diff --git a/src/__tests__/index.test.ts b/src/__tests__/index.test.ts
index 23c8b78..6decf34 100644
--- a/src/__tests__/index.test.ts
+++ b/src/__tests__/index.test.ts
@@ -454,6 +454,179 @@ describe('createChannel integration tests', () => {
   })
 })
 
+describe('origin validation', () => {
+  function createMockWindowTarget() {
+    const target: any = { self: null as any }
+    target.self = target
+    target.postMessage = vi.fn()
+    return target
+  }
+
+  function mockPort() {
+    return {
+      postMessage: vi.fn(),
+      start: vi.fn(),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    }
+  }
+
+  it('should reject connections from an origin other than the declared one', async () => {
+    const mockTarget = createMockWindowTarget()
+    const channel = createChannel(
+      mockTarget,
+      'reject-test',
+      'https://trusted.com'
+    )
+
+    const port = mockPort()
+    window.dispatchEvent(
+      new MessageEvent('message', {
+        data: {
+          type: 'bidc-connect',
+          channelId: 'bidc_reject-test',
+          timestamp: Date.now() + 1000,
+        },
+        origin: 'https://evil.com',
+        ports: [port] as any,
+      })
+    )
+
+    await new Promise((r) => setTimeout(r, 50))
+    expect(port.postMessage).not.toHaveBeenCalled()
+
+    channel.cleanup()
+  })
+
+  it('should accept connections from the declared origin', async () => {
+    const mockTarget = createMockWindowTarget()
+    const channel = createChannel(
+      mockTarget,
+      'accept-test',
+      'https://trusted.com'
+    )
+
+    const port = mockPort()
+    window.dispatchEvent(
+      new MessageEvent('message', {
+        data: {
+          type: 'bidc-connect',
+          channelId: 'bidc_accept-test',
+          timestamp: Date.now() + 1000,
+        },
+        origin: 'https://trusted.com',
+        ports: [port] as any,
+      })
+    )
+
+    await new Promise((r) => setTimeout(r, 50))
+    expect(port.postMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'bidc-confirm' })
+    )
+
+    channel.cleanup()
+  })
+
+  it('should accept any origin when none is declared (back-compat)', async () => {
+    const mockTarget = createMockWindowTarget()
+    const channel = createChannel(mockTarget, 'no-origin-test')
+
+    const port = mockPort()
+    window.dispatchEvent(
+      new MessageEvent('message', {
+        data: {
+          type: 'bidc-connect',
+          channelId: 'bidc_no-origin-test',
+          timestamp: Date.now() + 1000,
+        },
+        origin: 'https://any-origin.com',
+        ports: [port] as any,
+      })
+    )
+
+    await new Promise((r) => setTimeout(r, 50))
+    expect(port.postMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'bidc-confirm' })
+    )
+
+    channel.cleanup()
+  })
+
+  it('should use declared origin as targetOrigin in postMessage', () => {
+    const mockTarget = createMockWindowTarget()
+    const channel = createChannel(
+      mockTarget,
+      'target-origin-test',
+      'https://trusted.com'
+    )
+
+    expect(mockTarget.postMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'bidc-connect' }),
+      'https://trusted.com',
+      expect.any(Array)
+    )
+
+    channel.cleanup()
+  })
+
+  it('should use * as targetOrigin when no origin is declared', () => {
+    const mockTarget = createMockWindowTarget()
+    const channel = createChannel(mockTarget, 'wildcard-test')
+
+    expect(mockTarget.postMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'bidc-connect' }),
+      '*',
+      expect.any(Array)
+    )
+
+    channel.cleanup()
+  })
+
+  it('should validate origin without a target (iframe context)', async () => {
+    // Simulate an iframe context where window.parent is the embedding page
+    const fakeParent = { postMessage: vi.fn() }
+    Object.defineProperty(window, 'parent', {
+      value: fakeParent,
+      configurable: true,
+    })
+
+    try {
+      const channel = createChannel('iframe-origin-test', 'https://parent.com')
+
+      // Outbound connect message is restricted to the declared origin
+      expect(fakeParent.postMessage).toHaveBeenCalledWith(
+        expect.objectContaining({ type: 'bidc-connect' }),
+        'https://parent.com',
+        expect.any(Array)
+      )
+
+      // Inbound connect from a different origin is rejected
+      const port = mockPort()
+      window.dispatchEvent(
+        new MessageEvent('message', {
+          data: {
+            type: 'bidc-connect',
+            channelId: 'bidc_iframe-origin-test',
+            timestamp: Date.now() + 1000,
+          },
+          origin: 'https://evil.com',
+          ports: [port] as any,
+        })
+      )
+
+      await new Promise((r) => setTimeout(r, 50))
+      expect(port.postMessage).not.toHaveBeenCalled()
+
+      channel.cleanup()
+    } finally {
+      Object.defineProperty(window, 'parent', {
+        value: window,
+        configurable: true,
+      })
+    }
+  })
+})
+
 describe('edge cases', () => {
   it('should handle circular references with promises', async () => {
     const obj: any = { name: 'circular' }
diff --git a/src/index.ts b/src/index.ts
index df7db24..86830fb 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -335,22 +335,34 @@ type TChannel = {
  */
 function createChannel(): TChannel
 function createChannel(targetOrChannelId: MessageTarget | string): TChannel
+function createChannel(channelId: string, origin: string): TChannel
 function createChannel(maybeTarget: MessageTarget, channelId: string): TChannel
+function createChannel(
+  maybeTarget: MessageTarget,
+  channelId: string,
+  origin: string
+): TChannel
 function createChannel(
   targetOrChannelId?: MessageTarget | string,
-  channelId?: string
+  channelIdOrOrigin?: string,
+  maybeOrigin?: string
 ) {
   let maybeTarget: MessageTarget | undefined = undefined
-  if (
-    typeof channelId === 'undefined' &&
-    typeof targetOrChannelId === 'string'
-  ) {
-    // The first argument is channelId
-    channelId = targetOrChannelId
-    maybeTarget = undefined
-  } else if (typeof targetOrChannelId === 'object') {
-    // The first argument is a target
+  let channelId: string | undefined
+  // Expected origin of the other endpoint. When set, incoming connections
+  // from any other origin are rejected. Must be declared by the caller since
+  // it can't be derived from a cross-origin target.
+  let expectedOrigin: string | undefined
+
+  if (typeof targetOrChannelId === 'object') {
+    // (target, channelId?, origin?)
     maybeTarget = targetOrChannelId
+    channelId = channelIdOrOrigin
+    expectedOrigin = maybeOrigin
+  } else {
+    // (channelId?, origin?) — no target, uses window.parent / self
+    channelId = targetOrChannelId
+    expectedOrigin = channelIdOrOrigin
   }
 
