diff --git a/platforms/android/README.md b/platforms/android/README.md
index 30c4d85c5..1f9c1f46c 100644
--- a/platforms/android/README.md
+++ b/platforms/android/README.md
@@ -25,6 +25,7 @@
 - [Preload checkout](#preload-checkout)
 - [Configure checkout](#configure-checkout)
   - [Color schemes](#color-schemes)
+  - [Incoming message origin validation](#incoming-message-origin-validation)
   - [Title localization](#title-localization)
   - [Current configuration](#current-configuration)
 - [Checkout lifecycle](#checkout-lifecycle)
@@ -258,8 +259,8 @@ ShopifyCheckoutKit.configure {
 | `sheet` | `CheckoutSheetOptions()` | Customize native sheet presentation such as snap points, dismissal behavior, corner radius, title alignment, toolbar elevation, close icon styling, and the optional drag handle. |
 | `logLevel` | `LogLevel.WARN` | SDK logging verbosity. Use `LogLevel.DEBUG` during integration. |
 | `preloading` | `Preloading(enabled = true)` | Enables best-effort checkout preloading before presentation. |
-| `allowedMessageOrigins` | `emptySet()` | Extra origins allowed to send checkout protocol messages. |
-| `onMessageRejected` | `null` | Observes messages rejected by origin validation. |
+| `allowedMessageOrigins` | `emptySet()` | Origins trusted to send incoming checkout messages. Empty trusts every origin (open by default). See [Incoming message origin validation](#incoming-message-origin-validation). |
+| `onMessageRejected` | `null` | Callback invoked when a message is dropped by origin validation. Defaults to logging at debug level. |
 
 ### Color schemes
 
@@ -348,6 +349,48 @@ Set `dragHandle.visible = true` to show a fixed, visual-only drag handle at the
 when `dismissal.dragToDismissEnabled = false` so disabled drag gestures are not presented as available. Configure
 `dragHandleColor` in `ColorScheme` to override the default header-font-derived handle color.
 
+### Incoming message origin validation
+
+The native WebView is a private, app-controlled runtime, so Checkout Kit is
+**open by default**: with an empty `allowedMessageOrigins`, incoming
+checkout-protocol messages from any origin are accepted. Provide one or more
+origins to restrict which origins are trusted; the loaded checkout origin and
+`shop.app` (including its subdomains) are always trusted as well.
+
+```kotlin
+ShopifyCheckoutKit.configure {
+    it.allowedMessageOrigins = setOf(
+        "https://checkout.example.com",
+        "https://*.example.com",
+    )
+}
+```
+
+Each entry may be an exact origin (`https://example.com`), a wildcard subdomain
+(`https://*.example.com`, matching subdomains but not the apex), or `"*"` to
+explicitly trust every origin.
+
+Exact and wildcard entries accept an optional trailing slash. Exact entries
+must not include credentials, paths, queries, or fragments. For example,
+`https://example.com/` is accepted, while `https://user@example.com` and
+`https://example.com/path` are ignored.
+
+Messages dropped by origin validation are logged at debug level. To observe
+them instead, set `onMessageRejected`:
+
+```kotlin
+ShopifyCheckoutKit.configure {
+    it.onMessageRejected = { rejected ->
+        Log.w("Checkout", "Dropped ${rejected.origin}: ${rejected.reason}")
+    }
+}
+```
+
+> [!WARNING]
+> The `RejectedMessage` payload is untrusted — it was dropped precisely because
+> its origin was not in the allowlist. Incoming messages are advisory and are
+> never treated as an authoritative source of checkout state.
+
 ### Title localization
 
 Override `checkout_web_view_title` in your app resources:
@@ -364,30 +407,6 @@ Override `checkout_web_view_title` in your app resources:
 val configuration = ShopifyCheckoutKit.getConfiguration()
 ```
 
-### Incoming message origin validation
-
-Native checkout accepts messages from every origin by default. To restrict messages, configure one
-or more exact origins or wildcard subdomains. The checkout URL's origin and `shop.app` remain
-trusted automatically.
-
-```kotlin
-ShopifyCheckoutKit.configure {
-    it.allowedMessageOrigins = setOf(
-        "https://checkout.example.com",
-        "https://*.example.org",
-    )
-    it.onMessageRejected = { rejection ->
-        reportRejectedOrigin(rejection.origin, rejection.reason)
-    }
-}
-```
-
-Exact entries accept an optional trailing slash, but not credentials, paths, queries, or fragments.
-For example, `https://checkout.example.com/` is accepted, while
-`https://user@checkout.example.com` and `https://checkout.example.com/path` are ignored. Wildcard
-entries require the scheme and match subdomains only; `https://*.example.org` does not match
-`https://example.org`. Use `"*"` to explicitly disable origin validation.
-
 ## Checkout lifecycle
 
 Use `onFail` and `onDismiss` for checkout outcomes handled by your app. Use `CheckoutProtocol.Client` for typed checkout state, including completion. These descriptors wrap checkout protocol messages defined in the [protocol schema](../../protocol/services/shopping/embedded.openrpc.json).
diff --git a/platforms/react-native/README.md b/platforms/react-native/README.md
index 166e0a803..97e9ad5ba 100644
--- a/platforms/react-native/README.md
+++ b/platforms/react-native/README.md
@@ -37,6 +37,7 @@ experiences.
 - [Usage with the Shopify Storefront API](#usage-with-the-shopify-storefront-api)
 - [Configuration](#configuration)
   - [Colors](#colors)
+  - [Incoming message origin validation](#incoming-message-origin-validation)
   - [Localization](#localization)
     - [Checkout Sheet title](#checkout-sheet-title)
       - [iOS - Localization](#ios---localization)
@@ -345,6 +346,7 @@ instance of the `ShopifyCheckout` class.
 | `preloading`  |          | `true`      | Enable/disable [preloading](#preloading).                                                                                                                      |
 | `colors`      |          | `{}`        | An object with `ios` and `android` properties to override the colors for iOS and Android platforms individually. See [`colors`](#colors) for more information. |
 | `logLevel`    |          | `error`     | Sets the log level for the native SDK. Use `LogLevel.debug` for verbose logging during development, or `LogLevel.error` for production.                        |
+| `allowedMessageOrigins` |  | `[]`        | Origins trusted to send incoming checkout messages. Empty trusts every origin (open by default). See [Incoming message origin validation](#incoming-message-origin-validation). |
 
 Here's an example of how a fully customized configuration object might look:
 
@@ -387,6 +389,45 @@ function AppWithContext() {
 const shopifyCheckout = new ShopifyCheckout(config);
 ```
 
