diff --git a/src/app/api/auth/check-user/__tests__/route.test.ts b/src/app/api/auth/check-user/__tests__/route.test.ts
index 60b90f6f9..9c1a71389 100644
--- a/src/app/api/auth/check-user/__tests__/route.test.ts
+++ b/src/app/api/auth/check-user/__tests__/route.test.ts
@@ -31,6 +31,15 @@ vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi
     .fn()
     .mockResolvedValue({ allowed: true, remaining: 30, resetAt: 0 }),
+  // v1.4.43 W13 M-4 — `check-user` now routes through the auth-surface
+  // wrapper so the trust-violation branch can swap to a tighter global
+  // bucket. Default to a clean per-IP result.
+  checkAuthSurfaceRateLimit: vi.fn().mockResolvedValue({
+    allowed: true,
+    remaining: 30,
+    resetAt: 0,
+    ip: "203.0.113.1",
+  }),
   rateLimitHeaders: vi.fn(() => ({})),
 }));
 
@@ -72,7 +81,7 @@ describe("POST /api/auth/check-user — audit-log row (M-1)", () => {
     const args = vi.mocked(auditLog).mock.calls[0];
     expect(args[0]).toBe("auth.check-user");
     expect(args[1]).toEqual({
-      ipAddress: "unknown",
+      ipAddress: "203.0.113.1",
       details: {
         branch: "not_found",
         identifier_hash: expectedHash("ghost@example.com"),
@@ -99,7 +108,7 @@ describe("POST /api/auth/check-user — audit-log row (M-1)", () => {
     expect(auditLog).toHaveBeenCalledTimes(1);
     expect(vi.mocked(auditLog).mock.calls[0][1]).toEqual({
       userId: "user-1",
-      ipAddress: "unknown",
+      ipAddress: "203.0.113.1",
       details: {
         branch: "passkey_only",
         identifier_hash: expectedHash("marc"),
diff --git a/src/app/api/auth/check-user/route.ts b/src/app/api/auth/check-user/route.ts
index 6591231b8..b0a63373a 100644
--- a/src/app/api/auth/check-user/route.ts
+++ b/src/app/api/auth/check-user/route.ts
@@ -37,12 +37,15 @@
  */
 import { z } from "zod/v4";
 import { prisma } from "@/lib/db";
-import { apiSuccess, apiError, safeJson, getClientIp } from "@/lib/api-response";
+import { apiSuccess, apiError, safeJson } from "@/lib/api-response";
 import { apiHandler } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
 import { auditLog } from "@/lib/auth/audit";
 import { hashToken } from "@/lib/auth/hmac";
-import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import {
+  checkAuthSurfaceRateLimit,
+  rateLimitHeaders,
+} from "@/lib/rate-limit";
 import { NextResponse } from "next/server";
 import type { NextRequest } from "next/server";
 
@@ -59,12 +62,14 @@ export type CheckUserBranch =
   | "exists";
 
 export const POST = apiHandler(async (request: NextRequest) => {
-  const ip = getClientIp(request) ?? "unknown";
-  const rl = await checkRateLimit(
-    `auth:check-user:${ip}`,
+  // v1.4.43 W13 M-4 — tighter shared bucket on trust-chain misconfig.
+  const rl = await checkAuthSurfaceRateLimit(
+    request,
+    "auth:check-user",
     30,
     15 * 60 * 1000,
   );
+  const ip = rl.ip ?? "unknown";
   if (!rl.allowed) {
     return NextResponse.json(
       { data: null, error: "Too many requests. Please try again later." },
diff --git a/src/app/api/auth/login/__tests__/native-token.test.ts b/src/app/api/auth/login/__tests__/native-token.test.ts
index 5597ef3f2..65f2f809e 100644
--- a/src/app/api/auth/login/__tests__/native-token.test.ts
+++ b/src/app/api/auth/login/__tests__/native-token.test.ts
@@ -28,6 +28,15 @@ vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi
     .fn()
     .mockResolvedValue({ allowed: true, remaining: 5, reset: 0 }),
+  // v1.4.43 W13 M-4 — auth surfaces now route through the wrapper that
+  // returns `{ip, ...rate-limit-result}`. Default to a clean per-IP
+  // result so the existing tests don't need to set every IP up.
+  checkAuthSurfaceRateLimit: vi.fn().mockResolvedValue({
+    allowed: true,
+    remaining: 5,
+    resetAt: 0,
+    ip: "203.0.113.1",
+  }),
   rateLimitHeaders: vi.fn(() => ({})),
 }));
 
