Fix Broken User Authentication in Fastify
Broken Authentication remains a top-tier exploit vector. In the Fastify ecosystem, the 'fast-by-default' philosophy often leads developers to implement naive authentication logic, such as plaintext password comparisons or predictable session identifiers. To secure a Fastify instance, we must move away from manual credential checking and adopt hardened plugins that enforce cryptographic standards and prevent brute-force enumeration.
The Vulnerable Pattern
const fastify = require('fastify')();// VULNERABLE: Manual auth, plaintext storage, no rate limiting fastify.post(‘/login’, async (request, reply) => { const { username, password } = request.body; const user = await db.users.findOne({ username });
if (user && user.password === password) { // VULNERABLE: Hardcoded/Predictable session token return { success: true, token: ‘user-session-999’ }; }
// VULNERABLE: Verbose errors allow account enumeration return reply.code(401).send({ error: ‘Invalid password for ’ + username }); });
The Secure Implementation
The secure implementation mitigates three primary attack vectors: 1. Credential Stuffing: Argon2id is used for password hashing, providing high resistance against GPU-accelerated cracking. 2. Brute Force: @fastify/rate-limit throttles repeated login attempts at the middleware layer. 3. Account Enumeration: By returning a generic 'Authentication failed' message and ensuring the timing of the verification is consistent (even if the user doesn't exist), we prevent attackers from mapping valid usernames. Additionally, we use @fastify/jwt to issue cryptographically signed, short-lived tokens instead of predictable session IDs.
const fastify = require('fastify')(); const argon2 = require('argon2');// SECURE: Register hardened plugins fastify.register(require(‘@fastify/jwt’), { secret: process.env.JWT_SECRET }); fastify.register(require(‘@fastify/rate-limit’), { max: 5, timeWindow: ‘1 minute’ });
fastify.post(‘/login’, async (request, reply) => { const { username, password } = request.body; const user = await db.users.findOne({ username });
// SECURE: Use Argon2 for side-channel resistant verification const isValid = user ? await argon2.verify(user.passwordHash, password) : false;
if (!isValid) { // SECURE: Generic error message prevents enumeration return reply.code(401).send({ error: ‘Authentication failed’ }); }
// SECURE: Short-lived, signed JWT const token = fastify.jwt.sign({ sub: user.id }, { expiresIn: ‘15m’ }); return { token }; });
Your Fastify API
might be exposed to Broken User Authentication
74% of Fastify apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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