Protecting user data in a mobile app comes down to five habits: collect only what you need, encrypt it in transit and on the device, keep secrets off the phone, secure the server API as if every request were hostile, and be honest about data use in store listings. Most real breaches happen at the API, not inside the app.
This checklist is for business owners commissioning an app. You don't need to implement it yourself, but you should be able to ask your developers about every line and get a clear answer.
Where does mobile app data actually leak?
Usually at the backend. The app on the phone is a client; the valuable data lives on your server, and the server is what attackers probe.
The common failure points:
- The API trusts the app. It assumes requests come from your genuine app and skips checks. Anyone can call your API directly with a script, so the server must check every permission itself.
- Secrets shipped inside the app. API keys, admin tokens or payment credentials bundled into the app file can be extracted by anyone who downloads it.
- Data stored in plain text on the device. Login tokens or personal details saved in ordinary app storage, readable on a rooted or jailbroken phone or from an unencrypted backup.
- Over-collection. Asking for contacts, precise location or the photo library "just in case" increases what you can lose and what you must disclose.
- Third-party SDKs. Analytics, ad and chat libraries that collect more than you realised and send it somewhere you didn't approve.
The industry reference for this is the OWASP Mobile Top 10, a free list of the most common mobile risks. Ask your developer which items on it they test for.
What should the security checklist cover?
Cover data collection, storage, transport, authentication, the API and your suppliers. Use the table below as a working checklist in your project kick-off or before launch.
| Area | What "done" looks like | Question to ask your developer |
|---|---|---|
| Data minimisation | Every field and permission has a written reason | "Which permissions do we request, and why each one?" |
| Storage on device | Tokens in iOS Keychain / Android Keystore; no personal data in plain files or logs | "Where is the login token stored?" |
| Transport | HTTPS only (TLS); no plain-HTTP exceptions in production | "Are there any cleartext exceptions in the config?" |
| Authentication | Short-lived access tokens, refresh tokens, optional biometric unlock | "What happens when a phone is lost or a token is stolen?" |
| Server-side authorisation | Every endpoint checks the user may see that specific record | "If I change an ID in a request, can I see someone else's data?" |
| Rate limiting | Login, OTP and password-reset endpoints throttled per account and per IP | "How do we stop OTP or password guessing?" |
| Secrets | No private keys in the app; secrets live on the server | "Is there anything in the app bundle we'd regret leaking?" |
| Third-party SDKs | Inventory of every SDK and what data it collects | "Can you list every SDK and its data use?" |
| Logging and monitoring | Errors tracked without logging passwords, OTPs or full card data | "What do our crash reports contain?" |
| Updates | Plan for security patches and forced-update for critical fixes | "Can we force users onto a fixed version?" |
How should data be stored on the phone?
Store as little as possible, and put anything sensitive in the platform's secure storage. iOS has the Keychain and Android has the Keystore; both are designed for credentials and keys.
Practical rules:
- Don't store passwords at all. Store a token that can be revoked from the server.
- Cache personal data only when it improves the experience, such as an order history for offline viewing, and clear it on logout.
- Keep sensitive screens out of the app switcher preview for banking-style apps, so a passer-by can't see account details.
- Strip personal data from logs. Debug logging that prints full API responses is a common accidental leak, especially when logs are sent to a crash-reporting service.
Why is the API the most important part of app security?
Because the API is the front door to your data, and it's reachable by anyone, not just your app. Hiding a button in the app does not protect the data behind it.
The single most common serious flaw is broken object-level authorisation: the app asks for /orders/1043, and the server returns order 1043 without checking it belongs to the logged-in user. Change the number and you see a stranger's order. Every request must be checked on the server against the user's identity and role. Our guide to user authentication and roles in web apps explains how those roles should be designed.
Rate limiting matters just as much. On our own website we found and fixed a login rate-limit bypass: the limiter trusted an IP-address header that a client can set to anything, so an attacker could appear to be a new visitor on every attempt. The fix was to read the IP only from our own trusted proxy. Mobile backends have the same weakness when they trust headers the client controls, and OTP endpoints are a favourite target because guessing a six-digit code is cheap without limits.
