Null 字符问题

由于 PHP 的文件系统操作是基于 C 语言的函数的,所以它可能会以您意想不到的方式处理 Null 字符。 Null字符在 C 语言中用于标识字符串结束,一个完整的字符串是从其开头到遇见 Null 字符为止。 以下代码演示了类似的攻击:

Example #1 会被 Null 字符问题攻击的代码

<?php
$file 
$_GET['file']; // "../../etc/passwd\0"
if (file_exists('/home/wwwrun/'.$file.'.php')) {
    
// file_exists will return true as the file /home/wwwrun/../../etc/passwd exists
    
include '/home/wwwrun/'.$file.'.php';
    
// the file /etc/passwd will be included
}
?>

因此,任何用于操作文件系统的字符串(译注:特别是程序外部输入的字符串)都必须经过适当的检查。以下是上述例子的改进版本:

Example #2 验证输入的正确做法

<?php
$file 
$_GET['file']; 

// 对字符串进行白名单检查
switch ($file) {
    case 
'main':
    case 
'foo':
    case 
'bar':
        include 
'/home/wwwrun/include/'.$file.'.php';
        break;
    default:
        include 
'/home/wwwrun/include/main.php';
}
?>
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User Contributed Notes 3 notes

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2
Anonymous
6 months ago
Looks like this issue was fixed in PHP 5.3 https://bugs.php.net/bug.php?id=39863
up
1
J.D. Grimes
18 days ago
This issue has been fixed for file_exists(): https://bugs.php.net/bug.php?id=39863

It still exists for include|require(_once) as of this writing.
up
-23
kpobococ at gmail dot com
5 years ago
Since problems with null bytes do not stretch to regular string functions, this should be enough to ensure no GET parameter contains them any more:

<?php
function getVar($name)
{
   
$value = isset($_GET[$name]) ? $_GET[$name] : null;
   
    if (
is_string($value)) {
       
$value = str_replace("\0", '', $value);
    }
}
?>

Modifying this to work with other superglobals should not be a problem, so I will leave it up to you.
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