It’s fairly easy to construct a filename that refers to something other than what you intended. For example, say you have a $username variable that contains the name the user wants to be called, which the user has specified through a form field. Now let’s say you want to store a welcome message for each user in the directory /usr/local/lib/ greetings so that you can output the message any time the user logs in to your application.

The code to print the current user’s greeting is:

This seems harmless enough, but what if the user chose the username ” ../../../../etc/passwd“? The code to include the greeting now includes this relative path instead: /etc/passwd. Relative paths are a common trick used by hackers against unsuspecting scripts.

Another trap for the unwary programmer lies in the way that, by default, PHP can open remote files with the same functions that open local files. The fopen() function and anything that uses it (e.g., include() and require()) can be passed an HTTP or FTP URL as a filename, and the document identified by the URL will be opened. For example:

If $username is set to, a remote file is opened, not a local one.

The situation is even worse if you let the user tell you which file to include():

If the user passes a theme parameter of and your variables_order includes GET or POST, your PHP script will happily load and run the remote code. Never use parameters as filenames like this.

There are several solutions to the problem of checking filenames. You can disable remote file access, check filenames with realpath() and basename(), and use the open_basedir option to restrict filesystem access outside your site’s document root.

Check for relative paths

When you need to allow the user to specify a filename in your application, you can use a combination of the realpath() and basename() functions to ensure that the filename is what it ought to be. The realpath() function resolves special markers such as .and … After a call to realpath(), the resulting path is a full path on which you can then use basename(). The basename() function returns just the filename portion of the path.

Going back to our welcome message scenario, here’s an example of realpath() and basename() in action:

In this case, we’ve resolved $filename to its full path and then extracted just the filename. If this value doesn’t match the original value of $filename, we’ve got a bad filename that we don’t want to use.

Once you have the completely bare filename, you can reconstruct what the file path ought to be, based on where legal files should go, and add a file extension based on the actual contents of the file:

Here is the list of of Article in this Series:

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Thanks Kevin Tatroe, Peter MacIntyre and Rasmus Lerdorf. Special Thanks to O’Relly.