Use Cryptographic Randomness for Tokens and IDs | BeforeMerge Rules | BeforeMerge

Use Cryptographic Randomness for Tokens and IDs | BeforeMerge Rules | BeforeMerge

Use Cryptographic Randomness for Tokens and IDs

Impact: CRITICAL (prevents predictable tokens that enable session hijacking or enumeration attacks)

Math.random() uses a pseudo-random number generator (PRNG) that is not cryptographically secure. Its output can be predicted if an attacker observes enough values, and some engines seed it with low-entropy sources. When used to generate session tokens, CSRF tokens, password reset links, unique IDs for access control, or nonces, an attacker can predict or brute-force subsequent values.

Incorrect (using Math.random() for security-sensitive values):

// ❌ Predictable token generation
function generateToken(): string {
  return Math.random().toString(36).substring(2)
}
 
// ❌ Predictable unique IDs used for access control
function createInviteLink(teamId: string): string {
  const code = Math.random().toString(36).slice(2, 10)
  return `https://app.example.com/invite/${teamId}/${code}`
}
 
// ❌ Predictable CSRF nonce
function generateNonce(): string {
  return `nonce-${Math.random()}`
}

Correct (using crypto APIs for secure randomness):

// ✅ crypto.randomUUID() — available in all modern browsers and Node 19+
function generateToken(): string {
  return crypto.randomUUID()
}
 
// ✅ crypto.getRandomValues() for custom-length tokens
function generateSecureCode(length: number = 32): string {
  const array = new Uint8Array(length)
  crypto.getRandomValues(array)
  return Array.from(array, (b) => b.toString(16).padStart(2, '0')).join('')
}
 
// ✅ Secure invite link
function createInviteLink(teamId: string): string {
  const code = crypto.randomUUID()
  return `https://app.example.com/invite/${teamId}/${code}`
}

Additional context:

Math.random() is fine for non-security purposes: shuffling UI elements, generating placeholder data, animations, or randomizing display order.
crypto.randomUUID() is available in browsers (all major since 2022) and Node.js 19+. For older Node.js, use require('crypto').randomUUID().
If you need a nanoid-style short ID, the nanoid package uses crypto.getRandomValues() internally and is a safe alternative.

Detection hints:

# Find Math.random() used near token/id/key generation
grep -rn "Math\.random()" src/ --include="*.ts" --include="*.tsx"

Reference: MDN crypto.randomUUID()