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Fix Broken User Authentication in Koa

Broken Authentication (OWASP A07:2021) in Koa applications typically manifests through weak password storage, lack of brute-force protection, and insecure session management. To harden a Koa stack, you must move away from manual credential checks and implement robust hashing, signed sessions, and rate limiting.

The Vulnerable Pattern

const Koa = require('koa');
const Router = require('@koa/router');
const bodyParser = require('koa-bodyparser');
const app = new Koa();
const router = new Router();

// VULNERABILITIES: Plaintext storage, no rate limiting, unsigned cookies
const users = [{ username: ‘admin’, password: ‘password123’ }];


router.post(‘/login’, async (ctx) => {
const { username, password } = ctx.request.body;
const user = users.find(u => u.username === username && u.password === password);


if (user) {
// Security Flaw: Cookie is not signed or encrypted; easily spoofed
ctx.cookies.set(‘user_id’, user.username, { httpOnly: false });
ctx.body = { message: ‘Authenticated’ };
} else {
ctx.status = 401;
ctx.body = { error: ‘Invalid credentials’ };
}
});


app.use(bodyParser()).use(router.routes());
app.listen(3000);

The Secure Implementation

The secure implementation addresses three critical vectors. First, it replaces plaintext comparison with Argon2id, a memory-hard hashing algorithm that resists GPU-based cracking. Second, it utilizes 'koa-session' with 'signed: true', ensuring the session cookie cannot be tampered with by the client without invalidating the signature. Third, it implements 'koa-ratelimit' to mitigate brute-force and credential stuffing attacks. Additionally, session cookies are hardened with 'httpOnly' to prevent XSS-based theft and 'secure' to ensure transmission only over HTTPS.

const Koa = require('koa');
const Router = require('@koa/router');
const bodyParser = require('koa-bodyparser');
const session = require('koa-session');
const ratelimit = require('koa-ratelimit');
const argon2 = require('argon2');

const app = new Koa();
const router = new Router();


// Secure session configuration
app.keys = [process.env.SESSION_SECRET || ‘super-secret-key’];
const CONFIG = { key: ‘koa.sess’, maxAge: 86400000, httpOnly: true, signed: true, secure: true, sameSite: ‘strict’ };


// Rate limiting to prevent brute force
const loginLimiter = ratelimit({
driver: ‘memory’, db: new Map(), duration: 60000, max: 5,
errorMessage: ‘Too many attempts. Try again later.’
});


// Mock DB with Argon2id hashes
const db = { admin: ‘$argon2id$v=19$m=65536,t=3,p=4$6i…hashed’ };


router.post(‘/login’, loginLimiter, async (ctx) => {
const { username, password } = ctx.request.body;
const hash = db[username];


if (hash && await argon2.verify(hash, password)) {
ctx.session.user = username;
ctx.body = { status: ‘ok’ };
} else {
ctx.status = 401;
ctx.body = { error: ‘Authentication failed’ };
}
});


app.use(bodyParser()).use(session(CONFIG, app)).use(router.routes());
app.listen(3000);

System Alert • ID: 1517
Target: Koa API
Potential Vulnerability

Your Koa API might be exposed to Broken User Authentication

74% of Koa apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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