security: stop trusting the leftmost x-forwarded-for for rate limits and abuse quotas (FRE-371)

getClientIP (the rate-limit and audit key) took the LEFTMOST x-forwarded-for
hop and read x-real-ip unconditionally. In our topology — client → Cloudflare
→ same-host origin proxy → app — both proxies APPEND, so the leftmost hop is
whatever the sender typed: a rotating X-Forwarded-For minted a fresh rate-limit
bucket per request, and unlimited bug-report quota via the same pattern in
routes/api/the-nook/report.ts. x-real-ip is client-settable too; nothing in
the chain overwrites it. It is no longer read at all.

New trust order (src/server/trusted-client-ip.ts, resolveTrustedClientIp):

  1. a non-loopback socket peer IS the client — its headers are ignored;
  2. cf-connecting-ip (Cloudflare overwrites it at the edge; parsed as an
     address or rejected);
  3. x-forwarded-for counted from the RIGHT, discounting one hop for the
     Cloudflare edge address the origin proxy appended (the topology has
     exactly one trusted proxy below Cloudflare);
  4. "unknown" — callers key stricter shared limits on it, so resolution
     fails closed toward a tighter bucket, never a looser one.

Addresses are never truncated: a /24-truncated key would pool attackers with
the innocents behind them.

Share-vs-copy, stated deliberately: this is a port of the parsing primitives
 FRE-366 reviewed in request-client.ts, NOT an import. resolveClientIp carries
a standing rule (1d9287a) that it never feeds a security decision; lifting
that rule for these callers would couple an access-control change to an
analytics branch under review, and the analytics helper only exposes
truncated prefixes while rate limiting needs the full address. If the two
drift, trusted-client-ip.ts is the one security paths follow. The deploy-side
precondition (origin reachable only via Cloudflare + the same-host proxy) is
documented in the file header; if the app is ever directly internet-reachable
no in-app read can recover the true client.

Tests updated deliberately, not to go green: rate-limit.test.ts now pins the
rightmost-minus-one order, cf-connecting-ip precedence, peer-over-headers,
x-real-ip rejection, and the spoof case (a rotating leftmost hop does not
change the resolved client); report.test.ts pins the quota key stability
under a rotating header. New trusted-client-ip.test.ts covers the helper's
own edges: APIEvent wrapper, loopback forms, unparseable hops failing closed.

src/routes/api/the-nook/report.test.ts

@@ -10,6 +10,8 @@ import { describe, expect, it, mock, beforeEach } from "bun:test";
 
 const sentEmails: { to: string; subject: string; html: string }[] = [];
 const tokenAnswers: { allowed: boolean; retryAfterSec?: number }[] = [];
+/** Every rate-limit key the route derived, in order. */
+const rateKeys: string[] = [];
 
 mock.module("~/server/email", () => ({
   default: async (to: string, subject: string, html: string) => {
@@ -19,7 +21,10 @@ mock.module("~/server/email", () => ({
 }));
 
 mock.module("~/server/bug-report-rate-limit", () => ({
-  takeBugReportToken: () => tokenAnswers.shift() ?? { allowed: true }
+  takeBugReportToken: (key: string) => {
+    rateKeys.push(key);
+    return tokenAnswers.shift() ?? { allowed: true };
+  }
 }));
 
 const { POST } = await import("./report");
@@ -72,6 +77,7 @@ describe("POST /api/the-nook/report", () => {
   beforeEach(() => {
     sentEmails.length = 0;
     tokenAnswers.length = 0;
+    rateKeys.length = 0;
   });
 
   it("emails the owner a report containing the snapshot, escaped", async () => {
@@ -133,10 +139,49 @@ describe("POST /api/the-nook/report", () => {
 
