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Restricting acceptable MIME types

From: Uploading Files Securely with PHP

Video: Restricting acceptable MIME types

The upload file class that we've been building accepts any type of file. And I'm just going to paste in here a list of image mime types.

Restricting acceptable MIME types

The upload file class that we've been building accepts any type of file. This is potentially insecure, particularly if the upload folder is publicly accessible. But you can use the type element of the file's superglobal array to filter uploads and restrict them to specific types. So let's create a class property to store an array of permitted types. So we'll create it here with all the other property definitions. We'll make it protected, and we'll call it permittedTypes, and it needs to be an array.

And I'm just going to paste in here a list of image mime types. You can find that list in the text file in the exercise files for this video. And then we just need to put a semicolon at the end there. By the way, this image/pjpg is what was used by old versions of Internet Explorer for JPEG files. That's why it's been included there. Next we need to create a method to check the file's type. So we'll put that down with our other check methods.

So we need to find checkSize, and we'll put it in after checkSize. It'll be a protected method. I will call it checkType, and we'll pass it file as an argument. This argument that we're passing here will be a reference to the current element in the file's superglobal array. So we can now use a conditional statement to check whether the file type is in the array of permitted types. So if, then we'll use the in_array function, which looks for a needle.

The needle we're looking for is file, and the element is type, and the haystack that we're looking for is that permitted types array, which is a property, so we use this, and then permittedTypes. So if it is one of the permitted types, we're happy, and we can return true. And if it's not one of the permitted types, we need to add a message to the messages array.

So else, then, this messages, will add an array element to it. We need the file name. And then add that message that it's not a permitted type. And of course, if it's not a permitted type, we need to return false. So now that we've defined that method, we need to call it in the checkFile method. So we need to scroll up to, checkFile. Here we are. So after checking the size, we will check type.

So if, and if it's going to return false, we don't want it, so we need to use the negative operator. So if not this, checkType, and then we pass it file. If it doesn't pass muster, we need to return false. So the check file method is now running several checks. It's checking the error code. If it isn't zero, it gets an error message, and returns false.

It checks the size. If it doesn't fall within our limits, it returns false. And if the type isn't one of our permitted types, it also returns false. It only returns true if the file passes muster with all the various checks that we're making. So let's just save that and go back to form.php, where we can test it. Now in the previous video, I changed max to 5,000 kilobytes, which was ridiculous, so we'll change that back to 50.

And then we can save that and run it in a browser. We'll select a file. This is a JPG, it's accepted it. Let's choose this WebP file, it's accepted that too. But let's try a different type of file, let's choose a Word document. Try to upload that, it's not a permitted type of file. And if we do the same with a JavaScript file, try to upload it, it's not a permitted type of file.

So this works really well if you want to restrict file uploads to a narrow range of MIME types. But it's important to realize that PHP doesn't actually verify the MIME type reported in the file's superglobal array. It takes the browser's word for it. So the MIME type can be spoofed and sometimes, browsers don't report what you actually expect. That WebP file worked fine in Chrome, but it wouldn't work in other browsers. An alternative approach is to allow all file types to be uploaded but to accord special treatment to those with filename extensions that could be risky.

And we'll look at that approach a little later in this chapter.

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This video is part of

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Uploading Files Securely with PHP

33 video lessons · 2252 viewers

David Powers
Author

 
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  1. 4m 49s
    1. Welcome
      57s
    2. What you should know before watching this course
      2m 0s
    3. Using the exercise files
      1m 52s
  2. 33m 2s
    1. How PHP handles file uploads
      6m 16s
    2. Examining the $_FILES array
      5m 8s
    3. Setting the maximum file size
      5m 36s
    4. Preparing the upload folder
      3m 18s
    5. Moving the file to its destination
      6m 51s
    6. Limitations on file uploads
      5m 53s
  3. 47m 3s
    1. Planning the class's features
      3m 15s
    2. Creating and using a namespaced class
      5m 25s
    3. Creating the class constructor
      7m 26s
    4. Getting a reference to the uploaded file
      5m 9s
    5. Checking the error level
      5m 7s
    6. Displaying errors and other messages
      4m 51s
    7. Setting and checking the maximum file size
      7m 19s
    8. Strengthening the setMaxSize() method
      8m 31s
  4. 35m 30s
    1. Restricting acceptable MIME types
      5m 27s
    2. Removing spaces from file names
      5m 21s
    3. Restricting acceptable file-name extensions
      6m 10s
    4. Neutralizing potentially dangerous uploads
      5m 54s
    5. Renaming files with duplicate names
      7m 58s
    6. Moving the file to its destination
      4m 40s
  5. 10m 25s
    1. Understanding how the $_FILES array handles multiple files
      4m 42s
    2. Adapting the class to handle both single and multiple uploads
      5m 43s
  6. 38m 15s
    1. Overview of the UploadFile class
      5m 9s
    2. Setting up to use the class
      4m 11s
    3. Using the class
      8m 12s
    4. Reporting errors with multiple uploads
      4m 0s
    5. Displaying the server limits
      4m 51s
    6. Alerting the user about exceeding the server limits
      6m 14s
    7. Changing the class's defaults
      5m 38s
  7. 1m 38s
    1. Goodbye
      1m 38s

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