Prevent malicious image uploads

If you allow users to upload images to your site, you may be opening up your site to malicious use. This is because particularly cunning hackers can hide commands to do just about anything in the data that makes up the image.

For a great example of an infected image file, see the following page:

Note that you’ll need to disable your anti-virus if you want to create this file on your computer.

If you allow users to upload this file, it will produce some interesting results!

To prevent the vulnerability (at least when using C#) you simply need to try and load the image data into an Image object. This will throw an exception if the image contains anything nasty. Here is an example:

var image = Image.FromStream(imageDataStream);
using (var memoryStream = new MemoryStream())
    image.Save(memoryStream, ImageFormat.Png);

Note that you’ll need to load the uploaded data into a stream (imageDataStream in the example above) to get this to work.

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