Make tokens from a secure random source

How to do it

Use crypto.randomBytes or crypto.randomUUID in Node, the secrets module in Python, random_bytes in PHP, SecureRandom in Java and Ruby, and crypto/rand in Go. Plain generators such as Math.random are fast but predictable, so an attacker who sees a few values can work out the next. API keys also follow Store API keys as hashes. Make each token as long as Lock down session cookies says. Links in email also follow single-use and short-lived links.

Examples

JavaScript example

import { randomBytes } from 'node:crypto';

// Bad: predictable
const resetCode = Math.random().toString(36).slice(2);
// Good
const resetToken = randomBytes(32).toString('base64url');

Python example

import secrets

# Bad: predictable
reset_code = str(random.randint(100000, 999999))
# Good
reset_token = secrets.token_urlsafe(32)

How to check

Search the code for Math.random, random.random, random.choice, mt_rand, uniqid and math/rand. No match makes a token, code or ID that grants access.

Sources

• OWASP Cryptographic Storage Cheat Sheet on secure random numbers

Related rules

• ECM-11 Let guests check out without an account in E-commerce and fulfilment
• API-42 Store API keys as hashes in API design
• APS-16 Lock down session cookies in Application security
• ACC-69 Make sign-in and reset links single-use and short-lived in Accounts and identity
• APS-56 Use only standard cryptography in Application security