Flutter App Security Best Practices for Fintech App

Building out secure Flutter apps for fintech is a must if your app deals with financial info, personal user data, or sensitive APIs.

I recently had a chance to work on a Flutter-based fintech project where security was an integral need right from the first day. Each line of code had to ask this one question: What if this gets exploited?

This blog goes over what I did to put in place Flutter app security best practices, which we put in for meet security standards. Also, we did a professional Vulnerability Assessment and Penetration Testing (VAPT) audit. If you are developing a fintech app or any app that deals with private info, these practices will help you build trust and also prevent potential threats.

Here is what I put together for the 4 core areas which cover Flutter app security best practices:

• Secure Data Storage
• SSL Pinning for API Communication
• AES Encryption for Requests/Responses
• Device Integrity & Root Detection

Each section presents a real-world example, the Flutter packages used, and some working code to get you started.

1. Secure Data Storage with flutter_secure_storage

Let’s be honest: Developers tend to use SharedPreferences which is a quick and easy solution. However when it comes to saving sensitive info like access tokens, user MPINs, or session keys plaintext storage is too risk.

For my fin tech app we use flutter_secure_storage that works with the Keychain on iOS and the Keystore/EncryptedSharedPreferences on Android. We keep data encrypted at rest and protected from other apps and tamper attempts.

Package

dependencies:

flutter_secure_storage: ^9.0.0

Sample Code

import 'package:flutter_secure_storage/flutter_secure_storage.dart';

final secureStorage = FlutterSecureStorage();

// Save data securely
await secureStorage.write(key: 'authToken', value: 'xyz123');

// Retrieve data
String? token = await secureStorage.read(key: 'authToken');

// Delete data when no longer needed
await secureStorage.delete(key: 'authToken');

Why it mattered in my case:

• I held MPINs and tokens that must be kept safe even in the event of a lost phone.
• Tokens were automatically cleared on logout or session expiration.
• Not a single sensitive value was ever touched in logs or insecure storage.

2. API Communication Security with SSL Pinning

Just using HTTPS is not sufficient. If an end-user installs a fake certificate on the device (intentionally or unintentionally), your HTTPS traffic can be intercepted through a Man-in-the-Middle (MITM) attack.

To avoid that, I used SSL pinning by accepting only a particular certificate. That is, the app talks to the server only if the certificate is identical to what I have pinned.

I used the Dio package and configured it to load a PEM certificate from assets. This aligns with Flutter security for fintech apps, offering safer communication without relying on third-party SSL pinning plugins.

How I Did It:

• Download the server certificate.
• Convert it to .pem format using OpenSSL.
• Place the file in your assets directory.
• Tell Dio to use it while making network requests.

Sample Code

import 'dart:io';
import 'package:dio/dio.dart';
import 'package:flutter/services.dart' show rootBundle;

Future<Dio> getPinnedDioClient() async {
  final sslCert = await rootBundle.load('assets/certificates/certificate.pem');
  final context = SecurityContext(withTrustedRoots: false);
  context.setTrustedCertificatesBytes(sslCert.buffer.asUint8List());

  final httpClient = HttpClient(context: context);
  httpClient.badCertificateCallback = (cert, host, port) => false;

  final dio = Dio();
  dio.httpClientAdapter = DefaultHttpClientAdapter()..onHttpClientCreate = (_) => httpClient;

  return dio;
}

Why it mattered in my case:

• The backend was sensitive, dealing with bank account information, wallets, and personal info.
• SSL pinning made sure that even if a device contained an insecure root cert installed, it could not intercept data from our app.

3. AES Encryption for API Payloads using encrypt

Most programmers presume HTTPS is sufficient. But I wanted to go one step further. What if I encrypted the payload before HTTPS even got a hold of it?

For this project, I used AES encryption with the encrypt package. This is a fantastic demonstration of Flutter app security best practices in action, making sure even if HTTPS gets compromised, your data stays unreadable to attackers.

All outgoing API request bodies were encrypted. The server decrypted it with the same AES key. Responses were also encrypted before sending back.

Package

dependencies:

encrypt: ^5.0.1

Sample Code

import 'package:encrypt/encrypt.dart';
import 'dart:convert';

final key = Key.fromUtf8("32charlongencryptionkey1234567890");
final iv = IV.fromLength(16);
final encrypter = Encrypter(AES(key, mode: AESMode.cbc));

String encryptPayload(Map<String, dynamic> data) {
  final jsonStr = jsonEncode(data);
  return encrypter.encrypt(jsonStr, iv: iv).base64;
}

Map<String, dynamic> decryptPayload(String encryptedBase64) {
  final decrypted = encrypter.decrypt(Encrypted.fromBase64(encryptedBase64), iv: iv);
  return jsonDecode(decrypted);
}

Why it mattered in my case:

• It safeguarded sensitive information such as KYC information, Aadhaar numbers, and banking information.
• Even in case the HTTPS layer was compromised, the encrypted body could not be read.
• The backend was set up to deal with AES encryption/decryption through the same key.

4. App Integrity Checks with app_integrity_checker

What good is all that backend protection if someone can install your APK on a rooted phone, bypass checks, and log API requests?

To tackle this, I implemented multiple device integrity checks using the app_integrity_checker package. It helps detect if:

• The device is rooted/jailbroken
• Developer mode is enabled
• The app is running in an emulator

Package

dependencies:

app_integrity_checker: ^1.0.1

Sample Code

import 'package:app_integrity_checker/app_integrity_checker.dart';

bool isSecure = await AppIntegrityChecker.isDeviceSecure();
bool isRooted = await AppIntegrityChecker.isRooted;
bool isDevMode = await AppIntegrityChecker.isDeveloperMode;
bool isEmulator = await AppIntegrityChecker.isEmulator;

if (!isSecure || isRooted || isDevMode || isEmulator) {
  // Show warning, restrict access, or logout user
}

Why it mattered in my case:

• I caught several test users trying to run the app on emulators.
• Rooted devices were automatically logged out and blocked from sensitive actions.
• Developer mode warnings were shown when needed.

Summary: My Security Checklist for Flutter + Fintech

Following these Flutter app security best practices will help you build safer, more reliable fintech apps that earn user trust and stay ahead of threats.

Final Thoughts

Security is not merely about staying out of trouble or checking boxes on a compliance list. It's about trusting users. In Flutter security for fintech apps, people trust you with their money. That means your app has to be as secure as your intention.

By adding levels of secure storage, encryption, pinning, and integrity checks, I created a Flutter app that didn't merely "work," but was prepared to withstand real-world threats.

These Flutter app security best practices are not complicated. You don't have to be a security specialist to apply them. All you need is the right mindset and the determination to go the extra mile.

If you're working on a sensitive or large-scale project, choose Flutter and consider these your starting blocks. And if you ever need to discuss securing your application or implementing these into your product, my inbox is open.