   it("still succeeds when no fingerprint header exists", async () => {
     const res = await POST(
-      request(validPayload, { "x-forwarded-for": "203.0.113.9, 10.0.0.1" })
+      request(validPayload, {
+        "cf-connecting-ip": "203.0.113.9",
+        "x-forwarded-for": "203.0.113.9, 10.0.0.1"
+      })
     );
     expect(res.status).toBe(200);
     expect(sentEmails).toHaveLength(1);
+    // The quota key is the Cloudflare-recorded client, not a header hop the
+    // sender chose.
+    expect(rateKeys).toEqual(["203.0.113.9"]);
+  });
+
+  it("a rotating leftmost x-forwarded-for does not mint a fresh quota key", async () => {
+    // FRE-371: the old read keyed on the leftmost hop, which the sender
+    // controls outright — one scripted loop with a rotating header had
+    // unlimited reports. With the same Cloudflare-recorded client and a
+    // different spoofed prefix per request, the key must be stable.
+    for (const spoof of ["1.1.1.1", "2.2.2.2", "3.3.3.3"]) {
+      const res = await POST(
+        request(validPayload, {
+          "cf-connecting-ip": "203.0.113.9",
+          "x-forwarded-for": `${spoof}, 203.0.113.9, 10.0.0.1`
+        })
+      );
+      expect(res.status).toBe(200);
+    }
+
+    expect(rateKeys).toEqual([
+      "203.0.113.9",
+      "203.0.113.9",
+      "203.0.113.9"
+    ]);
+  });
+
+  it("keys on the rightmost-minus-one hop when Cloudflare's header is absent", async () => {
+    const res = await POST(
+      request(validPayload, {
+        "x-forwarded-for": "6.6.6.6, 203.0.113.9, 10.0.0.1"
+      })
+    );
+    expect(res.status).toBe(200);
+    // Never the leftmost (sender-chosen) hop.
+    expect(rateKeys).toEqual(["203.0.113.9"]);
   });
 
   it("delivers the landing page's beta request with only the email", async () => {

src/routes/api/the-nook/report.ts

@@ -3,6 +3,7 @@ import { z } from "zod";
 import sendEmail from "~/server/email";
 import { CONTACT_RECIPIENT_EMAIL } from "~/lib/contact-config";
 import { takeBugReportToken } from "~/server/bug-report-rate-limit";
+import { resolveTrustedClientIp } from "~/server/trusted-client-ip";
 import { json, error } from "./_lib";
 
 /**
@@ -103,10 +104,9 @@ export async function POST(event: APIEvent): Promise<Response> {
   )
     .trim()
     .toLowerCase();
-  const forwarded = event.request.headers.get("x-forwarded-for");
-  const rateKey =
-    fingerprint ||
-    (forwarded ? forwarded.split(",")[0]!.trim() : "unknown");
+  // FRE-371: the IP fallback must be the spoof-resistant read — a rotating
+  // leftmost x-forwarded-for used to mint unlimited report quota.
+  const rateKey = fingerprint || resolveTrustedClientIp(event);
   const token = takeBugReportToken(rateKey);
   if (!token.allowed) {
     return new Response(JSON.stringify({ error: "Too many reports" }), {

src/server/security.ts

@@ -3,6 +3,7 @@ import { getCookie, setCookie } from "vinxi/http";
 import type { H3Event } from "vinxi/http";
 import { t } from "~/server/api/utils";
 import { logAuditEvent } from "~/server/audit";
+import { resolveTrustedClientIp } from "~/server/trusted-client-ip";
 import { env } from "~/env/server";
 import {
   AUTH_CONFIG,
@@ -328,39 +329,15 @@ async function cleanupExpiredRateLimits(): Promise<void> {
 }
 
