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I've run across the following snippet in themes from time to time:

if ( ! defined('ABSPATH')) exit('restricted access');

It's at the beginning of some (all?) PHP files in a theme and it's supposed to prevent direct access of the file by nefarious sources.

I see that this isn't included in Twenty Ten or Eleven and I've never seen it recommended in official WordPress documentation. It seems like a good idea to me, but I also don't know enough about security to judge it and can't find much with Google.

Is this something I should have in my custom themes? If so, should it be in all PHP files or just some?

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Just for later readers, this can be written shorter and nicer: defined('ABSPATH') OR exit; – kaiser Jan 14 '13 at 16:16
up vote 20 down vote accepted

Usually, you don’t need it. But … there is at least one edge case:

  • If a theme file is a template part,
  • and it is using global variables from the calling context (parent file),
  • and register_globals is on,
  • and it is just using these variables without any security check …

… an attacker can call this file, set the missing variables with GET or POST and make the theme file print those out. And then there is a security problem.

So … the best option is not a context check like the one from your example, but good code: avoid global variables, check their content before you print it out.

In some cases I add the context check when I think somebody else will use my code and change it without security in mind. It doesn’t hurt.

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If a template part still contained at least one function call that would cause a PHP fatal error would this scenario still be possible? – Chris_O Aug 25 '12 at 4:52
@Chris_O Depends on the order of appearance. – toscho Aug 25 '12 at 4:57
Makes sense and totally agree on another reason not to use global variables between file calls. – Chris_O Aug 25 '12 at 4:59
It's always best to be safe than sorry. Too much security can't hurt, can it? – Sean Berg Aug 25 '12 at 17:41
If you do everything right, you should not use code that isn’t needed. This question is the proof that it makes code harder to follow. – toscho Aug 25 '12 at 17:45

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