JWT authentication flow in Astro

Introduction

JWT authentication flow helps keep users logged in safely by using tokens. It makes sure only the right people can access certain parts of a website or app.

When you want users to log in and stay logged in without typing their password every time.

When you need to protect parts of your website or app so only logged-in users can see them.

When you want a simple way to check if a user is allowed to do something.

When building apps that talk to servers and need to send user identity securely.

When you want to avoid storing session info on the server and keep things fast.

Syntax

Astro

1. User sends login info (like username and password) to server.
2. Server checks info and creates a JWT token if correct.
3. Server sends JWT token back to user.
4. User stores token (usually in localStorage or cookies).
5. User sends token with requests to access protected data.
6. Server checks token to allow or deny access.

The JWT token is a string with three parts separated by dots: header, payload, and signature.

Tokens are usually sent in HTTP headers like Authorization: Bearer <token>.

Examples

User sends login info, server responds with JWT token.

Astro

POST /login
Request body: { username: 'user1', password: 'pass123' }
Response: { token: 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...' }

User sends token to get protected profile data.

Astro

GET /profile
Headers: Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
Response: { name: 'User One', email: '[e-mail]' }

Sample Program

This Astro example shows a simple login that returns a JWT token and a protected route that checks the token before greeting the user.

Astro

import jwt from 'jsonwebtoken';

const SECRET = 'mysecretkey';

// Simulate user database
const users = [{ username: 'user1', password: 'pass123' }];

// Login handler
export async function POST({ request }) {
  const { username, password } = await request.json();
  const user = users.find(u => u.username === username && u.password === password);
  if (!user) {
    return new Response(JSON.stringify({ error: 'Invalid login' }), { status: 401 });
  }
  const token = jwt.sign({ username: user.username }, SECRET, { expiresIn: '1h' });
  return new Response(JSON.stringify({ token }), { status: 200 });
}

// Protected route example
export async function GET({ request }) {
  const authHeader = request.headers.get('Authorization');
  if (!authHeader || !authHeader.startsWith('Bearer ')) {
    return new Response(JSON.stringify({ error: 'No token provided' }), { status: 401 });
  }
  const token = authHeader.split(' ')[1];
  try {
    const payload = jwt.verify(token, SECRET);
    return new Response(JSON.stringify({ message: `Hello, ${payload.username}!` }), { status: 200 });
  } catch {
    return new Response(JSON.stringify({ error: 'Invalid token' }), { status: 401 });
  }
}

Important Notes

Always keep your secret key safe and never share it publicly.

Tokens expire to keep things secure; users may need to log in again after expiration.

Store tokens securely in the browser to avoid security risks like cross-site scripting.

Summary

JWT authentication uses tokens to keep users logged in safely.

Tokens are sent with requests to access protected parts of an app.

Servers check tokens to allow or deny access.

Practice

1. What is the main purpose of a JWT (JSON Web Token) in authentication?

easy

A. To securely transmit user identity and permissions between client and server

B. To store user passwords on the client side

C. To encrypt all user data in the database

D. To replace cookies for storing session data only

Solution

1. Step 1: Understand JWT role in authentication

   JWTs carry encoded user identity and permissions safely between client and server.

2. Step 2: Compare options with JWT purpose

   Only To securely transmit user identity and permissions between client and server correctly describes JWT's role; others confuse JWT with password storage or encryption.

3. Final Answer:

   To securely transmit user identity and permissions between client and server -> Option A

4. Quick Check:

   JWT purpose = secure token for identity [OK]

Hint: JWTs carry user info safely between client and server [OK]

Common Mistakes:

• Thinking JWT stores passwords
• Confusing JWT with database encryption
• Assuming JWT replaces cookies entirely

2. Which of the following is the correct way to send a JWT token in an HTTP request header?

easy

A. Authorization: Bearer <token>

B. Token: JWT <token>

C. Auth-Token: <token>

D. JWT-Authorization: Bearer <token>

Solution

1. Step 1: Recall standard JWT header format

   The standard way to send JWT is in the Authorization header with 'Bearer' prefix.

