How to Make an App Secure: The Five Layers That Matter

Short answer

App security comes down to five layers: protect data at rest on the device, protect it in transit, authenticate people properly, control what the backend will accept, and keep dependencies current. Apple provides the device-side primitives through Keychain, the Secure Enclave and App Transport Security, and the OWASP Mobile Top 10 is the working checklist for the rest. Most real breaches trace to the backend or to secrets left in the app, not to iOS itself.

Where mobile apps actually get breached

The mental image of an attacker reverse-engineering a Swift binary is mostly wrong. The common failures are duller and more preventable.

Secrets shipped inside the app are the first. An API key, a database credential or a signing secret compiled into the binary is readable by anyone willing to extract it, and a public app is a public distribution of that secret. Anything the client holds should be treated as known to an attacker.

Backends that trust the client are the second, and the most damaging. If your server accepts “userId” from the app and returns that user’s data without checking who is authenticated, the app is irrelevant: a person with a proxy tool can request any account. Every authorisation decision belongs on the server, checked against the authenticated session rather than against a value the client supplied.

Insecure local storage is the third. Access tokens or personal data written to plain files or to UserDefaults are readable on a jailbroken device and can end up in backups. The Keychain exists for exactly this, and Apple’s security documentation covers how to store credentials so they benefit from hardware protection.

Weak or homemade authentication is the fourth: password rules that allow reuse, tokens that never expire, no rate limiting on login, or a password reset flow that leaks whether an account exists.

Old dependencies are the fifth, and the easiest to fix. Every third-party SDK is code you ship and cannot audit line by line; the ones that go unmaintained become the vulnerability you inherit.

Layer	What it protects	The common mistake
Local storage	Tokens, credentials, cached personal data	Writing tokens to UserDefaults or plain files
Transport	Data between app and server	Disabling ATS or trusting any certificate
Authentication	Who the user is	Tokens that never expire, no rate limiting
Authorisation	What that user may see	Trusting an ID sent by the client
Dependencies	Everything the SDKs touch	Never updating, no inventory of what ships

The device-side work, concretely

Store credentials in the Keychain, not in UserDefaults, and mark them with the strictest accessibility level the feature allows so they are unavailable while the device is locked. Where the app protects something sensitive, gate it behind Face ID or Touch ID with a passcode fallback, and treat biometric success as a local convenience rather than as proof of identity to your server.

Keep App Transport Security enabled. It enforces modern TLS for network calls by default, and the exceptions that disable it usually exist because a legacy endpoint was never fixed. Fixing the endpoint is cheaper than shipping an app that accepts weak connections.

Log with care. Crash reports and analytics are where personal data leaks accidentally, and a log line that seemed harmless in development becomes a data flow you must declare. Apple requires an accurate account of what you collect and why, published as privacy details on the product page, so the logging decision and the compliance decision are the same decision.

Assume the binary is readable. Obfuscation raises the cost of casual inspection and does not change the threat model: design so that nothing catastrophic happens when someone reads your code, because eventually someone will.

The backend is where most of the risk lives

Three rules carry most of the weight.

Authorise every request against the session, never against client input. The pattern to look for in a code review is any endpoint that takes an identifier and returns data without asking whether the authenticated user is entitled to it.

Validate and rate limit at the edge. Input validation stops injection classes; rate limiting stops credential stuffing, enumeration and the accidental cost of a runaway client. Both are cheap to add early and awkward to retrofit.

Keep secrets on the server and rotate them. Third-party keys, push certificates and database credentials belong in a secrets manager with an owner and a rotation date, not in a repository or a chat thread.

For teams choosing infrastructure, both major managed options handle these primitives well when configured correctly. The security question is rarely which platform, it is whether the rules on top are written and tested, since a misconfigured rule set in a managed backend exposes data just as effectively as a hand-rolled server with no checks.

Testing that the layers actually hold

Security work that is never tested is a set of intentions. Four checks catch most of what matters, and none of them needs a specialist.

Read your own binary. Extract the app bundle from a build and search the strings for anything resembling a key, a URL with credentials, or an internal hostname. Finding nothing takes ten minutes; finding something takes an afternoon to fix and saves a disclosure.

Proxy your own traffic. Point the app at a local intercepting proxy and watch what it sends. This surfaces two classes of problem at once: data you did not realise was leaving the device, and endpoints that answer questions they should refuse. While you are there, replay a request with a different user identifier and confirm the server rejects it.

Test the token lifecycle. Log in, capture the session token, log out, then replay a request with the old token. If it still works, the server is not invalidating sessions, which is a common gap in hand-rolled authentication and an easy one to miss because the app behaves correctly.

Inventory what ships. Every SDK in the project, with a version, a maintenance status and a reason it is there. The list itself is the deliverable, since it turns a vague concern about dependencies into a decision you can make each release. Anything on the list that nobody can justify should come out, because unused code still ships and still carries risk.

Compliance is a design input, not a final step

If your app handles personal data of people in the EU, GDPR applies regardless of where your company sits, which shapes the data model rather than the launch checklist: what you collect, how long you keep it, how a user deletes their account and what happens to their data when they do. Apple requires apps that support account creation to also support account deletion in-app, which turns a legal principle into a shipping requirement.

Payments follow the same logic. Taking card details in your own interface pulls you into PCI scope; using Apple Pay or a hosted provider keeps the card data out of your systems entirely, which is usually the right call for a small team.

The practical version: decide what you genuinely need to collect before the design phase, because every extra field is a permanent obligation. Apps that collect less are easier to secure, cheaper to comply with, and faster to review.

Check	When	Who owns it
No secrets in the client binary	Before first TestFlight build	Developer
Server authorises every request	Before beta with real users	Backend developer
Keychain for tokens and credentials	Before first TestFlight build	Developer
Privacy details match real data flows	Before App Store submission	Product owner
Account deletion works end to end	Before App Store submission	Product owner
Dependency inventory and update pass	Every release	Developer
Rate limiting on auth endpoints	Before public launch	Backend developer

What we do on AJBU projects, and the honest limit

We treat security as part of the architecture conversation in discovery rather than as a review at the end. That means deciding what data the app genuinely needs before designing screens, keeping tokens in the Keychain, putting every authorisation check on the server, and shipping an account deletion flow because Apple requires it and because it is the right default. Dependencies get reviewed at each release, and anything that stops being maintained gets replaced rather than pinned forever.

The honest limit: no studio can promise an app is secure. Security is a moving target, most of the risk after launch sits in operational practices we do not control, who has access to the production console, whether staff use a password manager, how support verifies a caller, and a serious threat model for a regulated product needs a specialist audit rather than a development team’s judgment. For fintech, health data or anything holding money, budget for an external penetration test before launch and treat the studio’s work as the foundation it tests, not as the certificate.

If you are planning an app that will hold personal or financial data and want a straight answer on what it needs before you commit to a scope, book a free app idea call and we will walk through the data model and the risks with you.