@@ -55,7 +64,11 @@ vi.mock("next/headers", () => ({
 import { POST } from "../route";
 import { prisma } from "@/lib/db";
 import { verifyPassword } from "@/lib/auth/password";
-import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import {
+  checkRateLimit,
+  checkAuthSurfaceRateLimit,
+  rateLimitHeaders,
+} from "@/lib/rate-limit";
 import { ensureDbCompatibility } from "@/lib/db-compat";
 import { auditLog } from "@/lib/auth/audit";
 import { createSession } from "@/lib/auth/session";
@@ -94,6 +107,12 @@ beforeEach(() => {
     remaining: 5,
     reset: 0,
   } as never);
+  vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValue({
+    allowed: true,
+    remaining: 5,
+    resetAt: 0,
+    ip: "203.0.113.1",
+  } as never);
   vi.mocked(rateLimitHeaders).mockReturnValue({} as never);
   vi.mocked(ensureDbCompatibility).mockResolvedValue(undefined);
   vi.mocked(auditLog).mockResolvedValue(undefined as never);
diff --git a/src/app/api/auth/login/route.ts b/src/app/api/auth/login/route.ts
index 21108ffa6..49619ded4 100644
--- a/src/app/api/auth/login/route.ts
+++ b/src/app/api/auth/login/route.ts
@@ -6,10 +6,9 @@ import { auditLog } from "@/lib/auth/audit";
 import {
   apiSuccess,
   apiError,
-  getClientIp,
   safeJson,
 } from "@/lib/api-response";
-import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import { checkAuthSurfaceRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
 import { ensureDbCompatibility } from "@/lib/db-compat";
 import { NextRequest, NextResponse } from "next/server";
 import { apiHandler } from "@/lib/api-handler";
@@ -22,8 +21,16 @@ import {
 import { issueAccessAndRefresh } from "@/lib/auth/refresh-token";
 
 export const POST = apiHandler(async (request: NextRequest) => {
-  const ip = getClientIp(request) ?? "unknown";
-  const rl = await checkRateLimit(`auth:login:${ip}`, 5, 15 * 60 * 1000);
+  // v1.4.43 W13 M-4 — `checkAuthSurfaceRateLimit` swaps to a tighter
+  // global bucket when the trust chain is misconfigured; otherwise it
+  // is byte-equivalent to the previous per-IP `auth:login:{ip}` key.
+  const rl = await checkAuthSurfaceRateLimit(
+    request,
+    "auth:login",
+    5,
+    15 * 60 * 1000,
+  );
+  const ip = rl.ip ?? "unknown";
   if (!rl.allowed) {
     return NextResponse.json(
       {
diff --git a/src/app/api/auth/passkey/login-options/route.ts b/src/app/api/auth/passkey/login-options/route.ts
index c50cd0f2a..8a5342ed3 100644
--- a/src/app/api/auth/passkey/login-options/route.ts
+++ b/src/app/api/auth/passkey/login-options/route.ts
@@ -1,14 +1,18 @@
 import { createAuthenticationOptions } from "@/lib/auth/passkey";
-import { apiSuccess, getClientIp } from "@/lib/api-response";
-import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import { apiSuccess } from "@/lib/api-response";
+import {
+  checkAuthSurfaceRateLimit,
+  rateLimitHeaders,
+} from "@/lib/rate-limit";
 import { NextResponse } from "next/server";
 import { apiHandler } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
 
