![]() Most image file types allow text comments added to them. If it isn't an image, this will fail and therefor the entire upload will fail: Using getimagesize(), we'll be processing the image with the GD library. Here's where the php GD library comes in handy. So instead of just relying on the Content-type header, it would be better to also validate the content of the uploaded file. Nothing will prevent him/her from modifying the request headers and simply change the Content type to "image/png". As I mentioned before, the attacker has full control over the request. So to prevent this, we implement Content-type verification: In the script above an attacker could upload anything they want, like a php script for example, and follow a direct link to execute it. So all of our security has to be done serverside. No matter how hard we try, we can't prevent an attacker from trying to upload something they're not supposed to. Think about browser addons, proxies, Perl scripts. POST requests can be intercepted in a number of ways. ![]() ![]() For this I'm gonna start with a very basic html form and php script that handles the files.Īttackers don't have to use the form on your website to upload files to your server. I'm gonna walk through the entire process I've been through here so you can follow along. Now I'm no where near an expert on this, but I've been asked to develop this once in the past. When you start working on a secure image upload script, there are many things to consider. Such manipulated images will probably simply turn out unreadable for GD. It's good policy for sites that host images to remove that info for the user.Īlso, copying the image will probably get rid of most exploits that use faulty image data to cause overflows in the viewer software, and inject malicious code. Users are often not even aware how much info gets put into the Metadata section of JPG files! Camera info, position, times, software used. That will slightly diminish image quality because content gets compressed twice, but it will remove any EXIF information present in the original image. If you want to make totally sure the content is clean, consider using GD or ImageMagick to copy the incoming image 1:1 into a new, empty one. Regarding file names, random names are definitely a good idea and take away a lot of headaches. So, let's move on and create our own, secure, file upload.Secure User Image Upload Capabilities in PHP With Uploadcare you can upload and manage files quickly and easily via their PHP integration. Maybe it's worth considering to use an already existing service. In other words, when you validate the upload with this method, attackers can pretend that their file has another file size or type.Īs you can see, there is a lot we need to take care of. the filesize with $_FILES, because this can be modified by the uploader in case of an attack. ![]() I hope it's useful for some of you and now happy coding :)įirst of all, the most important thing I want to tell you, the $_FILES variable in PHP (except tmp_name) can be modified. In this post, I'll show you how to upload files to your server using HTML and PHP and validate the files. But I recommend reading the article to understand why I'm doing things as I do and how it works. If you just want the sourcecode - scroll to the end of the page or click here. ![]()
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