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.
2026-10-30 05:06:18 +00:00
parent 134f41eac4
commit 2fd2a4a71a
7 changed files with 577 additions and 47 deletions
--- a/src/server/trusted-client-ip.ts
+++ b/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]}`;
+}