 export const POST = apiHandler(async (request: Request) => {
-  const ip = getClientIp(request) ?? "unknown";
-  const rl = await checkRateLimit(
-    `auth:passkey-login-options:${ip}`,
+  // v1.4.43 W13 M-4 — tighter shared bucket on trust-chain misconfig.
+  const rl = await checkAuthSurfaceRateLimit(
+    request,
+    "auth:passkey-login-options",
     10,
     15 * 60 * 1000,
   );
diff --git a/src/app/api/auth/passkey/login-verify/__tests__/native-token.test.ts b/src/app/api/auth/passkey/login-verify/__tests__/native-token.test.ts
index b4f5f4030..18fad6ff4 100644
--- a/src/app/api/auth/passkey/login-verify/__tests__/native-token.test.ts
+++ b/src/app/api/auth/passkey/login-verify/__tests__/native-token.test.ts
@@ -24,6 +24,9 @@ vi.mock("@/lib/auth/audit", () => ({
 
 vi.mock("@/lib/rate-limit", () => ({
   checkRateLimit: vi.fn(),
+  // v1.4.43 W13 M-4 — passkey/login-verify now uses the auth-surface
+  // wrapper. Default to a clean per-IP result.
+  checkAuthSurfaceRateLimit: vi.fn(),
 }));
 
 vi.mock("@/lib/db-compat", () => ({
@@ -50,7 +53,10 @@ vi.mock("next/headers", () => ({
 import { POST } from "../route";
 import { prisma } from "@/lib/db";
 import { verifyAuthentication } from "@/lib/auth/passkey";
-import { checkRateLimit } from "@/lib/rate-limit";
+import {
+  checkRateLimit,
+  checkAuthSurfaceRateLimit,
+} from "@/lib/rate-limit";
 
 const FAKE_USER = {
   id: "user-1",
@@ -81,6 +87,12 @@ beforeEach(() => {
     remaining: 9,
     reset: 0,
   } as never);
+  vi.mocked(checkAuthSurfaceRateLimit).mockResolvedValue({
+    allowed: true,
+    remaining: 9,
+    resetAt: 0,
+    ip: "203.0.113.1",
+  } as never);
   vi.mocked(verifyAuthentication).mockResolvedValue({
     verification: { verified: true },
     passkey: { userId: "user-1" },
diff --git a/src/app/api/auth/passkey/login-verify/route.ts b/src/app/api/auth/passkey/login-verify/route.ts
index bac870ed7..041cf3b90 100644
--- a/src/app/api/auth/passkey/login-verify/route.ts
+++ b/src/app/api/auth/passkey/login-verify/route.ts
@@ -4,12 +4,11 @@ import { auditLog } from "@/lib/auth/audit";
 import {
   apiSuccess,
   apiError,
-  getClientIp,
   safeJson,
 } from "@/lib/api-response";
 import { prisma } from "@/lib/db";
 import { ensureDbCompatibility } from "@/lib/db-compat";
-import { checkRateLimit } from "@/lib/rate-limit";
+import { checkAuthSurfaceRateLimit } from "@/lib/rate-limit";
 import { NextRequest } from "next/server";
 import { apiHandler } from "@/lib/api-handler";
 import { annotate } from "@/lib/logging/context";
@@ -23,12 +22,14 @@ import { issueAccessAndRefresh } from "@/lib/auth/refresh-token";
 export const POST = apiHandler(async (request: NextRequest) => {
   await ensureDbCompatibility();
 
