Server-side Receipt Validation (Purchase Verification)
A client reported a bug: a user purchased Premium, got a transaction token, restored the app from a backup on a different device — and Premium was active again without repaying. Classic. The reason is client-only validation: the app checks a local receipt or a StoreKit trust flag without consulting the server. We are a mobile development team with over 7 years of experience implementing in-app purchases. During this time, we have implemented server-side validation for 30+ projects and have learned that without it, fraud losses can reach 15% of revenue. The average savings for our clients range from 15% to 20% of revenue — a substantial figure.
What is Receipt Validation?
Receipt Validation is the process of verifying the authenticity of a purchase receipt on the server side using official app store APIs. Only server-side validation ensures that a purchase was actually made and has not been tampered with. Without it, any application is vulnerable to attacks.
Why is client-side validation insecure?
On iOS, StoreKit 2 returns a Transaction with an Apple signature. You can verify the signature locally via Transaction.verificationResult, but this does not protect against replay attacks: an attacker intercepts a valid receipt from one user and substitutes it into another account. The situation is similar on Android — BillingClient.queryPurchasesAsync() returns Purchase objects that the client should not treat as confirmation without server-side verification of the purchaseToken.
The most common fraud scheme is receipt sharing: one receipt is distributed among users through forums. Without a server-side database that records which originalTransactionId (iOS) or orderId (Android) has already been used, this cannot be caught.
How to protect against receipt sharing?
Create a purchase_receipts table with a unique index on original_transaction_id and product_id. On each verification, check whether the receipt has already been used by another user. If so, reject the request. This blocks the spread of one receipt across multiple accounts. Additionally, use Server Notifications to track refunds and cancellations.
Why are Server Notifications critical?
App Store Server Notifications V2 and Real-time Developer Notifications from Google send events in real time: DID_RENEW, EXPIRED, REFUND, GRACE_PERIOD_EXPIRED. Without them, your server learns about subscription status changes with a delay of up to 24 hours, allowing users with canceled subscriptions to continue accessing content. Setting up a webhook handler is a mandatory step to protect against stale data.
How normal server-side verification works
iOS side (App Store Server API). The old approach — POST to https://buy.itunes.apple.com/verifyReceipt with base64-encoded receipt-data — is deprecated. Apple promotes the App Store Server API v1: the client sends the server transactionId from Transaction.id (StoreKit 2), the server makes a GET /inApps/v1/history/{transactionId} with a JWT token (signed with an ES256 key from App Store Connect). The response is a JWSTransaction that must be decoded and the signature verified using the Apple Root CA.
In parallel, you need to subscribe to App Store Server Notifications V2: Apple pushes events to your endpoint. More details at App Store Server Notifications V2.
Android side (Google Play Developer API). For one-time purchases — purchases.products.get with packageName, productId, purchaseToken. For subscriptions — purchases.subscriptions.v2.get. Authorization via a Service Account with the role Financial data viewer — this is the minimum required permission. The response contains purchaseState (0 = Purchased, 1 = Canceled, 2 = Pending) and acknowledgementState — if it is 0, you need to call purchases.products.acknowledge, otherwise Google will automatically refund within 3 days. More details at Google Play Developer API.
Idempotency and replay protection. In the database, store a purchase_receipts table with a unique index on original_transaction_id + product_id. On each verification request, first check for an existing record — if already verified for a different user_id, return an error.
purchase_receipts id uuid PK user_id uuid FK platform enum('ios','android') original_transaction_id varchar UNIQUE (per product) product_id varchar purchase_state smallint expires_at timestamptz -- for subscriptions raw_payload jsonb -- original response from Apple/Google verified_at timestamptz Stack and integration
The server side is most often Node.js (library app-store-server-api) or Python (google-auth + googleapiclient). For Node, the package node-apple-receipt-verify is convenient for the legacy endpoint, but it is better to use app-store-server-api from Apple directly — it supports JWT authorization and JWS verification out of the box.
On the client side, iOS code is minimal: get Transaction.id from Transaction.all or from the updates stream, send it to the backend. Do not send the entire appStoreReceiptURL — it is legacy and the file may be invalid on the simulator.
On Android, the client sends purchaseToken and productId from Purchase.purchaseToken. Important: the token may be the same for multiple productId during subscription upgrades — account for this in your logic.
Comparison of iOS and Android approaches
| Criteria | iOS (App Store Server API) | Android (Google Play Developer API) |
|---|---|---|
| Protocol | REST with JWT authorization | REST with OAuth2 (Service Account) |
| Key type | ES256 (App Store Connect) | JSON key Service Account |
| Signature verification | JWS (Apple Root CA) | API response (HTTPS) |
| Notifications | Server Notifications V2 | Real-time Developer Notifications |
| Replay protection | unique transactionId |
unique purchaseToken |
Comparison of client-side vs server-side validation
| Criteria | Client-side validation | Server-side validation |
|---|---|---|
| Security | Vulnerable to replay and tampering | High, with store-side verification |
| Sharing protection | Impossible | Blocked by transaction uniqueness |
| Status synchronization | Device only | Across all devices via server |
| Change notifications | None | Server Notifications |
Process of work
We begin with an audit of the current validation scheme — where exactly the receipt is checked, whether a server-side purchase database exists, and whether Server Notifications are processed. Then we design the database schema and API endpoints, implement verification for each platform, configure a webhook handler for Server Notifications, and cover with tests using mock responses from Apple/Google. A separate stage is load testing of the verification endpoint, because during peak launches (promotion, feature in the top of App Store) it receives everything at once.
What is included in the work
- Audit of the current validation scheme and vulnerabilities.
- Design of the purchase database and API endpoints.
- Implementation of verification for iOS (App Store Server API v1) and Android (Google Play Developer API).
- Setup of Server Notifications (App Store Server Notifications V2 and Real-time Developer Notifications).
- Writing tests with mock responses from Apple/Google.
- Integration and load testing.
- API documentation and developer instructions.
Timelines — from 2 to 5 days, depending on the availability of server infrastructure and the number of purchase types (one-time, subscriptions, consumable, non-consumable). If a server already exists and only verification needs to be added — closer to 2 days. Full architecture from scratch plus migration of existing users — up to 5 days.
Contact us to discuss the details of your project. Order an audit of your current validation scheme — we will identify vulnerabilities and propose an optimal solution. Get a consultation on your project — we will estimate timelines and offer the best solution.