fix(ack-pay): validate receipt payment option

.changeset/validate-receipt-payment-option.md

@@ -0,0 +1,16 @@
+---
+"@agentcommercekit/ack-pay": patch
+---
+
+Validate that a PaymentReceipt's `paymentOptionId` matches an option offered by
+the verified Payment Request token.
+
+A receipt proves a `paymentRequestToken` and a selected `paymentOptionId`, but
+verification did not bind that selection back to an option actually offered by
+the request. `verifyPaymentReceipt` now rejects a receipt whose
+`paymentOptionId` is not present in the verified request's `paymentOptions`. The
+check reads from the proof-decoded credential, so it cannot be bypassed by
+mutating the outer object on the parsed-credential input path.
+
+Introduces `InvalidPaymentReceiptError` so callers can catch receipt-level
+validation failures explicitly.

packages/ack-pay/src/errors.ts

@@ -7,3 +7,10 @@ export class InvalidPaymentRequestTokenError extends Error {
     this.name = "InvalidPaymentRequestTokenError"
   }
 }
+
+export class InvalidPaymentReceiptError extends Error {
+  constructor(message = "Invalid payment receipt", options?: ErrorOptions) {
+    super(message, options)
+    this.name = "InvalidPaymentReceiptError"
+  }
+}

packages/ack-pay/src/verify-payment-receipt.test.ts

@@ -23,7 +23,10 @@ import { beforeEach, describe, expect, it } from "vitest"
 
 import { createPaymentReceipt } from "./create-payment-receipt"
 import { createSignedPaymentRequest } from "./create-signed-payment-request"
-import { InvalidPaymentRequestTokenError } from "./errors"
+import {
+  InvalidPaymentReceiptError,
+  InvalidPaymentRequestTokenError,
+} from "./errors"
 import type { PaymentRequestInit } from "./payment-request"
 import { isPaymentReceiptCredential } from "./receipt-claim-verifier"
 import { verifyPaymentReceipt } from "./verify-payment-receipt"
@@ -175,6 +178,27 @@ describe("verifyPaymentReceipt()", () => {
     })
   })
 
+  it("throws when the receipt payment option is not in the payment request", async () => {
+    const mismatchedReceipt = createPaymentReceipt({
+      paymentRequestToken,
+      paymentOptionId: "missing-payment-option-id",
+      issuer: receiptIssuerDid,
+      payerDid: createDidPkhUri(
+        "eip155:84532",
+        "0x7B3D8F2E1C9A4B5D6E7F8A9B0C1D2E3F4A5B6C",
+      ),
+    })
+
+    const mismatchedReceiptJwt = await signCredential(mismatchedReceipt, {
+      did: receiptIssuerDid,
+      signer: createJwtSigner(receiptIssuerKeypair),
+    })
+
+    await expect(
+      verifyPaymentReceipt(mismatchedReceiptJwt, { resolver }),
+    ).rejects.toThrow(InvalidPaymentReceiptError)
+  })
+
   it("throws for an invalid JWT receipt", async () => {
     await expect(
       verifyPaymentReceipt("invalid-jwt", { resolver }),

packages/ack-pay/src/verify-payment-receipt.ts

@@ -10,6 +10,7 @@ import {
   type W3CCredential,
 } from "@agentcommercekit/vc"
 
+import { InvalidPaymentReceiptError } from "./errors"
 import type { PaymentRequest } from "./payment-request"
 import {
   getReceiptClaimVerifier,
@@ -128,6 +129,20 @@ export async function verifyPaymentReceipt(
     },
   )
 
+  // Bind the receipt's selected option back to an option actually offered by
+  // the verified Payment Request. Reads from `verifiedReceipt` (proof-decoded),
+  // so a mutated outer credential cannot smuggle in an unoffered option.
+  const paymentOptionExists = paymentRequest.paymentOptions.some(
+    (paymentOption) =>
+      paymentOption.id === verifiedReceipt.credentialSubject.paymentOptionId,
+  )
+
+  if (!paymentOptionExists) {
+    throw new InvalidPaymentReceiptError(
+      "Receipt paymentOptionId does not match any payment option in the Payment Request token",
+    )
+  }
+
   return {
     receipt: verifiedReceipt,
     paymentRequestToken,