-  const ip = getClientIp(request);
-  const rl = await checkRateLimit(
-    `auth:passkey-verify:${ip}`,
+  // v1.4.43 W13 M-4 — tighter shared bucket on trust-chain misconfig.
+  const rl = await checkAuthSurfaceRateLimit(
+    request,
+    "auth:passkey-verify",
     10,
     15 * 60 * 1000,
   );
+  const ip = rl.ip;
   if (!rl.allowed) {
     return apiError("Too many attempts. Please wait 15 minutes.", 429);
   }
diff --git a/src/app/api/auth/refresh/route.ts b/src/app/api/auth/refresh/route.ts
index be698f106..cb3256813 100644
--- a/src/app/api/auth/refresh/route.ts
+++ b/src/app/api/auth/refresh/route.ts
@@ -7,12 +7,11 @@ import { apiHandler } from "@/lib/api-handler";
 import {
   apiSuccess,
   apiError,
-  getClientIp,
   safeJson,
 } from "@/lib/api-response";
 import { auditLog } from "@/lib/auth/audit";
 import { annotate } from "@/lib/logging/context";
-import { checkRateLimit } from "@/lib/rate-limit";
+import { checkAuthSurfaceRateLimit } from "@/lib/rate-limit";
 import { resolveTokenPolicy } from "@/lib/auth/native-client";
 import {
   rotateRefreshToken,
@@ -20,8 +19,14 @@ import {
 } from "@/lib/auth/refresh-token";
 
 export const POST = apiHandler(async (request: NextRequest) => {
-  const ip = getClientIp(request) ?? "unknown";
-  const rl = await checkRateLimit(`auth:refresh:${ip}`, 60, 15 * 60 * 1000);
+  // v1.4.43 W13 M-4 — tighter shared bucket on trust-chain misconfig.
+  const rl = await checkAuthSurfaceRateLimit(
+    request,
+    "auth:refresh",
+    60,
+    15 * 60 * 1000,
+  );
+  const ip = rl.ip ?? "unknown";
   if (!rl.allowed) {
     return apiError("Too many refresh attempts. Please retry later.", 429);
   }
diff --git a/src/app/api/auth/register/route.ts b/src/app/api/auth/register/route.ts
index a4d35ef90..26cfbd608 100644
--- a/src/app/api/auth/register/route.ts
+++ b/src/app/api/auth/register/route.ts
@@ -6,10 +6,9 @@ import { auditLog } from "@/lib/auth/audit";
 import {
   apiSuccess,
   apiError,
-  getClientIp,
   safeJson,
 } from "@/lib/api-response";
-import { checkRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
+import { checkAuthSurfaceRateLimit, rateLimitHeaders } from "@/lib/rate-limit";
 import { ensureDbCompatibility } from "@/lib/db-compat";
 import { NextRequest, NextResponse } from "next/server";
 import { apiHandler } from "@/lib/api-handler";
@@ -21,8 +20,15 @@ import {
 } from "@/lib/tz/resolver";
 
