How to securely handle user uploaded files using PHP

How to use PHP to safely process files uploaded by users

With the development of the Internet, the user interaction functions of websites have become more and more abundant, among which user uploading files is a very common function. However, how to safely handle files uploaded by users has become an important issue that developers must face. In this article, we’ll cover how to securely handle user-uploaded files using PHP.

1. Set the maximum size of file upload
   In PHP, you can use the two configuration items upload_max_filesize and post_max_size to control the maximum size of file upload. size. You can set it in the project's configuration file or .htaccess file.

For example, set the maximum file upload size to 10MB in the configuration file:

upload_max_filesize = 10M
post_max_size = 10M

2. Check the file type
   Users can upload malicious files by forging file extensions file, so we need to check the MIME type of the file to make sure it is a file type that is allowed to be uploaded. The type field in PHP's $_FILES array can obtain the MIME type of the uploaded file.

Use the finfo_open and finfo_file functions to check the MIME type of the file:

$file = $_FILES['file']['tmp_name'];
$finfo = finfo_open(FILEINFO_MIME_TYPE);
$mime = finfo_file($finfo, $file);
finfo_close($finfo);

$allowedTypes = array('image/jpeg', 'image/png');
if (!in_array($mime, $allowedTypes)) {
    // 文件类型不符合要求，执行相应操作
}

3. Check the file name
   user Malicious actions can be attempted by uploading special characters in file names, so we need to filter and validate file names to only allow safe characters. Regular expressions can be used for verification, and only file names are allowed to contain letters, numbers and some special characters:

$filename = $_FILES['file']['name'];
$pattern = '/^[a-zA-Z0-9-_.]+$/';
if (!preg_match($pattern, $filename)) {
    // 文件名不符合要求，执行相应操作
}

4. Move files to a secure directory
   Files uploaded by users should be saved in a In a safe directory, we can use the move_uploaded_file function to move files from the temporary directory to the specified directory.

$tempFile = $_FILES['file']['tmp_name'];
$targetDir = "uploads/";
$targetFile = $targetDir . basename($_FILES['file']['name']);

if (move_uploaded_file($tempFile, $targetFile)) {
    // 文件上传成功，执行相应操作
} else {
    // 文件上传失败，执行相应操作
}

5. Add suffix name verification
   In addition to checking the file type, you can also increase security by verifying the suffix name of the file. You can use the pathinfo function to get the file extension and then verify it.

$filename = $_FILES['file']['name'];
$extension = pathinfo($filename, PATHINFO_EXTENSION);
$allowedExtensions = array('jpg', 'png');
if (!in_array($extension, $allowedExtensions)) {
    // 文件后缀名不符合要求，执行相应操作
}

To summarize, here are some suggestions on how to use PHP to safely handle user-uploaded files. However, security is an ongoing process, and only continuously learned and updated security measures can ensure that files uploaded by users do not pose any threat to the system. It is recommended to use existing security libraries and functions during development. For example, the file upload function in the Laravel framework has higher security and usability.

Reference materials:

• PHP official documentation: http://php.net/manual/en/features.file-upload.php
• Laravel file upload : https://laravel.com/docs/5.8/filesystem#file-uploads

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