Web Security Basics Every Developer Must Know (2026)
Security isn't just for security teams. Every developer needs these fundamentals to protect their applications and users.
The Threat Landscape in 2026
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Most common attacks targeting web apps:
1. SQL Injection — Still #1, still devastating
2. XSS (Cross-Site Scripting) — Steals sessions, defaces sites
3. CSRF (Cross-Site Request Forgery) — Actions on behalf of users
4. Authentication bypass — Weak passwords, session fixation
5. Sensitive data exposure — API keys in code, unencrypted data
6. IDOR (Broken Access Control) — Accessing others' data
7. SSRF (Server-Side Request Forgery) — Internal network probing
8. Dependency vulnerabilities — Compromised npm/pip packages
Key principle: Defense in depth
→ Don't rely on one security layer
→ Multiple independent controls
→ If one fails, others catch it
#1 SQL Injection Prevention
CODE
// ❌ VULNERABLE: String concatenation
const query = `SELECT * FROM users WHERE email = '${email}'`;
// Attacker inputs: ' OR '1'='1' --
// Result: Returns ALL users!
// ✅ Parameterized queries (always!)
const user = db.prepare('SELECT * FROM users WHERE email = ?').get(email);
// The database treats the input as DATA, not code.
// With ORM (Sequelize/TypeORM/Prisma):
User.findOne({ where: { email } }); // Safe by default
// Even with parameterized queries, validate input first:
function isValidEmail(email) {
return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
}
if (!isValidEmail(email)) return res.status(400).json({ error: 'Invalid email' });
// ⚠️ Dangerous: Dynamic table/column names can't be parameterized!
const allowedTables = ['users', 'products', 'orders'];
if (!allowedTables.includes(table)) throw new Error('Invalid table');
#2 XSS (Cross-Site Scripting) Defense
CODE
// Types of XSS:
// 1. Stored XSS: Malicious script saved in DB, shown to all viewers
// → Comment sections, profiles, product reviews
// 2. Reflected XSS: Script in URL, reflected back in response
// → Search results, error messages
// 3. DOM-based XSS: Client-side script modifies DOM unsafely
// → innerHTML, document.write(), eval()
// ❌ Vulnerable:
app.get('/search', (req, res) => {
res.send(`Results for: ${req.query.q}`); // Script injection!
});
// ✅ Output encoding:
const escapeHtml = (str) => str
.replace(/&/g, '&')
.replace(/</g, '<')
.replace(/>/g, '>')
.replace(/"/g, '"')
.replace(/'/g, ''');
res.send(`Results for: ${escapeHtml(req.query.q)}`);
// ✅ For rich content, use a sanitizer:
import DOMPurify from 'dompurify';
const clean = DOMPurify.sanitize(userContent);
// ✅ Set Content-Type headers (prevents MIME sniffing):
res.setHeader('Content-Type', 'application/json; charset=utf-8');
// Browser won't try to render JSON as HTML
// ✅ CSP (Content Security Policy):
res.setHeader('Content-Security-Policy',
"default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'"
);
#3 CSRF Protection
CODE
// How CSRF works:
// 1. User is logged into your site (has valid cookie)
// 2. User visits attacker's site
// 3. Attacker's page submits a form to YOUR site
// 4. Browser automatically sends cookies → action executed as logged-in user!
// ✅ Solution 1: CSRF tokens (most common)
import crypto from 'crypto';
// Generate token on server:
function generateCsrfToken() {
return crypto.randomBytes(32).toString('hex');
}
// Store in session:
app.use((req, res, next) => {
if (!req.session.csrfToken) {
req.session.csrfToken = generateCsrfToken();
}
next();
});
// Send to client (cookie or template variable):
res.cookie('XSRF-TOKEN', req.session.csrfToken, { httpOnly: false });
// httpOnly: false so JavaScript can read it and send in header
// Validate on every state-changing request:
app.post('/api/update', (req, res) => {
const token = req.headers['x-csrf-token'] || req.body._csrf;
if (token !== req.session.csrfToken) {
return res.status(403).json({ error: 'CSRF token mismatch' });
}
// Process request...
});
// ✅ Solution 2: SameSite cookies (modern approach)
res.setHeader('Set-Cookie', [
'sessionId=abc123; Path=/; HttpOnly; Secure; SameSite=Strict',
// Strict: Never send cross-site (safest, breaks some legitimate use cases)
// Lax: Send on top-level navigation (GET), not on requests from other sites
// None: Always send (required for cross-site POSTs, must also set Secure)
]);
// ✅ Solution 3: Check Origin/Referer header
app.use((req, res, next) => {
if (['POST', 'PUT', 'PATCH', 'DELETE'].includes(req.method)) {
const origin = req.headers.origin || req.headers.referer?.split('/')[2];
const allowed = ['https://myapp.com', 'https://www.myapp.com'];
if (origin && !allowed.some(o => o.endsWith(origin))) {
return res.status(403).json({ error: 'Origin not allowed' });
}
}
next();
});
#4 Authentication Best Practices
CODE
// Password hashing (NEVER store plaintext!)