   // Namespaced channelId to avoid conflicts with other libraries / multiple
@@ -375,8 +387,8 @@ function createChannel(
     function sendMessageWithTransfer(message: any, transfer: MessagePort) {
       if (maybeTarget) {
         if ('self' in maybeTarget && maybeTarget.self === maybeTarget) {
-          // It's an iframe contentWindow
-          maybeTarget.postMessage(message, '*', [transfer])
+          // Restrict delivery to the expected origin when declared
+          maybeTarget.postMessage(message, expectedOrigin || '*', [transfer])
         } else {
           ;(maybeTarget as Exclude<MessageTarget, Window>).postMessage(
             message,
@@ -395,7 +407,7 @@ function createChannel(
           window.parent !== window
         ) {
           // Inside an iframe, we can use window.parent
-          window.parent.postMessage(message, '*', [transfer])
+          window.parent.postMessage(message, expectedOrigin || '*', [transfer])
         } else {
           throw new Error('No target provided and no global context available')
         }
@@ -419,6 +431,9 @@ function createChannel(
       const port = event.ports[0] as MessagePort | undefined
       if (!port) return
 
+      // Reject connections from any origin other than the expected one
+      if (expectedOrigin && event.origin !== expectedOrigin) return
+
       const data = event.data as typeof connectMessage | typeof confirmMessage
       if (data?.channelId !== channelId) return
       if (data.type !== connectMessage.type) return