2. Step 2: Match options to standard

   Only Authorization: Bearer <token> matches the correct syntax: 'Authorization: Bearer <token>'. Others use incorrect header names or formats.

3. Final Answer:

   Authorization: Bearer <token> -> Option A

4. Quick Check:

   JWT header = Authorization: Bearer token [OK]

Hint: JWTs go in 'Authorization: Bearer <token>' header [OK]

Common Mistakes:

• Using wrong header names like 'Token' or 'Auth-Token'
• Omitting 'Bearer' prefix
• Adding extra words like 'JWT' before token

3. Given this Astro server code snippet handling JWT verification, what will be the output if the token is invalid?

import jwt from 'jsonwebtoken';

export async function get({ request }) {
  const authHeader = request.headers.get('Authorization');
  if (!authHeader) return new Response('No token', { status: 401 });

  const token = authHeader.split(' ')[1];
  try {
    const user = jwt.verify(token, 'secretkey');
    return new Response(`User: ${user.name}`, { status: 200 });
  } catch {
    return new Response('Invalid token', { status: 403 });
  }
}

medium

A. Response with status 200 and user name

B. Response with status 401 and message 'No token'

C. Response with status 403 and message 'Invalid token'

D. Response with status 500 and server error message

Solution

1. Step 1: Analyze token verification flow

   If token is invalid, jwt.verify throws an error caught by catch block.

2. Step 2: Check catch block response

   The catch block returns status 403 with message 'Invalid token'.

3. Final Answer:

   Response with status 403 and message 'Invalid token' -> Option C

4. Quick Check:

   Invalid token triggers 403 response [OK]

Hint: Invalid JWT triggers catch block with 403 response [OK]

Common Mistakes:

• Assuming invalid token returns 401
• Confusing catch block with success path
• Expecting server error instead of 403

4. Identify the bug in this Astro JWT authentication snippet:

export async function get({ request }) {
  const authHeader = request.headers.get('Authorization');
  if (!authHeader) return new Response('No token', { status: 401 });

  const token = authHeader.split(' ')[0];
  try {
    const user = jwt.verify(token, 'secretkey');
    return new Response(`User: ${user.name}`, { status: 200 });
  } catch {
    return new Response('Invalid token', { status: 403 });
  }
}

medium

A. Missing secret key in jwt.verify call

B. Token extraction uses wrong index from split

C. No check for missing Authorization header

D. Response status codes are reversed

Solution

1. Step 1: Check how token is extracted

   Token is extracted using authHeader.split(' ')[0], which gets 'Bearer' instead of the token.

2. Step 2: Identify correct token index

   The token is at index 1 after splitting by space, so index 0 is wrong.

3. Final Answer:

   Token extraction uses wrong index from split -> Option B

4. Quick Check:

   Token index must be 1, not 0 [OK]

Hint: Token is after 'Bearer', so use split index 1 [OK]

Common Mistakes:

• Using split index 0 instead of 1
• Forgetting to check Authorization header
• Mixing up status codes 401 and 403

5. You want to implement a refresh token flow in Astro using JWT. Which approach correctly handles token expiration and refresh securely?

hard

A. Send access token only once at login and never refresh it; rely on long expiration time

B. Store both access and refresh tokens in localStorage and refresh access token automatically on every request

C. Store refresh token in localStorage and access token in URL query parameters for easy access

D. Store access token in memory and refresh token in HttpOnly cookie; use refresh token to get new access token when expired

Solution

1. Step 1: Understand secure token storage

   Access tokens should be stored in memory to reduce exposure; refresh tokens in HttpOnly cookies to prevent JavaScript access.

2. Step 2: Use refresh token to get new access token

   When access token expires, client uses refresh token securely to request a new access token.

3. Final Answer:

   Store access token in memory and refresh token in HttpOnly cookie; use refresh token to get new access token when expired -> Option D

4. Quick Check:

   Secure refresh flow = memory + HttpOnly cookie [OK]

Hint: Keep refresh token HttpOnly, access token in memory [OK]

Common Mistakes:

• Storing tokens in localStorage (XSS risk)
• Never refreshing tokens (causes logout)
• Putting tokens in URL (exposes tokens)