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Insecure file upload - Files Limit

Need​

Enforce a limit on the number of files that can be uploaded within a specific time frame and implement proper file management to delete old files from the server.

Context​

  • Requirement of PHP 7.0 or higher for running the application
  • Usage of Illuminate/Http for HTTP request handling in Laravel
  • Usage of Illuminate/Support for providing foundational support classes and utilities in Laravel framework
  • Usage of Symfony/HttpFoundation for handling HTTP requests and responses in a Symfony application

Description​

Non compliant code​

public function upload(Request $request)
{
    if($request->hasFile('files')) {
        $files = $request->file('files');

        foreach($files as $file) {
            $filename = $file->getClientOriginalName();
            $file->move(public_path('uploads'), $filename);
        }

        return response()->json([
            'message' => 'Files uploaded successfully.'
        ], 200);
    }

    return response()->json([
        'message' => 'No files to upload.'
    ], 400);
}

In the above code, the upload function is handling the file upload process. When a request comes in with files, it loops through each file and moves it to the 'uploads' directory in the public path of the Laravel application.

The vulnerability lies in the fact that there is no limit to the number of files that can be uploaded. This means that an attacker could potentially upload an unlimited number of files to the server, which could lead to a Denial of Service (DoS) attack by filling up the server's storage.

Furthermore, the current implementation does not delete any previously uploaded files when a new one is uploaded. This means that the server's storage could fill up over time, even if the number of files uploaded at any one time is limited.

The absence of file upload limits can also lead to other potential issues such as:

  • Overuse of bandwidth: If users are allowed to upload as many files as they want, it could consume a lot of network bandwidth.
  • Overuse of memory: Processing a large number of files could lead to high memory usage.
  • Overuse of CPU: If the files are being processed after being uploaded (e.g., resized, converted), it could lead to high CPU usage.

Therefore, it's crucial to implement a limit on the number of files that can be uploaded per unit of time and to handle the deletion of old files when new ones are uploaded.

Steps​

  • Introduce a limit to the number of files that can be uploaded at a time. This can be done by checking the size of the 'files' array from the request and comparing it to a predefined limit.
  • Implement a file management system that deletes or archives old files when a new one is uploaded. This can be done by checking if a file with the same name already exists in the 'uploads' directory before moving the new file there.
  • Consider implementing a rate limiting system to prevent users from uploading too many files in a short period of time. This can help prevent abuse of the file upload system.

Compliant code​

public function upload(Request $request)
{
    // Define the maximum number of files that can be uploaded at a time
    $maxFiles = 10;

    if($request->hasFile('files')) {
        $files = $request->file('files');

        // Check if the number of files exceeds the limit
        if(count($files) > $maxFiles) {
            return response()->json([
                'message' => 'Too many files. You can only upload up to ' . $maxFiles . ' files at a time.'
            ], 400);
        }

        foreach($files as $file) {
            $filename = $file->getClientOriginalName();

            // Check if a file with the same name already exists
            if(file_exists(public_path('uploads/' . $filename))) {
                // Delete the old file
                unlink(public_path('uploads/' . $filename));
            }

            $file->move(public_path('uploads'), $filename);
        }

        return response()->json([
            'message' => 'Files uploaded successfully.'
        ], 200);
    }

    return response()->json([
        'message' => 'No files to upload.'
    ], 400);
}

The updated code introduces a limit to the number of files that can be uploaded at a time. This is done by defining a variable $maxFiles and checking if the number of files in the request exceeds this limit. If it does, the function returns a response with a 400 status code and a message indicating that too many files have been uploaded.

The code also implements a file management system that deletes old files when a new one is uploaded. This is done by checking if a file with the same name already exists in the 'uploads' directory before moving the new file there. If such a file exists, it is deleted using the unlink function.

These changes help to prevent the system from being overwhelmed by a large number of file uploads and ensure that the 'uploads' directory does not become cluttered with old files.

References​

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