 export const POST = apiHandler(async (request: NextRequest) => {
-  const ip = getClientIp(request) ?? "unknown";
-  const rl = await checkRateLimit(`auth:register:${ip}`, 5, 15 * 60 * 1000);
+  // v1.4.43 W13 M-4 — tighten to a global bucket when the trust chain
+  // is misconfigured; otherwise byte-equivalent per-IP semantics.
+  const rl = await checkAuthSurfaceRateLimit(
+    request,
+    "auth:register",
+    5,
+    15 * 60 * 1000,
+  );
+  const ip = rl.ip ?? "unknown";
   if (!rl.allowed) {
     return NextResponse.json(
       {
diff --git a/src/lib/__tests__/rate-limit-auth-surface.test.ts b/src/lib/__tests__/rate-limit-auth-surface.test.ts
new file mode 100644
index 000000000..25558cae5
--- /dev/null
+++ b/src/lib/__tests__/rate-limit-auth-surface.test.ts
@@ -0,0 +1,213 @@
+/**
+ * v1.4.43 W13 M-4 — `checkAuthSurfaceRateLimit` is the wrapper every
+ * anonymous auth surface routes through. It guards against the
+ * trust-chain misconfiguration documented in `api-response.ts`:
+ *
+ *   - Clean chain → per-IP bucket `{prefix}:{ip}` with the caller's limit.
+ *   - Broken chain (`trustViolation === true`) → tight global bucket
+ *     `auth:anon:trust-violation` shared across every auth surface,
+ *     capped at 100/15min total. One attacker can no longer exhaust the
+ *     per-surface "unknown" bucket and lock every other anonymous caller
+ *     out of login + register + passkey-verify + …
+ */
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+
+import {
+  checkRateLimit as realCheckRateLimit,
+  checkAuthSurfaceRateLimit,
+} from "../rate-limit";
+import { _resetTrustViolationWarningForTests } from "../api-response";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    $queryRaw: vi.fn(),
+  },
+}));
+
+import { prisma } from "@/lib/db";
+
+const ORIGINAL_HOPS = process.env.TRUST_PROXY_HOPS;
+
+beforeEach(() => {
+  vi.mocked(prisma.$queryRaw).mockReset();
+  vi.mocked(prisma.$queryRaw).mockResolvedValue([
+    { count: 1, reset_at: new Date(Date.now() + 60_000) },
+  ] as never);
+  _resetTrustViolationWarningForTests();
+  delete process.env.TRUST_PROXY_HOPS;
+});
+
+afterEach(() => {
+  if (ORIGINAL_HOPS === undefined) delete process.env.TRUST_PROXY_HOPS;
+  else process.env.TRUST_PROXY_HOPS = ORIGINAL_HOPS;
+});
+
+function makeRequest(headers: Record<string, string>): Request {
+  return new Request("https://example.com/api/auth/login", { headers });
+}
+
+/**
+ * Pull the rate-limit key passed to `prisma.$queryRaw` by walking the
+ * tagged-template args. `$queryRaw\`INSERT … VALUES (${key}, …)\`` lands
+ * as `[stringsArray, key, windowInterval]`. The key is the first
+ * substitution argument.
+ */
+function lastKey(): string {
+  const calls = vi.mocked(prisma.$queryRaw).mock.calls;
+  const last = calls[calls.length - 1];
+  // First substitution is the bucket key — index 1 of the call arguments.
+  return last[1] as string;
+}
+
+/**
+ * Pull the limit passed to the underlying `checkRateLimit`. The wrapper
+ * itself doesn't expose it directly; we re-derive by counting the calls
+ * with the tight key and checking that the returned `allowed` flag flips
+ * at the documented 100/15min cap below.
+ */
+function lastWindowMs(): string {
+  const calls = vi.mocked(prisma.$queryRaw).mock.calls;
+  const last = calls[calls.length - 1];
+  return last[2] as string;
+}
+
+describe("checkAuthSurfaceRateLimit — per-IP path", () => {
+  it("routes to {prefix}:{ip} when the trust chain is clean", async () => {
+    // Default TRUST_PROXY_HOPS=1, single XFF entry → clean chain.
+    const result = await checkAuthSurfaceRateLimit(
+      makeRequest({ "x-forwarded-for": "203.0.113.5" }),
+      "auth:login",
+      5,
+      15 * 60 * 1000,
+    );
+    expect(result.ip).toBe("203.0.113.5");
+    expect(result.allowed).toBe(true);
+    expect(lastKey()).toBe("auth:login:203.0.113.5");
+    expect(lastWindowMs()).toBe(`${15 * 60 * 1000} milliseconds`);
+  });
+
+  it("falls back to {prefix}:unknown when getClientIp returns null but trustViolation is false", async () => {
+    // No XFF + no x-real-ip + no CF → null ip, no violation (no chain to violate).
+    const result = await checkAuthSurfaceRateLimit(
