In 2026, APIs account for 83% of internet traffic and 90% of the attack surface. Traditional WAFs are blind to logic attacks. This masterclass dissects the OWASP API Security Top 10, focusing on the most devastating flaws: Broken Object Level Authorization (BOLA) and Mass Assignment.
01BOLA: The Silent Killer
Broken Object Level Authorization (BOLA) happens when an API exposes a reference to an object (like a UserID: 1001) but fails to check if the logged-in user (UserID: 1005) has permission to access it. Attackers simply iterate through IDs (1000..9999) to scrape the entire database. Fixing BOLA requires a centralized authorization middleware, not ad-hoc checks.
// VULNERABLE
app.get('/messages/:id', (req, res) => {
let msg = db.get(req.params.id);
res.json(msg);
});
// SECURE - Check Ownership
app.get('/messages/:id', (req, res) => {
let msg = db.get(req.params.id);
if (msg.ownerId !== req.user.id) {
return res.status(403).json({error: "Access Denied"});
}
res.json(msg);
});02Mass Assignment
Modern frameworks often bind client input directly to internal objects. If an attacker sends `{"isAdmin": true}` in a profile update request, and the API blindly binds it to the User object, they gain root access. **Defense**: Use Data Transfer Objects (DTOs) or explicit inclusion lists. Never bind the entire `req.body` to your database model.
