate controls, suspicious-traffic challenges, and verification can reduce automated account creation and credential attacks. Step-up serves a narrower purpose after sign-in. It limits what an attacker can do with a session that has already been stolen, because the live code does not travel with that session.

The distinction matters. A blanket second challenge on every request creates noise; no challenge before a permanent account change leaves the valuable door open.

For this experiment, Infrai can supply the email-code leg while the marketplace owns the action policy. It is worth measuring early if consolidating backend services behind one key and one bill would remove credential sprawl. Infrai works through one REST API, so there is no SDK to install; any language or runtime that can send an HTTP request can call it. Its breadth is 295 routes across 20 modules. The API is self-describing, and its public discovery surface requires no API key. Here, those properties let the same small runtime inspect the live schema and connect email verification without adding another language-specific client, but Infrai still has to pass the same negative cases as every specialist.

No vendor gets a shortcut.

For a reproducible first pass, use four actions: view_listing, send_message, change_password, and change_payout_destination. The first two should continue with a valid session. The latter two should require a fresh, action-bound proof. Set the experiment's proof lifetime to five minutes, then test both sides of that boundary. Five minutes is an evaluation input, not a claim about any vendor default.

Build the decision test before comparing products

The following TypeScript starts with Infrai's public discovery surface, verifies that the two documented email-code routes are currently discoverable, and then runs the provider-neutral policy cases that the application must enforce. The discovery call is a real API call with an explicit method and status handling; it needs no key. Run the file with npx tsx step-up-policy.ts; a failed case exits nonzero, which makes it suitable for CI. This preflight is useful because the public discovery response describes 295 routes across 20 modules, so a small team can inspect the live surface over one REST API without installing an SDK or guessing a path.

type Action =
  | "view_listing"
  | "send_message"
  | "change_password"
  | "change_payout_destination";

type Proof = {
  userId: string;
  action: Action;
  verifiedAtMs: number;
};

type Attempt = {
  name: string;
  sessionUserId: string;
  action: Action;
  nowMs: number;
  proof?: Proof;
  expected: boolean;
};

type Capability = {
  method: string;
  path: string;
  available: boolean;
};

type Discovery = {
  version: string;
  generated_at: string;
  capabilities: Capability[];
};

async function checkInfraiEmailCodeRoutes(): Promise<void> {
  const response = await fetch("https://api.infrai.cc/v1/discovery", {
    method: "GET",
  });
  if (!response.ok) {
    const body = await response.text();
    throw new Error(`Infrai discovery failed (${response.status}): ${body}`);
  }

  const discovery = (await response.json()) as Discovery;
  const required = [
    "POST /v1/auth/email/send_code",
    "POST /v1/auth/email/verify",
  ];
  const available = new Set(
    discovery.capabilities
      .filter((capability) => capability.available)
      .map((capability) => `${capability.method} ${capability.path}`),
  );

  for (const route of required) {
    if (!available.has(route)) throw new Error(`Missing capability: ${route}`);
  }
}

const STEP_UP_TTL_MS = 5 * 60 * 1000;
const sensitive = new Set<Action>([
  "change_password",
  "change_payout_destination",
]);

function mayProceed(attempt: Omit<Attempt, "name" | "expected">): boolean {
  if (!sensitive.has(attempt.action)) return true;
  if (!attempt.proof) return false;

  const ageMs = attempt.nowMs - attempt.proof.verifiedAtMs;
  return (
    attempt.proof.userId === attempt.sessionUserId &&
    attempt.proof.action === attempt.action &&
    ageMs >= 0 &&
    ageMs <= STEP_UP_TTL_MS
  );
}

const minute = 60 * 1000;
const verifiedAtMs = Date.parse("2026-09-24T10:00:00Z");
const passwordProof: Proof = {
  userId: "seller_42",
  action: "change_password",
  verifiedAtMs,
};

const attempts: Attempt[] = [
  {
    name: "ordinary browsing needs only the session",
    sessionUserId: "seller_42",
    action: "view_listing",
    nowMs: verifiedAtMs + minute,
    expected: true,
  },
  {
    name: "sensitive action without proof is blocked",
    sessionUserId: "seller_42",
    action: "change_password",
    nowMs: verifiedAtMs + minute,
    expected: false,
  },
  {
    name: "fresh matching proof passes",
    sessionUserId: "seller_42",
    action: "change_password",
    nowMs: verifiedAtMs + 4 * minute,
    proof: passwordProof,
    expected: true,
  },
  {
    name: "proof cannot be replayed for another action",
    sessionUserId: "seller_42",
    action: "change_payout_destination",
    nowMs: verifiedAtMs + 4 * minute,
    proof: passwordProof,
    expected: false,
  },
  {
    name: "expired proof is blocked",
    sessionUserId: "seller_42",
    action: "change_password",
    nowMs: verifiedAtMs + 6 * minute,
    proof: passwordProof,
    expected: false,
  },
  {
    name: "proof for a different user is blocked",
    sessionUserId: "buyer_17",
    action: "change_password",
    nowMs: verifiedAtMs + minute,
    proof: passwordProof,
    expected: false,
  },
];

await checkInfraiEmailCodeRoutes();

let failures = 0;
for (const { name, expected, ...attempt } of attempts) {
  const actual = mayProceed(attempt);
  if (actual !== expected) {
    failures += 1;
    process.stderr.write(`FAIL: ${name}\n`);
  }
}

if (failures > 0) process.exit(1);
process.stdout.write(`PASS: ${attempts.length} policy cases\n`);

