Predictable Token

Also Known As

TL;DR

Explanation

Security tokens (password reset links, session IDs, API keys) must be unpredictable. Using md5(time()) produces only 86,400 distinct values per day — an attacker can brute-force all of them in seconds. rand() and mt_rand() are pseudorandom and their output can be predicted from a small number of observed values. The only correct source for security-sensitive random data is a CSPRNG: bin2hex(random_bytes(32)) in PHP.

Common Misconception

Why It Matters

Common Mistakes

• Using rand() or mt_rand() which are seeded by time and guessable.
• Using MD5 or SHA1 of a timestamp — the timestamp is known or estimable.
• Sequential integer tokens for anything security-sensitive.
• Short tokens (less than 128 bits) that make brute force feasible even with secure randomness.

Code Examples

// Predictable — attacker can guess or brute-force
\$token = md5(\$userId . time());
\$token = uniqid();   // only 7 bytes of entropy
\$token = rand();     // seeded from time — predictable

// 32 bytes = 256 bits of entropy — computationally unguessable
\$token = bin2hex(random_bytes(32)); // 64 hex chars

// Store hash in DB, send raw token to user
\$stored = hash('sha256', \$token);
\$db->insert('password_resets', [
    'user_id'    => \$userId,
    'token_hash' => \$stored,
    'expires_at' => date('Y-m-d H:i:s', time() + 3600),
]);

// Verify with constant-time comparison:
if (!hash_equals(\$stored, hash('sha256', \$inputToken))) abort(400);
if (\$record->expires_at < now()) abort(400);

References

• ↗ https://cwe.mitre.org/data/definitions/338.html
• ↗ https://www.php.net/manual/en/function.random-bytes.php

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