Implementing JWT Authentication in Flutter Web

Introduction

JWT (JSON Web Token) is a compact, URL-safe means of representing claims between two parties. For Flutter Web applications, JWTs provide a stateless authentication model that scales well for mobile development teams who want a single backend powering web and mobile clients. This tutorial walks through the practical steps to implement JWT authentication in Flutter Web, focusing on token acquisition, storage considerations for the browser, attaching tokens to requests, and refresh strategies.

Why JWT For Flutter Web

JWTs are self-contained: they carry user identity and expiry information in a signed token. For Flutter Web, that means fewer server-side session lookups and consistent auth between web and mobile clients. Use JWT when your APIs are microservice-friendly and when you can protect tokens from client-side vulnerabilities. JWTs are not a silver bullet — they must be paired with secure storage and refresh strategies to mitigate theft and XSS risks.

Backend Token Handling

The backend should issue two tokens where possible: a short-lived access token (JWT) and a longer-lived refresh token. Access tokens authenticate API calls and should have a small TTL (minutes). Refresh tokens are used to request new access tokens and should be stored and handled with greater care (ideally in HTTP-only cookies when used by web clients).

Design notes for the backend:

• Sign access tokens with a strong secret or asymmetric keys.

• Encode only non-sensitive claims (no passwords, no PII).

• Provide an endpoint for token refresh that validates the refresh token and rotates it on use.

Storing Tokens Securely

Browser storage choices for Flutter Web:

• localStorage/sessionStorage: convenient but accessible from JavaScript; vulnerable to XSS.

• HTTP-only cookies: cannot be read by JS and are recommended for refresh tokens, but need CSRF protection and same-site settings.

• In-memory: safest against XSS but lost on full page reload (you can combine with a short-lived cookie to rehydrate).

Practical approach for many Flutter Web apps:

• Store access tokens in memory while the app is active.

• Store refresh tokens in an HTTP-only, Secure, SameSite cookie set by the backend.

• If you must persist something client-side (e.g., offline support), be explicit about XSS exposure and consider encryption with careful key management.

Example: storing a token in browser localStorage (only if you accept XSS risk):

import 'dart:html' as html;
void saveAccessToken(String token) {
  html.window.localStorage['access_token'] = token;
}
String? readAccessToken() => html.window.localStorage['access_token'];

Attaching Tokens To Requests

Use an HTTP client layer that adds the Authorization header to outgoing requests. Do not sprinkle header logic throughout UI code — centralize it in a client or interceptor. If you use the http package, implement a BaseClient that injects the header.

import 'package:http/http.dart' as http;
class AuthClient extends http.BaseClient {
  final http.Client _inner;
  final String Function() _getToken;
  AuthClient(this._inner, this._getToken);
  @override
  Future<http.StreamedResponse> send(http.BaseRequest request) {
    final token = _getToken();
    if (token.isNotEmpty) request.headers['Authorization'] = 'Bearer $token';
    return _inner.send(request);
  }
}

Refreshing Tokens And Middleware

Implement automatic refresh on 401 responses. The client should pause outgoing requests, attempt a refresh, update the stored token, then retry the failed request. Be careful to avoid refresh storms: when multiple requests fail concurrently, ensure only one refresh is in-flight and others await the result.

On the backend, issue rotated refresh tokens and invalidate used ones. For web, prefer storing refresh tokens in HTTP-only cookies and call a /refresh endpoint that uses the cookie to validate rotation; this avoids exposing the refresh token to JavaScript.

Security Checklist

• Use short-lived access tokens and rotate refresh tokens.

• Prefer HTTP-only cookies for refresh tokens on web.

• Centralize Authorization header injection.

• Protect refresh endpoint against CSRF when necessary.

• Log and monitor token misuse and implement revocation where practical.

Conclusion

Implementing JWT in Flutter Web requires balancing usability and security. Use short-lived JWTs for API calls, place refresh tokens where JavaScript cannot read them (HTTP-only cookies), centralize token handling in an HTTP client, and implement a robust refresh flow with rotation and concurrency control. Following these practical steps will keep your Flutter web apps aligned with secure mobile development practices while providing a consistent auth model across platforms.

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