Insecure Randomness

Using Math.random() for security-sensitive values like tokens or IDs is predictable and can be brute-forced.

How It Works

Math.random() uses a deterministic algorithm seeded from system time. It's not cryptographically random — given a few observed outputs, an attacker can predict past and future values. Using it for password reset tokens, session IDs, or CSRF tokens makes those values guessable.

Vulnerable Code

// BAD: Math.random is not cryptographically secure
const resetToken = Math.random().toString(36).slice(2);
const userId = 'user_' + Math.random().toString(36).slice(2);

Secure Code

// GOOD: use crypto.randomBytes for all security-sensitive values
import { randomBytes } from 'crypto';
const resetToken = randomBytes(32).toString('hex'); // 256 bits of real entropy
const userId = 'user_' + randomBytes(16).toString('hex');

Real-World Example

Multiple password reset vulnerabilities have been reported where tokens generated with Math.random() were brute-forced in seconds using V8 engine seed prediction techniques documented in academic research.

How to Prevent It

Always use crypto.randomBytes() (Node.js) or crypto.getRandomValues() (browser) for tokens and IDs
Use uuid v4 from the 'uuid' package for IDs — it uses crypto.randomUUID() internally
Minimum 128 bits of entropy for tokens, 256 bits preferred for high-value operations
Search your codebase for Math.random() and audit every usage