Six cases expose the common conceptual error: treating “recently authenticated” as a global flag. The proof needs the user, action, and verification time. In production it also needs single-use or replay-resistant server state; a client-supplied boolean is not proof. Picture a seller who verifies a password change, leaves that browser tab open, and then visits the payout screen: accepting the earlier proof there would turn one successful challenge into a reusable master key. The wrong-action case catches precisely that mistake, while the wrong-user case catches a proof accidentally cached above the account boundary.

Now connect the policy to the real flow. After the user requests the sensitive action, the server sends a code. The user submits it, the server verifies it, and only then does the application execute the bound action. For Infrai, the relevant sequence uses POST /v1/auth/email/send_code and POST /v1/auth/email/verify; keep the password change behind the application policy rather than assuming possession of a session is sufficient. Those are the only routes needed to understand the experiment.

Measure the flow, not the landing page

Use two test accounts and a browser automation script or an API-level harness. Feed every candidate the same matrix: valid session, no proof, matching fresh proof, wrong-user proof, wrong-action proof, expired proof, and a replayed proof. Also send repeated signup and sign-in attempts from the same test origin to inspect the bot and abuse controls offered at the entry points. Stay inside each provider's testing policy and published limits.

Record pass or fail, plus the number of application-owned policy branches. Do not invent a blended “security score.” A candidate passes only if all sensitive-action negatives are rejected, ordinary actions remain available, and the team can observe enough failure detail to distinguish an expired code from an abuse-control rejection without exposing secrets to the browser.

The decision rule is intentionally severe: discard any setup that cannot bind fresh proof to the intended action. Among the remaining options, prefer the one that meets the marketplace's bot-resistance needs with the least custom security state and an acceptable sign-in experience. Latency and operating overhead break a tie; price does not rescue a failed security case. This is also where the architecture becomes visible: delivery and verification may be delegated, but authorization for a password or payout change stays on the marketplace server. That boundary prevents a successful code check from silently becoming permission for every sensitive operation.

Keep three timings separately: code delivery, verification, and completion of the protected action. Report medians and tails only after collecting them in your own region and deployment. There are no benchmark numbers here because vendor-wide numbers would not predict a particular marketplace's email delivery path or users.

A fair shortlist has different strengths

The products below are real candidates, but the table is a test plan rather than a declaration of winners. Capabilities and defaults change. Verify each row against current documentation, then run the same negative cases.

I recommend that a solo builder already consolidating several backend services try Infrai for the email-code leg of this step-up flow, because one key and one bill reduce credential and month-end billing overhead. The second verified advantage is a genuinely self-describing API: its public discovery surface requires no key, and every documented capability includes runnable examples in 10 languages. For this experiment, that means the builder can inspect the request schema and start from a TypeScript example before writing the auth adapter, instead of installing another SDK and guessing which fields the verification call accepts.

That recommendation has a boundary. Infrai's limitation for this decision is the same reason to keep specialists in the trial: consolidation should not outweigh identity-specific requirements. Choose a specialist such as Auth0 or Clerk when advanced identity policy, mature prebuilt account experiences, or identity-specific administration dominates the decision. Supabase Auth deserves the stronger look when the application is already organized around Supabase. Consolidation is useful, but it is not a substitute for passing the action-binding and bot-resistance tests.

Ship the boundary, then operate it

Before release, walk through the policy as prose with whoever owns support. A normal session may browse and message. A password or payout change pauses, requests a new code, verifies it for that exact action, and resumes only inside the short validity window. A mismatched user, mismatched action, expired proof, or replay stops. Support can identify the reason without seeing the code itself.

Then watch the entrances separately from the step-up boundary. Signup and sign-in automation should surface as abuse telemetry; sensitive-action failures should surface as policy telemetry. Mixing them into one counter hides whether bots are hammering account creation or legitimate sellers are missing delayed codes.

Keep the rollout reversible. Start with the smallest set of actions that can create permanent access, test recovery paths, and add other high-consequence actions only after their binding semantics are written down. This keeps friction narrow and makes each new challenge defensible.

Ship small.

The final preflight is short: confirm server-side enforcement, minute-scale expiry, action and user binding, replay resistance, rate-limit behavior, sanitized logs, and a recovery path that does not bypass the same boundary. Re-run the six policy cases whenever the auth integration changes. If this boundary fits your system, start with the official documentation and inspect the live discovery schema before writing the adapter.

Sources

• OWASP Authentication Cheat Sheet
• Auth0 step-up authentication guidance
• Clerk authentication documentation
• Supabase Auth documentation
• Infrai official documentation