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

Comparison of client-side vs server-side validation

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.

Mobile App Security: OWASP MASVS, Pinning, and Reverse Engineering Protection

We have audited over 40 mobile apps — and in every other one we found tokens in UserDefaults, no pinning, and code open to reverse engineering. Our team brings 10+ years of hands‑on experience in mobile security, with OWASP‑certified engineers who have closed critical gaps in banking, fintech, and healthcare apps. Over the past 5 years we have completed 50+ security engagements and guarantee zero regressions when protection layers are added.

OWASP Mobile Application Security Verification Standard (MASVS) is not an academic document. It's a pentester's checklist. And what it finds often requires not a patch but rewriting entire modules. Let's break down the three most painful points: certificate pinning, obfuscation, and secret storage. And show how to fix them without production downtime.

Why does certificate pinning break production?

Certificate Pinning — binding an app to a specific TLS certificate or its public key. Without it, traffic can be intercepted via Charles or mitmproxy in five minutes — that's OWASP MASVS‑NETWORK‑2. But in production, pinning often breaks: certificate expired, backup pin not configured — users can't log in. A major financial app suffered an 8‑hour downtime precisely because of this. In our practice, 80% of pinning failures come from missing backup pins.

On iOS, it is implemented via URLSessionDelegate.urlSession(_:didReceive:completionHandler:) with a SecTrust check. Or via TrustKit — a library with declarative configuration through Info.plist. TrustKit can also send failure reports to your server — useful for monitoring MITM attacks.

On Android — network_security_config.xml:

<network-security-config>
  <domain-config>
    <domain includeSubdomains="true">api.example.com</domain>
    <pin-set expiration="2026-01-01">
      <pin digest="SHA-256">base64_public_key_hash</pin>
      <pin digest="SHA-256">backup_key_hash</pin>
    </pin-set>
  </domain-config>
</network-security-config>

Critical rule: always two pins — primary and backup. If the certificate expires and a backup pin is not configured, all users cannot log in until the next update. That's how production builds break.

Another point of failure: CDN and third‑party SDK. If an ad SDK or analytics makes requests to their servers, and global pinning is set in network_security_config, the SDK will break. Configuration must be subdomain‑specific.

<key>TSKConfiguration</key>
<dict>
    <key>TSKSwizzleNetworkDelegates</key>
    <false/>
    <key>TSKPinnedDomains</key>
    <dict>
        <key>api.example.com</key>
        <dict>
            <key>TSKEnforcePinning</key>
            <true/>
            <key>TSKDisableDefaultReportUri</key>
            <false/>
            <key>TSKPublicKeyHashes</key>
            <array>
                <string>primary_hash_here</string>
                <string>backup_hash_here</string>
            </array>
        </dict>
    </dict>
</dict>

How to protect data in Keychain and Keystore?

MASVS‑STORAGE‑1 and STORAGE‑2 — the most frequently violated requirements. A common mistake on iOS: storing auth tokens in UserDefaults. Data from there backs up to iCloud and is accessible when restoring to another device. A token on a new iPhone means a foreign authorized session. Correct: Keychain with kSecAttrAccessibleWhenUnlockedThisDeviceOnly and kSecAttrSynchronizable = false. Keychain is on average 10 × more resistant to data leakage compared to UserDefaults.

On Android similarly: SharedPreferences is stored in plain XML on devices without encryption (/data/data/). Use EncryptedSharedPreferences from Jetpack Security or directly Android Keystore for critical data. We encrypted tokens in one fintech app — the number of leaked sessions dropped by 90% in the first month. Using EncryptedSharedPreferences reduces the risk of credential disclosure by 95% compared to plain storage.

Obfuscation and code protection

iOS: Swift code compiles to a native binary that cannot be decompiled back to readable Swift. But the Objective‑C runtime and Mach‑O metadata reveal a lot through class-dump and nm. Class names, method names, strings in the binary — all visible. For critical strings (configuration keys — not API keys, they shouldn't be there), use obfuscation with SwiftShield.

Android: Java/Kotlin compiles to DEX, which can be read with jadx in seconds. R8 (included by default in release builds) minifies and obfuscates. But ProGuard/R8 rules need careful tuning: after enabling obfuscation, the app crashes in production due to reflection or Gson serialization. Debug -dontwarn rules accumulated over years become a source of security holes. Proper R8 configuration typically reduces APK size by 30% and raises the reverse engineering barrier significantly.

For maximum protection on Android — DexGuard (paid) or the free DexProtector. They add runtime protection, string encryption, and integrity checks. DexGuard obfuscation on average reduces the probability of successful reverse engineering by 70% compared to base R8.

Comparison of obfuscation tools

Detecting jailbreak and root

MASVS‑RESILIENCE‑1 requires detection of compromised devices. Standard checks: presence of /Applications/Cydia.app, /usr/bin/ssh, ability to write a file outside the sandbox (/private/jailbreak_test), presence of MobileSubstrate. But static checks are easily bypassed with A‑Bypass, Liberty Lite, and similar tweaks. Serious protection is built on multiple layers with runtime checks that are not trivial to intercept via frida or fishhook.

Ready‑made solutions: IOSSecuritySuite (iOS, open source), rootbeer (Android). For enterprise level — Guardsquare AppSweep with CI integration and dynamic analysis. Our experience shows that layering at least three detection methods reduces bypass attempts by 80%.

Mobile app security engagement deliverables

How we implement protection: a case study from our practice

One of our clients came with a banking app that failed a security audit. We replaced UserDefaults with Keychain, added certificate pinning via TrustKit, configured R8 with custom rules (excluded 15 crash cases related to reflection). Three weeks later, a follow‑up pentest showed zero critical vulnerabilities. Since implementation — zero incidents in two years. Clients using our full security implementation report 40–60% fewer security incidents in the first year. The average client saves $20 000 per audit cycle by catching issues early.

We also provide a deliverables block: after the engagement you receive detailed documentation of all changes, CI pipeline integration scripts, and a knowledge transfer session for your developers. This ensures your team can maintain security independently.

Timeline and cost

• Security audit per OWASP MASVS L1 — from 1 to 2 weeks.
• Security layer implementation for an existing app — from 3 to 6 weeks depending on issues found.
• Full cycle "audit + implementation + test" — from 4 to 8 weeks.

Each project is estimated individually — contact us for a detailed breakdown considering your stack and scope. We work turnkey: from analysis to store deployment.

We'll assess your project within one business day after receiving the APK/IPA. Get in touch — we'll tell you which holes to close first. Schedule a consultation to discuss your mobile app security needs. Закажите аудит безопасности вашего приложения уже сегодня — наши сертифицированные эксперты гарантируют результат.