import bcrypt from 'bcryptjs';
// Hash on registration:
const hash = await bcrypt.hash(password, 12); // Cost factor 12 (adjust for CPU)
// Verify on login:
const valid = await bcrypt.compare(password, storedHash);
// bcrypt handles salting automatically (unique salt per password)
// Session management:
import { sign, verify } from 'jsonwebtoken';
const token = sign(
{ userId: user.id, role: user.role },
process.env.JWT_SECRET,
{ expiresIn: '15m' } // Short-lived access tokens!
);
// Refresh token pattern (longer-lived, stored securely):
const refreshToken = crypto.randomBytes(40).toString('hex');
await db.storeRefreshToken(userId, refreshToken, { expiresIn: '7d' });
// Rate limit auth endpoints:
import rateLimit from 'express-rate-limit';
const loginLimiter = rateLimit({
windowMs: 15 * 60 * 1000,
max: 10, // 10 attempts per 15 minutes per IP
message: { error: 'Too many attempts, try again later' },
});
app.post('/api/login', loginLimiter, handleLogin);
// Account lockout after failed attempts:
async function checkAccountLock(email) {
const attempts = await getFailedAttempts(email);
if (attempts.count >= 5 && attempts.lastAttempt > Date.now() - 15 * 60 * 1000) {
throw new Error('Account temporarily locked');
}
}
#5 Input Validation Framework
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// Validate EVERY external input (form data, API params, file uploads)
const Joi = require('joi');
const schemas = {
register: Joi.object({
email: Joi.string().email().max(254).required(),
password: Joi.string()
.min(8).max(128)
.pattern(/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)/)
.required()
.messages({
'string.pattern.base': 'Password must contain uppercase, lowercase, and number'
}),
name: Joi.string().min(1).max(100).trim().required(),
role: Joi.string().valid('user', 'creator').default('user'),
}),
pagination: Joi.object({
page: Joi.number().integer().min(1).default(1),
limit: Joi.number().integer().min(1).max(100).default(20),
sort: Joi.string().valid('name', 'date', 'popularity').default('date'),
order: Joi.string().valid('asc', 'desc').default('desc'),
}),
};
// Middleware:
function validate(schema) {
return (req, res, next) => {
const { error, value } = schema.validate(req.body, {
abortEarly: false, // Return ALL errors, not just first
stripUnknown: true, // Remove unexpected fields
});
if (error) {
return res.status(400).json({
error: 'Validation failed',
details: error.details.map(d => ({
field: d.path.join('.'),
message: d.message,
})),
});
}
req.body = value; // Use sanitized values
next();
};
}
app.post('/api/register', validate(schemas.register), handleRegister);
#6 Security Headers Checklist
CODE
import helmet from 'helmet';
app.use(helmet({
contentSecurityPolicy: { // Prevents XSS, data injection
directives: {
defaultSrc: ["'self'"],
scriptSrc: ["'self'"],
styleSrc: ["'self'", "'unsafe-inline'"],
imgSrc: ["'self'", "data:", "https:"],
},
},
hsts: { maxAge: 31536000, includeSubDomains: true }, // Force HTTPS
noSniff: true, // Prevent MIME sniffing
frameguard: { action: 'deny' }, // Prevent clickjacking
xssFilter: true, // Basic XSS filter
referrerPolicy: { policy: 'strict-origin-when-cross-origin' },
}));
// Additional custom headers:
app.use((req, res, next) => {
res.removeHeader('X-Powered-By'); // Hide server info
// Rate limiting info
res.setHeader('X-RateLimit-Limit', '100');
res.setHeader('X-RateLimit-Remaining', String(99 - getUsedQuota(req.ip)));
next();
});
Pre-Deployment Security Checklist
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□ All user input validated AND sanitized?
□ Database uses parameterized queries everywhere?
□ Passwords hashed with bcrypt/argon2?
□ JWT tokens short-lived + refresh token rotation?
□ Auth endpoints rate-limited?
□ CSRF protection enabled (tokens or SameSite)?
□ Security headers configured (helmet.js)?
□ Secrets in environment variables (never in code)?
□ Error messages don't leak internals (stack traces, paths)?
□ CORS configured with explicit allowlist?
□ Dependencies audited (npm audit --json)?
□ Access controls checked (users access only THEIR data)?
□ File uploads validated (type AND content, not just extension)?
□ HTTPS enforced everywhere?
□ Logging doesn't capture passwords/tokens?
□ Admin endpoints have additional protection?
Score yourself: Each ☑️ blocks an entire attack category.
What's the most important security practice you follow? What's one you always forget?
Follow @armorbreak for more practical developer guides.
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