OWASP Top 10: A Developer's Practical Guide to Web Security

Every web developer writes code that could be exploited. The difference between secure and vulnerable applications isn't luck — it's awareness. The OWASP Top 10 represents the most critical security risks to web applications, compiled from real-world breach data and expert consensus. Understanding these vulnerabilities is the first step toward writing code that attackers can't easily break.

A01: Broken Access Control

The number one vulnerability in web applications. Broken access control occurs when users can act outside their intended permissions. This includes accessing other users' data by modifying a URL parameter, elevating privileges from user to admin, or bypassing access checks by manipulating API requests. The fix is straightforward in principle: deny by default, implement server-side access control checks on every request, and never trust client-side enforcement alone.

// BAD: Trusting the user-supplied ID without authorization check
app.get('/api/users/:id/profile', async (req, res) => {
  const profile = await db.getProfile(req.params.id);
  return res.json(profile);
});
 
// GOOD: Verify the requesting user owns this resource
app.get('/api/users/:id/profile', authenticate, async (req, res) => {
  if (req.user.id !== req.params.id && req.user.role !== 'admin') {
    return res.status(403).json({ error: 'Forbidden' });
  }
  const profile = await db.getProfile(req.params.id);
  return res.json(profile);
});

A03: Injection

SQL injection, NoSQL injection, command injection, and LDAP injection all share the same root cause: untrusted data is sent to an interpreter as part of a command or query. The attacker's hostile data tricks the interpreter into executing unintended commands. Parameterized queries, ORMs, and input validation are your primary defenses.

If you're concatenating user input into SQL strings in 2026, you're not just writing vulnerable code — you're ignoring three decades of hard-learned lessons.

A07: Cross-Site Scripting (XSS)

XSS flaws occur when an application includes untrusted data in a web page without proper escaping. Attackers can execute scripts in the victim's browser to hijack sessions, deface websites, or redirect users to malicious sites. Modern frameworks like React and Angular escape output by default, but using dangerouslySetInnerHTML or bypassing template security defeats these protections.

Defense in Depth

• Implement Content Security Policy (CSP) headers to restrict script sources
• Use parameterized queries for all database access — no exceptions
• Apply input validation with allowlists, not blocklists
• Enable HTTPS everywhere and set secure cookie flags
• Keep dependencies updated and scan for known vulnerabilities
• Log security events and monitor for anomalous patterns

Security is not a feature you add at the end — it's a practice you weave into every line of code. Make the OWASP Top 10 part of your code review checklist, and you'll catch the vast majority of vulnerabilities before they reach production.