security: atomic compare-and-swap for TOTP replay window (#43, part 1)

The previous SELECT → compare → UPDATE sequence let two concurrent login
requests with the same valid 6-digit code both observe a stale lastTotpAt,
both pass the in-JS replay check, and both succeed. A stolen TOTP (shoulder-
surf, phishing-proxy replay) was usable twice within its 30 s window.

Replace the three callsites (login authorize, self-service enable, self-
service verify) with a shared consumeTotpWindow() helper: a single
updateMany() expresses "window unused" as a SQL WHERE clause, so Postgres'
row lock serialises concurrent writers and whichever commits second sees
count=0 and is treated as a replay.

Backup codes (ticket part 2) are tracked as follow-up work.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/api/src/lib/__tests__/totp-consume.test.ts

@@ -0,0 +1,58 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { consumeTotpWindow } from "../totp-consume.js";
+
+describe("consumeTotpWindow — atomic replay guard", () => {
+  let updateMany: ReturnType<typeof vi.fn>;
+  let db: { user: { updateMany: typeof updateMany } };
+
+  beforeEach(() => {
+    updateMany = vi.fn();
+    db = { user: { updateMany } };
+  });
+
+  it("returns true when the update affected a row", async () => {
+    updateMany.mockResolvedValue({ count: 1 });
+    await expect(consumeTotpWindow(db, "user-1")).resolves.toBe(true);
+  });
+
+  it("returns false when another concurrent request already consumed the window", async () => {
+    updateMany.mockResolvedValue({ count: 0 });
+    await expect(consumeTotpWindow(db, "user-1")).resolves.toBe(false);
+  });
+
+  it("issues a WHERE clause that only updates null or older-than-30-s rows", async () => {
+    updateMany.mockResolvedValue({ count: 1 });
+    const now = new Date("2026-04-17T12:00:30.000Z");
+    await consumeTotpWindow(db, "user-1", now);
+
+    expect(updateMany).toHaveBeenCalledTimes(1);
+    const call = updateMany.mock.calls[0]![0] as {
+      where: { id: string; OR: Array<{ lastTotpAt: unknown }> };
+      data: { lastTotpAt: Date };
+    };
+    expect(call.where.id).toBe("user-1");
+    expect(call.where.OR).toEqual([
+      { lastTotpAt: null },
+      { lastTotpAt: { lt: new Date("2026-04-17T12:00:00.000Z") } },
+    ]);
+    expect(call.data.lastTotpAt).toEqual(now);
+  });
+
+  it("simulated race: two parallel calls — exactly one wins", async () => {
+    // Model Postgres row-lock serialisation: the first updateMany to land
+    // sees count=1, the second (in the same 30-s window) sees count=0.
+    let served = 0;
+    updateMany.mockImplementation(async () => {
+      await new Promise((r) => setTimeout(r, 1));
+      return { count: served++ === 0 ? 1 : 0 };
+    });
+
+    const [a, b] = await Promise.all([
+      consumeTotpWindow(db, "user-1"),
+      consumeTotpWindow(db, "user-1"),
+    ]);
+
+    expect([a, b].sort()).toEqual([false, true]);
+    expect(updateMany).toHaveBeenCalledTimes(2);
+  });
+});

