Weak Random Function

Also Known As

TL;DR

Explanation

PHP's rand() and mt_rand() are Mersenne Twister-based PRNGs — fast and deterministic, suitable for simulations and games, but predictable given enough outputs. For security: tokens, session IDs, password reset links, CSRF tokens, and API keys must use cryptographically secure randomness. PHP 7+ provides random_bytes() and random_int() which use the OS CSPRNG (/dev/urandom on Linux). Never use weak PRNGs for anything an attacker must not be able to predict.

Common Misconception

Why It Matters

Common Mistakes

• rand() or mt_rand() for password reset tokens, CSRF tokens, or session IDs.
• uniqid() for security tokens — uses microtime(), highly predictable.
• base64_encode(mt_rand()) — encoding does not add entropy.
• str_shuffle() for token generation — relies on mt_rand() internally.

Avoid When

• Never use rand(), mt_rand(), or uniqid() for security tokens — they are predictable.
• Do not use microtime() or time() as a seed for token generation — trivially guessable.

When To Use

• Use random_bytes() for any security-sensitive random value — tokens, nonces, CSRF values, API keys.
• Use random_int() for cryptographically secure random integers within a range.

Code Examples

// Predictable — never use for security:
$token = md5(uniqid(mt_rand(), true));  // Both predictable
$resetToken = rand(100000, 999999);      // 900000 possibilities — brute-forceable
$apiKey = base64_encode(mt_rand());      // Predictable state

// Cryptographically secure:
$token    = bin2hex(random_bytes(32));        // 256 bits of entropy
$resetUrl = bin2hex(random_bytes(16));        // 128 bits — URL-safe
$pin      = random_int(100000, 999999);       // Secure integer range
$apiKey   = base64_encode(random_bytes(32));  // Secure API key

References

• ↗ https://www.php.net/manual/en/function.random-bytes.php

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