+      makeRequest({}),
+      "auth:register",
+      5,
+      15 * 60 * 1000,
+    );
+    expect(result.ip).toBeNull();
+    expect(lastKey()).toBe("auth:register:unknown");
+  });
+
+  it("returns the underlying RateLimitResult plus the resolved ip", async () => {
+    vi.mocked(prisma.$queryRaw).mockResolvedValueOnce([
+      { count: 6, reset_at: new Date(Date.now() + 60_000) },
+    ] as never);
+    const result = await checkAuthSurfaceRateLimit(
+      makeRequest({ "x-forwarded-for": "203.0.113.5" }),
+      "auth:login",
+      5,
+      15 * 60 * 1000,
+    );
+    expect(result.allowed).toBe(false);
+    expect(result.remaining).toBe(0);
+    expect(result.ip).toBe("203.0.113.5");
+  });
+});
+
+describe("checkAuthSurfaceRateLimit — trust-violation tightening", () => {
+  beforeEach(() => {
+    // TRUST_PROXY_HOPS=2 with a 1-entry chain → trust violation.
+    process.env.TRUST_PROXY_HOPS = "2";
+  });
+
+  it("routes every login attempt under a violation into the global bucket", async () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      await checkAuthSurfaceRateLimit(
+        makeRequest({ "x-forwarded-for": "5.6.7.8" }),
+        "auth:login",
+        5,
+        15 * 60 * 1000,
+      );
+      expect(lastKey()).toBe("auth:anon:trust-violation");
+      // Window matches the 15-minute global cap, NOT the caller's window.
+      expect(lastWindowMs()).toBe(`${15 * 60 * 1000} milliseconds`);
+    } finally {
+      warn.mockRestore();
+    }
+  });
+
+  it("routes every auth surface (login + register + passkey-*) into the SAME bucket under violation", async () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      const surfaces: Array<[string, number]> = [
+        ["auth:login", 5],
+        ["auth:register", 5],
+        ["auth:passkey-login-options", 10],
+        ["auth:passkey-verify", 10],
+        ["auth:refresh", 60],
+        ["auth:check-user", 30],
+      ];
+      const seenKeys = new Set<string>();
+      for (const [prefix, limit] of surfaces) {
+        await checkAuthSurfaceRateLimit(
+          makeRequest({ "x-forwarded-for": "5.6.7.8" }),
+          prefix,
+          limit,
+          15 * 60 * 1000,
+        );
+        seenKeys.add(lastKey());
+      }
+      // Every surface collapsed to the single tight bucket.
+      expect(seenKeys).toEqual(new Set(["auth:anon:trust-violation"]));
+    } finally {
+      warn.mockRestore();
+    }
+  });
+
+  it("returns ip from the helper even when the tightened bucket fires (so call sites can audit-log the same IP)", async () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      const result = await checkAuthSurfaceRateLimit(
+        makeRequest({
+          "x-forwarded-for": "5.6.7.8",
+          "x-real-ip": "9.9.9.9",
+        }),
+        "auth:login",
+        5,
+        15 * 60 * 1000,
+      );
+      // Falls through to x-real-ip when the chain is too short.
+      expect(result.ip).toBe("9.9.9.9");
+      expect(lastKey()).toBe("auth:anon:trust-violation");
+    } finally {
+      warn.mockRestore();
+    }
+  });
+
+  it("flips to denied once the global cap is hit (count > 100)", async () => {
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+    try {
+      vi.mocked(prisma.$queryRaw).mockResolvedValueOnce([
+        { count: 101, reset_at: new Date(Date.now() + 60_000) },
+      ] as never);
+      const result = await checkAuthSurfaceRateLimit(
+        makeRequest({ "x-forwarded-for": "5.6.7.8" }),
+        "auth:login",
+        5,
+        15 * 60 * 1000,
+      );
+      expect(result.allowed).toBe(false);
+      expect(lastKey()).toBe("auth:anon:trust-violation");
+    } finally {
+      warn.mockRestore();
+    }
+  });
+});
+
+describe("checkRateLimit (existing semantics — regression guard)", () => {
+  it("still creates the per-key bucket via the unchanged SQL path", async () => {
+    await realCheckRateLimit("custom:key:1.2.3.4", 5, 60_000);
+    expect(lastKey()).toBe("custom:key:1.2.3.4");
+  });
+});
diff --git a/src/lib/rate-limit.ts b/src/lib/rate-limit.ts
index e336099b0..4ee45cac4 100644
--- a/src/lib/rate-limit.ts
+++ b/src/lib/rate-limit.ts
@@ -4,6 +4,7 @@
  */
 