 /**
- * Get client IP address from request headers.
- * Only trusts X-Forwarded-For outside of local development (set by the Vercel
- * edge network in production; trusted in tests so the header-parsing path is
- * exercised). In local development, uses the socket address to prevent header
- * spoofing.
+ * Get client IP address for a security decision (rate limiting, audit).
+ *
+ * Thin alias over `resolveTrustedClientIp` (./trusted-client-ip), which owns
+ * the trust order: the previous leftmost `x-forwarded-for` read was
+ * client-spoofable in our append-only proxy topology, and `x-real-ip` was
+ * read unconditionally — both fixed there (FRE-371).
  */
 export function getClientIP(event: H3Event): string {
-  // In production on Vercel, X-Forwarded-For is set by the edge network
-  // and cannot be spoofed by clients. In dev, ignore it.
-  if (env.NODE_ENV !== "development") {
-    const forwarded = getHeaderValue(event, "x-forwarded-for");
-    if (forwarded) {
-      return forwarded.split(",")[0].trim();
-    }
-    const realIP = getHeaderValue(event, "x-real-ip");
-    if (realIP) {
-      return realIP;
-    }
-  }
-
-  // Fallback: try socket remote address
-  try {
-    const nodeReq = event.node.req;
-    if (nodeReq?.socket?.remoteAddress) {
-      const addr = nodeReq.socket.remoteAddress;
-      // Clean up IPv6-mapped IPv4 addresses
-      return addr.replace(/^::ffff:/, "");
-    }
-  } catch {
-    // socket access failed
-  }
-
-  return "unknown";
+  return resolveTrustedClientIp(event);
 }
 
 /**

src/server/security/rate-limit.test.ts

@@ -150,29 +150,100 @@ describe("Rate Limiting", () => {
   });
 
   describe("getClientIP", () => {
-    it("should extract IP from x-forwarded-for header", () => {
+    // FRE-371: the read is spoof-resistant now. In the pinned topology
+    // (client → Cloudflare → origin proxy → app) both proxies APPEND, so
+    // x-forwarded-for arrives as `<spoof…>, <true client>, <CF edge>` and the
+    // leftmost hop is whatever the sender typed. Trust order:
+    // cf-connecting-ip, then x-forwarded-for rightmost-minus-one, then the
+    // socket peer — never the leftmost hop, never x-real-ip.
+
+    it("takes x-forwarded-for rightmost-minus-one, never the leftmost hop", () => {
       const event = createMockEvent({
-        headers: { "x-forwarded-for": "192.168.1.1, 10.0.0.1" }
+        headers: {
+          "x-forwarded-for": "6.6.6.6, 192.168.1.1, 10.0.0.1"
+        }
       });
 
       const ip = getClientIP(event);
       expect(ip).toBe("192.168.1.1");
     });
 
-    it("should extract IP from x-real-ip header", () => {
+    it("prefers cf-connecting-ip (Cloudflare overwrites it at the edge)", () => {
+      const event = createMockEvent({
+        headers: {
+          "cf-connecting-ip": "203.0.113.7",
+          "x-forwarded-for": "6.6.6.6, 192.168.1.1, 10.0.0.1"
+        }
+      });
+
+      const ip = getClientIP(event);
+      expect(ip).toBe("203.0.113.7");
+    });
+
+    it("a rotating leftmost x-forwarded-for does not change the resolved client", () => {
+      // The spoof the old read fell for: same Cloudflare-recorded client,
+      // different sender-chosen prefix per request. The key must be stable,
+      // or each rotation mints a fresh rate-limit bucket.
+      const first = getClientIP(
+        createMockEvent({
+          headers: {
+            "cf-connecting-ip": "203.0.113.7",
+            "x-forwarded-for": "1.1.1.1, 203.0.113.7, 10.0.0.1"
+          }
+        })
+      );
+      const second = getClientIP(
+        createMockEvent({
+          headers: {
+            "cf-connecting-ip": "203.0.113.7",
+            "x-forwarded-for": "2.2.2.2, 203.0.113.7, 10.0.0.1"
+          }
+        })
+      );
+
+      expect(first).toBe("203.0.113.7");
+      expect(second).toBe(first);
+    });
+
+    it("ignores x-real-ip entirely (client-settable in this topology)", () => {
       const event = createMockEvent({
         headers: { "x-real-ip": "192.168.1.2" }
       });
 
       const ip = getClientIP(event);
-      expect(ip).toBe("192.168.1.2");
+      expect(ip).toBe("unknown");
     });
 