Other API basics:
- Validate every input on the server, not just in the app.
- Return only the fields the screen needs, not the whole database record.
- Version the API so you can retire insecure old versions.
What do Apple and Google require on privacy?
Both stores require you to disclose what data you collect and why, and both reject or remove apps whose behaviour doesn't match the disclosure. This is now part of shipping, not an optional extra.
- Apple asks for privacy "nutrition label" details in App Store Connect, requires a privacy policy link, and requires the App Tracking Transparency prompt before you track users across other companies' apps and websites. Apps that let users create an account must also let them delete it from within the app.
- Google Play requires a Data safety section describing collection, sharing and security practices, plus a privacy policy, and also has account-deletion requirements.
Your third-party SDKs count towards these disclosures. If an analytics library collects device identifiers, you must declare it. Our App Store and Google Play submission checklist covers the rejection reasons that most often catch first-time publishers.
Which privacy laws apply to my app?
The laws of the places where your users live usually apply, regardless of where your company is based. For most GRWW clients, that means some combination of the following:
- India: the Digital Personal Data Protection Act 2023 (DPDP Act) governs personal data, with consent and purpose limitation at its core.
- Australia: the Privacy Act 1988 and the Australian Privacy Principles, which apply to many businesses above a turnover threshold and to some smaller ones in specific sectors.
- Europe and the UK: GDPR and UK GDPR, with strict rules on lawful basis, consent and data-subject rights.
- USA: no single federal privacy law; state laws such as California's apply, plus sector rules.
This isn't legal advice; for health, financial or children's data, talk to a privacy lawyer. The engineering response is the same everywhere: collect less, record consent, let users access and delete their data, and know where it's stored.
How much does good app security cost?
Most of it costs very little if planned from the start, and a lot if retrofitted. Secure storage, HTTPS, server-side checks and rate limits are standard practice and should be inside any reasonable quote.
What can add cost:
- An independent penetration test before launch, typically a separate engagement whose price depends on scope.
- Extra controls for regulated data, such as audit logs, encrypted databases and stricter access reviews.
- Ongoing patching. Security updates are part of the roughly 15–25% of build cost per year we suggest budgeting for maintenance; see our breakdown of mobile app maintenance costs.
If a quote is noticeably cheaper than others, ask what security work was left out.
What should I do before launch day?
Run a short review against the checklist, then fix anything touching authentication, authorisation or secrets before you submit. In particular, ask a developer who didn't build the API to try reading another user's data by changing IDs, inspect the release build for embedded keys, and confirm in-app account deletion works end to end.
For the wider picture of how an app gets planned, built and launched, see our complete guide to mobile app development.
Frequently asked questions
Are iOS apps more secure than Android apps?
Both platforms provide strong security features, including sandboxing, secure key storage and app review. The bigger differences come from how your app and its backend are built. A badly designed API is equally exposed whichever phone the user carries.
Do I need certificate pinning?
Certificate pinning makes the app trust only your specific server certificate, which helps against interception on hostile networks. It's worth considering for banking, health or high-value apps, but it adds maintenance because certificates must be rotated carefully. For most business apps, correct HTTPS and strong server-side checks matter more.
Is a cross-platform app (Flutter or React Native) less secure?
Not inherently. Flutter and React Native apps can use the same Keychain and Keystore storage and the same HTTPS protections as native apps. Security depends on implementation and the backend far more than on the framework.
Should I store payment card details in my app?
No. Use a payment provider's SDK or hosted checkout so card details go straight to the provider, and keep only a token on your side. This reduces both your risk and your compliance burden considerably.
How often should an app get security updates?
Plan for regular dependency and OS-compatibility updates through the year, plus an immediate fix whenever a serious vulnerability is found in your code or a library you use. Apps that go untouched for a year or more tend to accumulate known vulnerabilities in outdated libraries.
Build it secure from the first sprint
Security is far cheaper to design in than to bolt on after an incident. If you're planning an app and want the data handling, API checks and store disclosures scoped properly from day one, talk to us about our app development services or get a fixed-price quote.