diff --git a/apps/docs/content/docs/shopify/index.mdx b/apps/docs/content/docs/shopify/index.mdx
index 85bee7d5..3a672fc1 100644
--- a/apps/docs/content/docs/shopify/index.mdx
+++ b/apps/docs/content/docs/shopify/index.mdx
@@ -46,7 +46,7 @@ All read operations are cached with `"use cache"` and `cacheLife("max")`. Withou
 POST /api/webhooks/shopify
 ```
 
-The handler verifies the HMAC signature using the `SHOPIFY_WEBHOOK_SECRET` environment variable, then calls `revalidateTag()` for the affected cache tags.
+The handler requires the `SHOPIFY_WEBHOOK_SECRET` environment variable, verifies the HMAC signature, then calls `revalidateTag()` for the affected cache tags.
 
 ### Supported topics
 
@@ -74,7 +74,7 @@ In your Shopify admin, go to **Settings → Notifications → Webhooks**:
 SHOPIFY_WEBHOOK_SECRET="your-webhook-signing-secret"
 ```
 
-> **Important:** Without `SHOPIFY_WEBHOOK_SECRET`, the handler skips signature verification. Always set this in production.
+> **Required:** `SHOPIFY_WEBHOOK_SECRET` must be set with a non-empty value. Requests hitting the endpoint without the secret configured return `500`; requests with a bad or missing `x-shopify-hmac-sha256` header return `401`.
 
 ### How it works
 
diff --git a/apps/template/.env.example b/apps/template/.env.example
index b50052b7..425445a2 100644
--- a/apps/template/.env.example
+++ b/apps/template/.env.example
@@ -4,3 +4,7 @@ SHOPIFY_STOREFRONT_ACCESS_TOKEN="your-storefront-access-token"
 
 # Store display name (shown in header, metadata, etc.)
 NEXT_PUBLIC_SITE_NAME="Your Store Name"
+
+# Required if using Shopify webhooks for cache invalidation
+# Copy from Shopify Admin > Settings > Notifications > Webhooks
+SHOPIFY_WEBHOOK_SECRET="your-webhook-signing-secret"
diff --git a/apps/template/app/api/webhooks/shopify/route.ts b/apps/template/app/api/webhooks/shopify/route.ts
index b958ce8a..1bd13ea3 100644
--- a/apps/template/app/api/webhooks/shopify/route.ts
+++ b/apps/template/app/api/webhooks/shopify/route.ts
@@ -4,22 +4,29 @@ import { revalidateTag } from "next/cache";
 
 import { getNumericShopifyId } from "@/lib/shopify/utils";
 
-const SHOPIFY_WEBHOOK_SECRET = process.env.SHOPIFY_WEBHOOK_SECRET;
-
 /**
  * Verify Shopify webhook HMAC signature
  */
-async function verifyWebhook(body: string, hmacHeader: string | null): Promise<boolean> {
-  if (!SHOPIFY_WEBHOOK_SECRET || !hmacHeader) {
+async function verifyWebhook(
+  secret: string,
+  body: string,
+  hmacHeader: string | null,
+): Promise<boolean> {
+  if (!hmacHeader) {
     return false;
   }
 
-  const hash = crypto
-    .createHmac("sha256", SHOPIFY_WEBHOOK_SECRET)
-    .update(body, "utf8")
-    .digest("base64");
+  const expected = Buffer.from(
+    crypto.createHmac("sha256", secret).update(body, "utf8").digest("base64"),
+    "base64",
+  );
+  const received = Buffer.from(hmacHeader, "base64");
+
+  if (expected.length !== received.length) {
+    return false;
+  }
 
-  return crypto.timingSafeEqual(Buffer.from(hash, "base64"), Buffer.from(hmacHeader, "base64"));
+  return crypto.timingSafeEqual(expected, received);
 }
 
 /**
@@ -33,17 +40,20 @@ async function verifyWebhook(body: string, hmacHeader: string | null): Promise<b
  * Settings > Notifications > Webhooks
  */
 export async function POST(request: Request) {
+  const secret = process.env.SHOPIFY_WEBHOOK_SECRET;
+  if (!secret) {
+    console.error("SHOPIFY_WEBHOOK_SECRET is not set");
+    return Response.json({ error: "Webhook secret not configured" }, { status: 500 });
+  }
+
   const body = await request.text();
   const hmacHeader = request.headers.get("x-shopify-hmac-sha256");
   const topic = request.headers.get("x-shopify-topic");
 
-  // Verify webhook signature in production
-  if (SHOPIFY_WEBHOOK_SECRET) {
-    const isValid = await verifyWebhook(body, hmacHeader);
-    if (!isValid) {
-      console.error("Invalid Shopify webhook signature");
-      return Response.json({ error: "Invalid signature" }, { status: 401 });
-    }
+  const isValid = await verifyWebhook(secret, body, hmacHeader);
+  if (!isValid) {
+    console.error("Invalid Shopify webhook signature");
+    return Response.json({ error: "Invalid signature" }, { status: 401 });
   }
 
   if (!topic) {
diff --git a/packages/plugin/template-rollout-log/2026-04-18-webhook-secret-required.md b/packages/plugin/template-rollout-log/2026-04-18-webhook-secret-required.md
new file mode 100644
index 00000000..df1ded68
--- /dev/null
+++ b/packages/plugin/template-rollout-log/2026-04-18-webhook-secret-required.md
@@ -0,0 +1,33 @@
+---
+title: Require SHOPIFY_WEBHOOK_SECRET for webhook handler
+changeKey: webhook-secret-required
+introducedOn: 2026-04-18
+changeType: breaking
+defaultAction: adopt
+appliesTo:
+  - all
+paths:
+  - app/api/webhooks/shopify/route.ts
+---
+
+## Summary
+
+The Shopify webhook handler no longer skips HMAC verification when `SHOPIFY_WEBHOOK_SECRET` is unset. It now returns `500 Webhook secret not configured` when the env var is missing or empty, and `401 Invalid signature` when the signature does not match.
+
+## Why it matters
+
+The previous fallback of "no secret = accept every request" was a silent cache-invalidation vector. Any unauthenticated caller could flush product, collection, inventory, and CMS caches by POSTing to `/api/webhooks/shopify`. Failing closed removes that footgun.
+
+## Apply when
+
+Any storefront that exposes `/api/webhooks/shopify` to the internet, which is every storefront deployed to Vercel. Adopt this unconditionally.
+
+## Safe to skip when
+
+Never safe to skip. The previous behavior was unsafe in production.
+
+## Validation
+
+- Set `SHOPIFY_WEBHOOK_SECRET` in Vercel project env vars (copy from Shopify Admin → Settings → Notifications → Webhooks).
+- `curl -X POST https://your-domain/api/webhooks/shopify` with no headers → expect `401` (or `500` if the env var is not set).
+- Trigger a real Shopify webhook → expect `200` with `tagsInvalidated` in the response.