diff --git a/lib/url-guard.ts b/lib/url-guard.ts
new file mode 100644
index 0000000..d809325
--- /dev/null
+++ b/lib/url-guard.ts
@@ -0,0 +1,51 @@
+/* ---- SSRF guard: never POST to internal/loopback/link-local addresses ---- */
+export function isInternalUrl(raw: string): boolean {
+  let u: URL;
+  try { u = new URL(raw); } catch { return true; }
+  if (u.protocol !== "https:" && u.protocol !== "http:") return true;
+  if (process.env.NODE_ENV === "production" && u.protocol !== "https:") return true;
+  const h = u.hostname.toLowerCase();
+  if (h === "localhost" || h.endsWith(".localhost") || h === "metadata.google.internal") return true;
+  if (h === "0.0.0.0" || h === "::1" || h === "[::1]") return true;
+  const ipv6 = h.replace(/^\[|\]$/g, "");
+  if (ipv6.includes(":")) {
+    const mapped = ipv6.match(/^::ffff:([0-9a-f]{1,4}):([0-9a-f]{1,4})$/);
+    if (mapped) {
+      const high = Number.parseInt(mapped[1]!, 16);
+      const low = Number.parseInt(mapped[2]!, 16);
+      const a = high >> 8;
+      const b = high & 255;
+      const c = low >> 8;
+      if (
+        a === 127 ||
+        a === 10 ||
+        a === 0 ||
+        (a === 172 && b >= 16 && b <= 31) ||
+        (a === 192 && b === 168) ||
+        (a === 169 && b === 254)
+      ) return true;
+      if (a || b || c || low) return false;
+    }
+    if (
+      ipv6 === "::" ||
+      ipv6 === "::1" ||
+      ipv6.startsWith("fe80:") ||
+      ipv6.startsWith("fc") ||
+      ipv6.startsWith("fd") ||
+      ipv6.startsWith("::ffff:10.") ||
+      ipv6.startsWith("::ffff:127.") ||
+      ipv6.startsWith("::ffff:192.168.") ||
+      /^::ffff:172\.(1[6-9]|2\d|3[01])\./.test(ipv6) ||
+      ipv6.startsWith("::ffff:169.254.")
+    ) return true;
+  }
+  const m = h.match(/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/);
+  if (m) {
+    const [a, b] = [Number(m[1]), Number(m[2])];
+    if (a === 127 || a === 10 || a === 0) return true;
+    if (a === 172 && b >= 16 && b <= 31) return true;
+    if (a === 192 && b === 168) return true;
+    if (a === 169 && b === 254) return true;
+  }
+  return false;
+}
diff --git a/lib/webhooks.ts b/lib/webhooks.ts
index 3c82e09..4cdbd01 100644
--- a/lib/webhooks.ts
+++ b/lib/webhooks.ts
@@ -1,5 +1,6 @@
 import crypto from "node:crypto";
 import { db, activeDomainWebhooks } from "./db";
+import { isInternalUrl } from "./url-guard";
 
 const TOLERANCE = 300; // seconds
 
@@ -33,25 +34,7 @@ export function newSecret(prefix = "whsec_") {
   return prefix + crypto.randomBytes(24).toString("base64url");
 }
 
-/* ---- SSRF guard: never POST to internal/loopback/link-local addresses ---- */
-export function isInternalUrl(raw: string): boolean {
-  let u: URL;
-  try { u = new URL(raw); } catch { return true; }
-  if (u.protocol !== "https:" && u.protocol !== "http:") return true;
-  if (process.env.NODE_ENV === "production" && u.protocol !== "https:") return true;
-  const h = u.hostname.toLowerCase();
-  if (h === "localhost" || h.endsWith(".localhost") || h === "metadata.google.internal") return true;
-  if (h === "0.0.0.0" || h === "::1" || h === "[::1]") return true;
-  const m = h.match(/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/);
-  if (m) {
-    const [a, b] = [Number(m[1]), Number(m[2])];
-    if (a === 127 || a === 10 || a === 0) return true;
-    if (a === 172 && b >= 16 && b <= 31) return true;
-    if (a === 192 && b === 168) return true;
-    if (a === 169 && b === 254) return true;
-  }
-  return false;
-}
+export { isInternalUrl };
 
 /* ---- per-domain outbound delivery (best-effort, no owner server needed) ---- */
 /**
diff --git a/tests/url-guard.test.mjs b/tests/url-guard.test.mjs
new file mode 100644
index 0000000..e53aa27
--- /dev/null
+++ b/tests/url-guard.test.mjs
@@ -0,0 +1,17 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { isInternalUrl } from "../lib/url-guard.ts";
+
+test("SSRF guard blocks private IPv6 webhook targets", () => {
+  assert.equal(isInternalUrl("http://[::1]/hook"), true);
+  assert.equal(isInternalUrl("http://[fe80::1]/hook"), true);
+  assert.equal(isInternalUrl("http://[fc00::1]/hook"), true);
+  assert.equal(isInternalUrl("http://[fd12:3456::1]/hook"), true);
+  assert.equal(isInternalUrl("http://[::ffff:192.168.1.10]/hook"), true);
+});
+
+test("SSRF guard allows public IPv6 webhook targets", () => {
+  assert.equal(isInternalUrl("https://[2606:4700:4700::1111]/hook"), false);
+  assert.equal(isInternalUrl("https://[::ffff:8.8.8.8]/hook"), false);
+});