-    it("should prefer x-forwarded-for over x-real-ip", () => {
+    it("ignores a client-supplied single-hop x-forwarded-for with no trusted proxy above it", () => {
+      // One hop only = the sender's own invention; discounting the trusted
+      // hop leaves nothing to believe.
       const event = createMockEvent({
+        headers: { "x-forwarded-for": "6.6.6.6" }
+      });
+
+      const ip = getClientIP(event);
+      expect(ip).toBe("unknown");
+    });
+
+    it("a non-loopback socket peer is the client and beats every header", () => {
+      const event = createMockEvent({
+        remoteAddress: "198.51.100.23",
         headers: {
-          "x-forwarded-for": "192.168.1.1",
-          "x-real-ip": "192.168.1.2"
+          "cf-connecting-ip": "6.6.6.6",
+          "x-forwarded-for": "6.6.6.6, 192.168.1.1, 10.0.0.1",
+          "x-real-ip": "6.6.6.6"
+        }
+      });
+
+      const ip = getClientIP(event);
+      expect(ip).toBe("198.51.100.23");
+    });
+
+    it("a loopback socket peer lets the forwarded headers speak", () => {
+      const event = createMockEvent({
+        remoteAddress: "127.0.0.1",
+        headers: {
+          "x-forwarded-for": "6.6.6.6, 192.168.1.1, 10.0.0.1"
         }
       });
 
@@ -198,13 +269,23 @@ describe("Rate Limiting", () => {
     it("should handle IPv6 addresses", () => {
       const event = createMockEvent({
         headers: {
-          "x-forwarded-for": "2001:0db8:85a3:0000:0000:8a2e:0370:7334"
+          "x-forwarded-for":
+            "2001:0db8:85a3:0000:0000:8a2e:0370:7334, 10.0.0.1"
         }
       });
 
       const ip = getClientIP(event);
       expect(ip).toBe("2001:0db8:85a3:0000:0000:8a2e:0370:7334");
     });
+
+    it("rejects a candidate hop that is not an address", () => {
+      const event = createMockEvent({
+        headers: { "x-forwarded-for": "not-an-ip, 10.0.0.1" }
+      });
+
+      const ip = getClientIP(event);
+      expect(ip).toBe("unknown");
+    });
   });
 
   describe("rateLimitLogin", () => {

src/server/security/test-utils.ts

@@ -14,12 +14,15 @@ export function createMockEvent(options: {
   cookies?: Record<string, string>;
   method?: string;
   url?: string;
+  /** Socket peer the server observed; omit for a peer-less synthetic event. */
+  remoteAddress?: string;
 }): H3Event {
   const {
     headers = {},
     cookies = {},
     method = "POST",
-    url = "http://localhost:3000/"
+    url = "http://localhost:3000/",
+    remoteAddress
   } = options;
 
   // Build the cookie header string from the cookies object only
@@ -49,7 +52,10 @@ export function createMockEvent(options: {
     headers: headersObj,
     node: {
       req: {
-        headers: allHeaders
+        headers: allHeaders,
+        ...(remoteAddress
+          ? { socket: { remoteAddress } }
+          : {})
       },
       res: {
         cookies: {}