+### Incoming message origin validation
+
+Checkout Kit runs on the native iOS and Android web views, which are private,
+app-controlled runtimes. It is therefore **open by default**: with an empty
+`allowedMessageOrigins`, incoming checkout-protocol messages from any origin are
+accepted. Provide one or more origins to restrict which origins are trusted; the
+loaded checkout origin and `shop.app` (including its subdomains) are always
+trusted as well.
+
+```tsx
+const config: Configuration = {
+  allowedMessageOrigins: [
+    'https://checkout.example.com',
+    'https://*.example.com',
+  ],
+};
+```
+
+Each entry may be an exact origin (`https://example.com`), a wildcard subdomain
+(`https://*.example.com`, matching subdomains but not the apex), or `'*'` to
+explicitly trust every origin. Exact and wildcard entries accept an optional
+trailing slash. Exact entries must not include credentials, paths, queries, or
+fragments. Messages dropped by origin validation are logged by the native SDK
+at debug level. To observe them instead, configure `onMessageRejected`:
+
+```tsx
+const config: Configuration = {
+  allowedMessageOrigins: ['https://checkout.example.com'],
+  onMessageRejected: ({origin, message, reason}) => {
+    console.warn(`Dropped message from ${origin}: ${reason}`, message);
+  },
+};
+```
+
+> [!NOTE]
+> Incoming messages are advisory (lifecycle/UI signals) and are never treated as
+> an authoritative source of checkout state, so origin validation is defense in
+> depth.
+
 ### Colors
 
 The SDK defaults to the `automatic` color scheme option, will switches between
diff --git a/platforms/web/README.md b/platforms/web/README.md
index 849ec075e..66b2ba009 100644
--- a/platforms/web/README.md
+++ b/platforms/web/README.md
@@ -36,6 +36,8 @@ Check out our blog to
   - [`target`](#target)
   - [`appearance`](#appearance)
   - [`log-level`](#log-level)
+  - [`allowed-origins`](#allowed-origins)
+  - [`onMessageRejected`](#onmessagerejected)
   - [Popup dimensions](#popup-dimensions)
   - [Overlay scrim](#overlay-scrim)
 - [Checkout lifecycle](#checkout-lifecycle)
@@ -397,6 +399,59 @@ Use `"debug"` while wiring up `src` and event handlers during integration, or
 checkout.logLevel = 'debug';
 ```
 
+### `allowed-origins`
+
+Controls which origins are trusted to post incoming checkout-protocol messages
+to the component. On web, checkout is **closed by default**: with no configured
+origins, only the cart URL origin (from `src`) and `shop.app` (including its
+subdomains) are trusted. Messages from any other origin are dropped.
+
+Provide extra origins as a space- or comma-separated list. Each entry may be:
+
+| Entry                     | Matches                                                        |
+| ------------------------- | -------------------------------------------------------------- |
+| `https://example.com`     | That exact origin.                                             |
+| `https://*.example.com`   | Any subdomain of `example.com` (not the apex `example.com`).   |
+| `*`                       | Every origin — disables origin validation entirely.            |
+
+Exact and wildcard entries accept an optional trailing slash. Exact entries
+must not include credentials, paths, queries, or fragments. For example,
+`https://example.com/` is accepted, while `https://user@example.com` and
+`https://example.com/path` are ignored.
+
+```html
+<shopify-checkout src="..." allowed-origins="https://checkout.example.com https://*.example.com" />
+```
+
+```ts
+checkout.allowedOrigins = ['https://checkout.example.com', 'https://*.example.com'];
+```
+
+> [!NOTE]
+> Incoming messages are advisory (lifecycle/UI signals) and are never treated
+> as an authoritative source of checkout state, so origin validation is defense
+> in depth. Use `*` only when you understand the trade-off — in the shared
+> browser, other pages and extensions can post to the component.
+
+Invalid entries are ignored, and a warning is logged at `log-level="warn"` or
+more verbose.
+
+### `onMessageRejected`
+
+A property-only callback invoked whenever an incoming message is dropped by
+origin validation. The smart default logs a warning; assign a function to
+observe rejected messages instead (for example, to report them).
+
+```ts
+checkout.onMessageRejected = ({ origin, data, reason }) => {
+  console.warn(`Dropped message from ${origin}: ${reason}`, data);
+};
+```
+
+> [!WARNING]
+> The payload is untrusted — it was dropped precisely because its origin was
+> not in the allowlist. Do not derive checkout state from it.
+
 ### Popup dimensions
 
 When `target="popup"`, the popup is centered over the host window. Defaults