From 5a8ff270907e4a4f66332481fe57922df52995a7 Mon Sep 17 00:00:00 2001
From: Nick Szaunig <nick.szaunig@encry.de>
Date: Mon, 12 Oct 2026 09:59:09 +0200
Subject: [PATCH] fix(security): Rate-Limits zaehlen IPv6 je /64-Netz

Seit CF-Connecting-IP gilt, sieht der Limiter echte IPv6-Adressen. Ein
Anschluss bekommt meist ein ganzes /64 und koennte durch Adresswechsel
jedes Limit umgehen (Login, 2FA, Checkout). getClientIp liefert IPv6 deshalb
als /64-Praefix, IPv4-gemappte Adressen wie IPv4 (wie VyCept, #94).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
---
 src/lib/rate-limit-ip.test.ts | 17 ++++++++++++++++-
 src/lib/rate-limit.ts         | 34 +++++++++++++++++++++++++++++++++-
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/src/lib/rate-limit-ip.test.ts b/src/lib/rate-limit-ip.test.ts
index 672656e..85f94f3 100644
--- a/src/lib/rate-limit-ip.test.ts
+++ b/src/lib/rate-limit-ip.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { resolveClientIp } from "./rate-limit";
+import { rateLimitSubject, resolveClientIp } from "./rate-limit";
 
 const h = (values: Record<string, string>) => new Headers(values);
 
@@ -23,3 +23,18 @@ describe("resolveClientIp (Client-IP hinter Cloudflare und Traefik)", () => {
     expect(resolveClientIp(h({}))).toBe("unknown");
   });
 });
+
+describe("rateLimitSubject (IPv6 je /64)", () => {
+  it("fasst alle Adressen eines /64-Netzes zusammen, sonst ließe sich jedes Limit durch Adresswechsel umgehen", () => {
+    expect(rateLimitSubject("2001:db8:1:2::1")).toBe("2001:db8:1:2::/64");
+    expect(rateLimitSubject("2001:db8:1:2:aaaa:bbbb:cccc:dddd")).toBe("2001:db8:1:2::/64");
+    expect(rateLimitSubject("2001:0DB8:0001:0002:ffff:ffff:ffff:ffff")).toBe("2001:db8:1:2::/64");
+    expect(rateLimitSubject("2001:db8:1:3::1")).not.toBe(rateLimitSubject("2001:db8:1:2::1"));
+  });
+
+  it("lässt IPv4 unverändert und behandelt IPv4-gemappte Adressen wie IPv4", () => {
+    expect(rateLimitSubject("198.51.100.9")).toBe("198.51.100.9");
+    expect(rateLimitSubject("::ffff:198.51.100.9")).toBe("198.51.100.9");
+    expect(rateLimitSubject("unknown")).toBe("unknown");
+  });
+});
diff --git a/src/lib/rate-limit.ts b/src/lib/rate-limit.ts
index e724f23..4b9ecb1 100644
--- a/src/lib/rate-limit.ts
+++ b/src/lib/rate-limit.ts
@@ -49,7 +49,39 @@ export function rateLimit(key: string, limit: number, windowMs: number): boolean
  * Rate-Limits in dieser Datei faelschen (Login, 2FA, Checkout, Reset, ...).
  */
 export async function getClientIp(): Promise<string> {
-  return resolveClientIp(await headers());
+  return rateLimitSubject(resolveClientIp(await headers()));
+}
+
+/** Die acht 16-Bit-Gruppen einer gueltigen IPv6-Adresse; eine IPv4-Endung (::ffff:a.b.c.d) wird umgerechnet. */
+function ipv6Groups(ip: string): number[] {
+  let text = ip.toLowerCase().split("%")[0] ?? "";
+  const dotted = /(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/.exec(text);
+  if (dotted) {
+    const [a = 0, b = 0, c = 0, d = 0] = dotted.slice(1).map(Number);
+    text = `${text.slice(0, dotted.index)}${((a << 8) | b).toString(16)}:${((c << 8) | d).toString(16)}`;
+  }
+  const parse = (part: string) => (part ? part.split(":").map((group) => Number.parseInt(group, 16)) : []);
+  const [head = "", tail] = text.split("::");
+  const left = parse(head);
+  if (tail === undefined) return left;
+  const right = parse(tail);
+  return [...left, ...new Array<number>(8 - left.length - right.length).fill(0), ...right];
+}
+
+/**
+ * Schluessel fuer Rate-Limits: IPv4 unveraendert, IPv6 als /64-Netz. Ein
+ * Anschluss bekommt meist ein ganzes /64; einzeln gezaehlt liesse sich jedes
+ * Limit durch Wechsel der Adresse innerhalb des eigenen Netzes umgehen.
+ * IPv4-gemappte Adressen zaehlen wie die IPv4-Adresse selbst.
+ */
+export function rateLimitSubject(ip: string): string {
+  if (isIP(ip) !== 6) return ip;
+  const groups = ipv6Groups(ip);
+  if (groups.slice(0, 5).every((group) => group === 0) && groups[5] === 0xffff) {
+    const [high = 0, low = 0] = groups.slice(6);
+    return [high >> 8, high & 0xff, low >> 8, low & 0xff].join(".");
+  }
+  return `${groups.slice(0, 4).map((group) => group.toString(16)).join(":")}::/64`;
 }
 
 /** https://www.cloudflare.com/ips/ (Stand 2026-10). */