src/server/trusted-client-ip.test.ts

@@ -0,0 +1,171 @@
+import { describe, expect, it } from "bun:test";
+
+import {
+  resolveTrustedClientIp,
+  isIpAddress,
+  isLoopbackAddress,
+  UNKNOWN_CLIENT_IP,
+  type ClientIpEventLike
+} from "./trusted-client-ip";
+
+/**
+ * Unit coverage for the security-path client-IP read (FRE-371). The caller
+ * suites (rate-limit.test.ts, report.test.ts) pin the behaviour through
+ * getClientIP and the route; this file pins the helper's own edges: the peer
+ * forms a real socket can present, the APIEvent wrapper, and failing closed.
+ */
+function event(opts: {
+  headers?: Record<string, string>;
+  remoteAddress?: string;
+  wrapped?: boolean;
+}): ClientIpEventLike {
+  const inner: ClientIpEventLike = {
+    ...(opts.remoteAddress
+      ? { node: { req: { socket: { remoteAddress: opts.remoteAddress } } } }
+      : {}),
+    request: { headers: new Headers(opts.headers ?? {}) }
+  };
+  // An APIEvent wraps its H3Event as `nativeEvent` (routes/api/* shape).
+  return opts.wrapped ? { nativeEvent: inner } : inner;
+}
+
+describe("resolveTrustedClientIp", () => {
+  it("trust order: non-loopback peer, then cf-connecting-ip, then rightmost-minus-one x-forwarded-for", () => {
+    // Peer wins.
+    expect(
+      resolveTrustedClientIp(
+        event({
+          remoteAddress: "198.51.100.23",
+          headers: {
+            "cf-connecting-ip": "203.0.113.7",
+            "x-forwarded-for": "6.6.6.6, 203.0.113.7, 10.0.0.1"
+          }
+        })
+      )
+    ).toBe("198.51.100.23");
+
+    // Loopback peer → Cloudflare's record.
+    expect(
+      resolveTrustedClientIp(
+        event({
+          remoteAddress: "127.0.0.1",
+          headers: {
+            "cf-connecting-ip": "203.0.113.7",
+            "x-forwarded-for": "6.6.6.6, 203.0.113.7, 10.0.0.1"
+          }
+        })
+      )
+    ).toBe("203.0.113.7");
+
+    // No CF header → the hop below the trusted proxy.
+    expect(
+      resolveTrustedClientIp(
+        event({
+          remoteAddress: "127.0.0.1",
+          headers: { "x-forwarded-for": "6.6.6.6, 203.0.113.7, 10.0.0.1" }
+        })
+      )
+    ).toBe("203.0.113.7");
+  });
+
+  it("reads the wrapped H3Event of an APIEvent-shaped input", () => {
+    expect(
+      resolveTrustedClientIp(
+        event({
+          wrapped: true,
+          remoteAddress: "127.0.0.1",
+          headers: { "cf-connecting-ip": "203.0.113.7" }
+        })
+      )
+    ).toBe("203.0.113.7");
+  });
+
+  it("never returns a truncated address", () => {
+    // The full /128 is the bucket key; a truncated one would pool attackers
+    // with their neighbours.
+    const ip = resolveTrustedClientIp(
+      event({
+        remoteAddress: "127.0.0.1",
+        headers: { "cf-connecting-ip": "2001:0db8:85a3:0000:0000:8a2e:0370:7334" }
+      })
+    );
+    expect(ip).toBe("2001:0db8:85a3:0000:0000:8a2e:0370:7334");
+  });
+
+  it("fails closed to the stricter unknown key", () => {
+    // One hop = the sender's own invention; a non-address hop; nothing at all.
+    expect(
+      resolveTrustedClientIp(
+        event({ headers: { "x-forwarded-for": "6.6.6.6" } })
+      )
+    ).toBe(UNKNOWN_CLIENT_IP);
+    expect(
+      resolveTrustedClientIp(
+        event({ headers: { "cf-connecting-ip": "not-an-ip" } })
+      )
+    ).toBe(UNKNOWN_CLIENT_IP);
+    expect(resolveTrustedClientIp(event({}))).toBe(UNKNOWN_CLIENT_IP);
+  });
+
+  it("recognises the loopback forms a real peer presents", () => {
+    expect(
+      resolveTrustedClientIp(
+        event({
+          // IPv6-mapped IPv4 loopback from a node socket.
+          remoteAddress: "::ffff:127.0.0.1",
+          headers: { "cf-connecting-ip": "203.0.113.7" }
+        })
+      )
+    ).toBe("203.0.113.7");
+    expect(
+      resolveTrustedClientIp(
+        event({
+          remoteAddress: "::1",
+          headers: { "cf-connecting-ip": "203.0.113.7" }
+        })
+      )
+    ).toBe("203.0.113.7");
+    // A non-loopback peer that merely looks odd is still the client.
+    expect(
+      resolveTrustedClientIp(
+        event({
+          remoteAddress: "::ffff:203.0.113.7",
+          headers: { "cf-connecting-ip": "6.6.6.6" }
+        })
+      )
+    ).toBe("203.0.113.7");
+  });
+
+  it("never believes x-real-ip", () => {
+    expect(
+      resolveTrustedClientIp(
+        event({ headers: { "x-real-ip": "203.0.113.7" } })
+      )
+    ).toBe(UNKNOWN_CLIENT_IP);
+  });
+});
+
+describe("address helpers", () => {
+  it("isIpAddress", () => {
+    expect(isIpAddress("203.0.113.7")).toBe(true);
+    expect(isIpAddress("::1")).toBe(true);
+    expect(isIpAddress("::ffff:203.0.113.7")).toBe(true);
+    expect(isIpAddress("2001:db8::1")).toBe(true);
+    expect(isIpAddress("not-an-ip")).toBe(false);
+    expect(isIpAddress("256.1.1.1")).toBe(false);
+    expect(isIpAddress("")).toBe(false);
+    expect(isIpAddress(null)).toBe(false);
+    expect(isIpAddress(undefined)).toBe(false);
+  });
+
+  it("isLoopbackAddress", () => {
+    expect(isLoopbackAddress("127.0.0.1")).toBe(true);
+    expect(isLoopbackAddress("127.255.0.3")).toBe(true);
+    expect(isLoopbackAddress("::1")).toBe(true);
+    expect(isLoopbackAddress("::ffff:127.0.0.1")).toBe(true);
+    expect(isLoopbackAddress("203.0.113.7")).toBe(false);
+    expect(isLoopbackAddress("::2")).toBe(false);
+    // Unparseable input is not loopback — fails closed.
+    expect(isLoopbackAddress("garbage")).toBe(false);
+  });
+});

