PHP GET variable array injection

Question

I've recently learned that it's possible to inject arrays into PHP GET variables to perform code execution?

.php?a[]=asd&a[]=asdasd&b[]=$a

That was the example I was given. I have no idea how it works and was wondering if this is even possible?

Answer 1

PHP will parse the query string, and inject those values in the $_GET super-global array (same for $_POST if this was done in a form using POST, btw).

In your case, the $_GET array will contain this :

array
  'a' => 
    array
      0 => string 'asd' (length=3)
      1 => string 'asdasd' (length=6)
  'b' => 
    array
      0 => string '$a' (length=2)

Each value passed in the query string will be put by PHP in the $_GET array, creating sub-arrays if necessary, when there are [] used in the query string.

But this doesn't cause any kind of "code execution" : as long as you deal with input properly (i.e. don't trust the input and use eval on it, or any kind of bad idea like this), there is no risk of code-injection.

Answer 2

If you are not sure how to get secure, the least you can do is to filter the $_GET array. Here is the function:

function filter_url($url)
{
  if (is_array($url))
  {
    foreach ($url as $key => $value)
    {
      // recurssion
      $url[$key] = filter_url($value);
    }
    return $url;
  }
  else
  {
    // remove everything except for a-zA-Z0-9_.-&=
    $url = preg_replace('/[^a-zA-Z0-9_\.\-&=]/', '', $url);
    return $url;
  }
}

Now you can filter the $_GET like this:

$_GET = filter_url($_GET);

This will essentially clean up your $_GET array from suspicious characters such as [ ].

Thanks

Answer 3

The above does not strictly allow code execution, but it may alter the control flow of your existing code if it does not take into account the fact the data may be an array.

The reason the above works is because PHP interprets variables ending in [] as arrays. So if you provide multiple GET variables with same name ending in [], PHP creates an array containing all the values.