How to protect against direct access to images?

Question

I would like to create a web site with many images. But I would like to protect against direct access to images, e.g. direct links to images without visiting the web site.

What is the preferred way to do this? And what are the alternatives with Pros and cons?

I have some ideas (I don't know if they are possible):

• File permissions
• PHP Sessions
• Temporary file names or URLs
• HTTP Redirection?

Maybe this isn't practiced on many web sites? E.g. I tried to access a private photo on Facebook without beeing logged in, but I could still visit the photo.

The platform will probably be a Ubuntu machine with NginX and PHP.

Answer 1

http://us3.php.net/image

You link the img element to a php file. This file checks if the user has the right permission, if so it can send an img response back.

<img src="url/LoadImg.php?id=1337" alt="" />

Still someone with the permission can download the image and provide it to other people somewhere else (webspace/mail/whatever). To make it a bit harder to steal it you can disable right clicking on the image, but still a user who knows a little bit about http should not have any problems to steal it. You can place a signature over the image (for example the logo/name of your website) so people can see that you where the source. This can be done with php aswell.

If you want to be funy you can setup an other image (porn is great for this :P) that is sent if the link comes from an other page :P

Answer 2

This is going to be hard to do. In order for your clients' web browsers to access the pictures, they need to be readable. File permissions won't work because you'll need to grant access to the browser. You won't be able to stop someone from downloading them and doing something with them.

If you only want to stop direct linking, if you change the filenames on a regular basis and update your pages to reflect this, other pages will have their links broken.

Answer 3

This might be useful: Allow/deny image hotlinking with .htaccess

Edit: One thing to note about this method is that some Browser/AV/Firewall software removes Referer data when you browse, which would cause potentially legitimate users to be treated as hotlinkers.

If your site already uses some kind of authentication or session system, then it would be better to use the method given in @Mark Baijens' answer.

Update: NGiNX rewrite rule to prevent hotlinking:

location ~* (\.jpg|\.png|\.css)$ {
    valid_referers blocked mydomain.com www.mydomain.com;
    if ($invalid_referer) {
        return 444;
    }
}

Answer 4

You could use a PHP script to retrieve the images using something like:

<img src="mysite.com/getimage.php?id=001" />

and have the PHP script return the image data only after confirming that the domain of the HTTP_REFERER is your's.

If you have an account-oriented site, I suggest using PHP sessions as you stated and have the PHP script verify the session before returning the image data.