src/server/trusted-client-ip.ts

@@ -0,0 +1,250 @@
+/**
+ * The client address, resolved for security decisions (FRE-371).
+ *
+ * This is the ONLY client-IP read allowed to feed a security decision — rate
+ * limiting, abuse quotas, account lockout, audit trails. It exists separately
+ * from `request-client.ts`'s `resolveClientIp` (the analytics read) on
+ * purpose:
+ *
+ *   - `resolveClientIp` carries a standing rule (FRE-369 review, 1d9287a):
+ *     it never feeds a security decision. Its blast radius is a poisoned
+ *     analytics row; ours is granting or denying access.
+ *   - Rate limiting needs the UN-truncated address; the analytics module only
+ *     ever hands out truncated prefixes.
+ *   - `request-client.ts` ships on FRE-366's branch, which is under review;
+ *     a security fix must not merge-depend on an analytics diff.
+ *
+ * The address-parsing primitives below are therefore a deliberate port of the
+ * reviewed ones, not an import. If the two ever drift, this file's version is
+ * the one security paths follow.
+ *
+ * WHY the old read was spoofable: `security.ts`'s `getClientIP` took the
+ * LEFTMOST `x-forwarded-for` hop, believing the header "cannot be spoofed".
+ * In the pinned topology — client → Cloudflare → same-host origin proxy →
+ * app — both proxies *append*, so the header arrives as
+ * `<client-supplied spoof…>, <true client>, <Cloudflare edge>` and the
+ * leftmost hop is exactly the one the sender chose. Rotating
+ * `X-Forwarded-For: <random>` per request minted a fresh rate-limit bucket
+ * and unlimited bug-report quota. `x-real-ip` was read unconditionally too;
+ * nothing in this topology overwrites it, so it is equally client text and is
+ * no longer read at all.
+ *
+ * Trust order (matches the convention FRE-366 landed after its security
+ * review, re-derived here for security stakes):
+ *
+ *   1. The direct socket peer, when it is a real routable address: it IS the
+ *      client (dev, direct hit), and any forwarded header from it is the
+ *      client describing itself — ignored.
+ *   2. `cf-connecting-ip` — reachable only when the peer is the loopback
+ *      origin proxy (or unreadable, e.g. a synthetic event in tests).
+ *      Cloudflare *overwrites* this header at the edge, so a client-supplied
+ *      value never survives. Must parse as an address.
+ *   3. `x-forwarded-for` counted from the RIGHT: drop the rightmost hop (the
+ *      Cloudflare edge address the origin proxy appended) and take the hop
+ *      before it. Never the leftmost. Must parse as an address.
+ *   4. "unknown" — callers give unknown-IP keys stricter shared limits, so
+ *      an unresolvable client fails toward a tighter bucket, never a looser
+ *      one.
+ *
+ * Deploy-side precondition this code cannot verify (same as the analytics
+ * read): the origin must be reachable only through Cloudflare and the
+ * same-host proxy. If the app is directly internet-reachable, a client can
+ * set every header read here and no in-app read can recover the truth.
+ */
+
+/** Cloudflare's "who actually connected" header: overwritten at the edge. */
+const CF_CONNECTING_IP = "cf-connecting-ip";
+const X_FORWARDED_FOR = "x-forwarded-for";
+
+/**
+ * Hops discounted from the right of `x-forwarded-for` before the value is
+ * taken: the Cloudflare edge address the origin proxy appended. Pinned to 1
+ * because there is exactly one trusted proxy between Cloudflare and the app;