 import { prisma } from "@/lib/db";
+import { getClientIpOrTrustWarning } from "@/lib/api-response";
 
 export interface RateLimitResult {
   allowed: boolean;
@@ -62,3 +63,65 @@ export function rateLimitHeaders(
     "X-RateLimit-Reset": new Date(result.resetAt).toISOString(),
   };
 }
+
+/**
+ * v1.4.43 W13 M-4 — anonymous/trust-violation bucket key shared by every
+ * caller of `checkAuthSurfaceRateLimit`. When `TRUST_PROXY_HOPS` and the
+ * actual proxy chain don't agree, `getClientIp` returns null and every
+ * caller falls back to a literal `"unknown"` rate-limit bucket. Any
+ * attacker can then exhaust the `auth:login:unknown` bucket and lock
+ * every other legitimate anonymous caller out of the surface.
+ *
+ * The tighter bucket is shared across every auth surface — a misconfigured
+ * deployment with the trust chain broken accepts at most this many
+ * anonymous attempts across login + register + passkey-verify + … per
+ * window. Operators see the once-per-process `console.warn` from
+ * `getClientIpOrTrustWarning` and fix the proxy chain; the tight bucket
+ * caps the blast radius while they do.
+ */
+const TIGHT_ANON_KEY = "auth:anon:trust-violation";
+const TIGHT_ANON_LIMIT = 100;
+const TIGHT_ANON_WINDOW_MS = 15 * 60 * 1000;
+
+/**
+ * Rate-limit helper for anonymous auth surfaces (login, register,
+ * passkey-login-{options,verify}, refresh, check-user).
+ *
+ * Per-IP path: when the trust chain resolves cleanly, route the request
+ * to a bucket keyed by `{prefix}:{ip ?? "unknown"}` with the caller's
+ * supplied limit + window. Existing semantics, byte-for-byte.
+ *
+ * Tightened path: when `getClientIpOrTrustWarning` reports
+ * `trustViolation === true`, the configured proxy hops and the actual
+ * chain length don't match. Every anonymous caller already shares a
+ * single `"unknown"` bucket on the per-IP path; this helper instead
+ * routes them to a fixed global bucket with a strict 100/15min cap.
+ * One attacker can no longer exhaust the per-surface bucket — they hit
+ * the global cap first, and every other anonymous caller on any auth
+ * surface shares the same cap.
+ *
+ * The returned shape matches `checkRateLimit` so call sites can attach
+ * `rateLimitHeaders(rl)` unchanged.
+ */
+export async function checkAuthSurfaceRateLimit(
+  request: Request,
+  prefix: string,
+  perIpLimit: number,
+  windowMs: number,
+): Promise<RateLimitResult & { ip: string | null }> {
+  const { ip, trustViolation } = getClientIpOrTrustWarning(request);
+  if (trustViolation) {
+    const result = await checkRateLimit(
+      TIGHT_ANON_KEY,
+      TIGHT_ANON_LIMIT,
+      TIGHT_ANON_WINDOW_MS,
+    );
+    return { ...result, ip };
+  }
+  const result = await checkRateLimit(
+    `${prefix}:${ip ?? "unknown"}`,
+    perIpLimit,
+    windowMs,
+  );
+  return { ...result, ip };
+}