packages/api/src/lib/totp-consume.ts

@@ -0,0 +1,48 @@
+// Atomic compare-and-swap for TOTP replay-window consumption.
+//
+// The old code path was: SELECT lastTotpAt → compare in JS → UPDATE. Two
+// concurrent requests with the same valid 6-digit code both see a stale
+// (or null) lastTotpAt, both pass the in-JS check, and both succeed. A
+// stolen TOTP (shoulder-surf, phishing-proxy replay) is therefore usable
+// twice within its 30 s window — the MFA design promise is violated.
+//
+// A single `updateMany` expresses the entire precondition in SQL: the WHERE
+// clause guarantees the row has not been consumed in the last 30 s, and the
+// SET sets the new timestamp. PostgreSQL's row-level lock serialises the two
+// racing writes; whichever commits second sees rows-affected = 0 and the
+// caller treats it as a replay.
+//
+// The 30 000 ms window matches the TOTP period (RFC 6238) — codes are
+// validated with `window: 1` so adjacent periods are still accepted; the
+// anti-replay check is the tighter per-code, per-user bound.
+
+// Intentionally loose structural type — Prisma's generated signature is a
+// deeply-inferred generic that does not simplify to a friendly shape; we only
+// need updateMany() with the documented args and a `{ count }` result.
+// Keeping the internal cast isolated here means every callsite stays
+// strictly typed.
+interface TotpConsumeDb {
+  user: {
+    updateMany: (args: {
+      where: { id: string; OR: Array<{ lastTotpAt: Date | { lt: Date } | null }> };
+      data: { lastTotpAt: Date };
+    }) => Promise<{ count: number }>;
+  };
+}
+
+export async function consumeTotpWindow(
+  db: { user: { updateMany: (...args: never[]) => unknown } },
+  userId: string,
+  now: Date = new Date(),
+): Promise<boolean> {
+  const typed = db as unknown as TotpConsumeDb;
+  const windowStart = new Date(now.getTime() - 30_000);
+  const result = await typed.user.updateMany({
+    where: {
+      id: userId,
+      OR: [{ lastTotpAt: null }, { lastTotpAt: { lt: windowStart } }],
+    },
+    data: { lastTotpAt: now },
+  });
+  return result.count > 0;
+}

packages/api/src/router/user-self-service-procedure-support.ts

@@ -5,6 +5,7 @@ import { TRPCError } from "@trpc/server";
 import { z } from "zod";
 import { findUniqueOrThrow } from "../db/helpers.js";
 import { createAuditEntry } from "../lib/audit.js";
+import { consumeTotpWindow } from "../lib/totp-consume.js";
 import { totpRateLimiter } from "../middleware/rate-limit.js";
 import type { TRPCContext } from "../trpc.js";
 
@@ -235,8 +236,10 @@ export async function verifyAndEnableTotp(
     throw new TRPCError({ code: "BAD_REQUEST", message: "Invalid TOTP token." });
   }
 
-  // Replay-attack prevention: reject if the same 30-second window was already used
-  if (user.lastTotpAt != null && Date.now() - user.lastTotpAt.getTime() < 30_000) {
+  // Atomic replay-guard: single UPDATE with WHERE-guard on lastTotpAt. See
+  // packages/api/src/lib/totp-consume.ts for rationale.
+  const accepted = await consumeTotpWindow(ctx.db, user.id);
+  if (!accepted) {
     throw new TRPCError({
       code: "BAD_REQUEST",
       message: "TOTP code already used. Wait for the next code.",
@@ -245,7 +248,7 @@ export async function verifyAndEnableTotp(
 
   await (ctx.db.user.update as Function)({
     where: { id: user.id },
-    data: { totpEnabled: true, lastTotpAt: new Date() },
+    data: { totpEnabled: true },
   });
 
   void createAuditEntry({
@@ -309,17 +312,12 @@ export async function verifyTotp(
     throw new TRPCError({ code: "UNAUTHORIZED", message: "Invalid TOTP token." });
   }
 
-  // Replay-attack prevention: reject if the same 30-second window was already used
-  if (user.lastTotpAt != null && Date.now() - user.lastTotpAt.getTime() < 30_000) {
+  // Atomic replay-guard — see packages/api/src/lib/totp-consume.ts.
+  const accepted = await consumeTotpWindow(ctx.db, user.id);
+  if (!accepted) {
     throw new TRPCError({ code: "UNAUTHORIZED", message: "Invalid TOTP token." });
   }
 
-  // Record successful TOTP use to prevent replay within the same window
-  await (ctx.db.user.update as Function)({
-    where: { id: user.id },
-    data: { lastTotpAt: new Date() },
-  });
-
   return { valid: true };
 }