+ * raise it only if the topology gains a trusted hop.
+ */
+const FORWARDED_HOPS_TO_DISCOUNT = 1;
+
+/** Returned when no trustworthy address exists. Callers rate-limit it harder. */
+export const UNKNOWN_CLIENT_IP = "unknown";
+
+/**
+ * The shapes this module reads: an H3Event (`security.ts` callers) or a
+ * SolidStart APIEvent (`routes/api/*`), which wraps its H3Event as
+ * `nativeEvent`. Every field is optional because test doubles are minimal;
+ * anything unreadable simply falls through to the next trust step.
+ */
+export interface ClientIpEventLike {
+  request?: { headers?: unknown };
+  headers?: unknown;
+  node?: {
+    req?: {
+      headers?: Record<string, unknown>;
+      socket?: { remoteAddress?: string };
+    };
+  };
+  nativeEvent?: ClientIpEventLike;
+}
+
+/**
+ * The client address for a security decision, per the trust order above.
+ * Never truncated: rate-limit buckets are per-address, and truncating would
+ * let an attacker share a bucket with the innocents behind their /24.
+ */
+export function resolveTrustedClientIp(event: ClientIpEventLike): string {
+  // An APIEvent's socket and (authoritative) headers live on the wrapped
+  // H3Event; a bare APIEvent-shaped double without one is read as-is.
+  const source = event.nativeEvent ?? event;
+
+  const peer = directPeer(source);
+  if (peer && isIpAddress(peer) && !isLoopbackAddress(peer)) return peer;
+
+  // The peer is the loopback origin proxy (or unreadable): forwarded headers
+  // may be believed, in the order above.
+  const connecting = cleanHop(headerValue(source, CF_CONNECTING_IP));
+  if (isIpAddress(connecting)) return connecting;
+
+  const hops = (headerValue(source, X_FORWARDED_FOR) ?? "")
+    .split(",")
+    .map(cleanHop)
+    .filter(Boolean);
+  const index = hops.length - 1 - FORWARDED_HOPS_TO_DISCOUNT;
+  if (index >= 0 && isIpAddress(hops[index])) return hops[index]!;
+
+  return UNKNOWN_CLIENT_IP;
+}
+
+/** The socket peer, IPv6-mapped IPv4 flattened (`::ffff:1.2.3.4` → `1.2.3.4`). */
+function directPeer(event: ClientIpEventLike): string | null {
+  try {
+    const addr = event.node?.req?.socket?.remoteAddress;
+    return addr ? addr.replace(/^::ffff:/, "") : null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Header read tolerant of every event shape in play: Fetch-style
+ * `request.headers.get`, an h3 `event.headers` (Headers or plain object), and
+ * the raw node request headers. Header names are case-insensitive per HTTP;
+ * the plain-object fallbacks lowercase to match how node and the test doubles
+ * store them.
+ */
+function headerValue(event: ClientIpEventLike, name: string): string | null {
+  try {
+    const fromHeaders = (h: unknown): string | null => {
+      if (!h) return null;
+      if (typeof (h as Headers).get === "function") {
+        return (h as Headers).get(name) ?? null;
+      }
+      if (typeof h === "object") {
+        const record = h as Record<string, unknown>;
+        const value = record[name] ?? record[name.toLowerCase()];
+        return typeof value === "string" ? value : null;
+      }
+      return null;
+    };
+
+    return (
+      fromHeaders(event.request?.headers) ??
+      fromHeaders(event.headers) ??
+      fromHeaders(event.node?.req?.headers)
+    );
+  } catch {
+    return null;
+  }
+}
+
+function cleanHop(value: string | null | undefined): string {
+  return (value ?? "").trim();
+}
+
+// ── Addresses: parse and validate ───────────────────────────────────────────
+// Ported from the reviewed `request-client.ts` (FRE-366 @ 1d9287a); see the
+// header comment for why this is a copy and not an import.
+
+/** Any address this file understands: a dotted quad or an IPv6 literal. */
+export function isIpAddress(value: string | null | undefined): boolean {
+  return value !== null && value !== undefined && addressBytes(value) !== null;
+}
+
+/**
+ * Whether an address is the loopback the production proxy presents.
+ *
+ * Parses rather than string-matches, so the forms a real peer can present in —
+ * `::ffff:127.0.0.53`, `0:0:0:0:0:0:0:1`, an IPv4-mapped literal — are all
+ * recognised. An address this does not parse is *not* loopback, which fails
+ * closed: forwarded headers from an unrecognised peer are ignored rather than
+ * believed (a non-loopback peer short-circuits before headers are read).
+ */
+export function isLoopbackAddress(ip: string): boolean {
+  const bytes = addressBytes(ip);
+  if (!bytes) return false;
+  if (bytes.length === 4) return bytes[0] === 127;
+
+  // IPv6 loopback ::1 — and ::ffff:127/8, which is loopback too.
+  const mapped = ipv4FromBytes(bytes);
+  if (mapped) return isLoopbackAddress(mapped);
+  const tail = bytes[15]!;
+  return bytes.slice(0, 15).every((byte) => byte === 0) && tail === 1;
+}
+
+function isIpv4(value: string): boolean {
+  const parts = value.split(".");
+  if (parts.length !== 4) return false;
+  return parts.every(
+    (part) => /^\d{1,3}$/.test(part) && Number(part) <= 255 && !/^0\d/.test(part)
+  );
+}
+
+/** 4 bytes for IPv4, 16 for IPv6 (including `::ffff:a.b.c.d`), else null. */
+function addressBytes(value: string): number[] | null {
+  const raw = value.trim().replace(/^\[|\]$/g, "").split("%")[0]!;
+  if (isIpv4(raw)) return raw.split(".").map(Number);
+  if (!raw.includes(":")) return null;
+
+  const halves = raw.split("::");
+  if (halves.length > 2) return null;
+  const compressed = halves.length === 2;
+
+  const head = groupsToBytes(halves[0] ?? "");
+  const tail = groupsToBytes(halves[1] ?? "");
+  if (!head || !tail) return null;
+
+  const total = head.length + tail.length;
+  if (compressed) {
+    if (total > 8) return null;
+    return [...head, ...new Array(8 - total).fill(0), ...tail].flatMap(split16);
+  }
+  if (total !== 8) return null;
+  return [...head, ...tail].flatMap(split16);
+}
+
+/** One side of `::` as 16-bit groups; a trailing `1.2.3.4` counts as two. */
+function groupsToBytes(side: string): number[] | null {
+  if (side === "") return [];
+  const out: number[] = [];
+  for (const part of side.split(":")) {
+    if (/^[0-9a-fA-F]{1,4}$/.test(part)) {
+      out.push(parseInt(part, 16));
+    } else if (isIpv4(part)) {
+      const o = part.split(".").map(Number);
+      out.push((o[0]! << 8) | o[1]!, (o[2]! << 8) | o[3]!);
+    } else {
+      return null;
+    }
+  }
+  return out;
+}
+
+function split16(group: number): number[] {
+  return [(group >> 8) & 0xff, group & 0xff];
+}
+
+/** The embedded IPv4 of an IPv4-mapped IPv6 address (`::ffff:1.2.3.4`), else null. */
+function ipv4FromBytes(bytes: number[]): string | null {
+  const prefix = bytes.slice(0, 10).every((byte) => byte === 0);
+  const marker = bytes[10] === 0xff && bytes[11] === 0xff;
+  if (!prefix || !marker) return null;
+  return `${bytes[12]}.${bytes[13]}.${bytes[14]}.${bytes[15]}`;
+}