Programming PHP, 2nd Edition

PHP Code

With the eval( ) function, PHP allows a script to execute arbitrary PHP code. Although it can be useful in a few limited cases, allowing any user-supplied data to go into an eval( ) call is asking to be hacked. For instance, the following code is a security nightmare:

<html>
  <head>
    <title>Here are the keys...</title>
  </head>
  <body>
    <?php if ($code) {
      echo "Executing code...";

      eval(stripslashes($code));           // BAD!
    } ?>

  <form>
      <input type="text" name="code" />
      <input type="submit" name="Execute Code" />
  </form>
  </body>
</html>

This page takes some arbitrary PHP code from a form and runs it as part of the script. The running code has access to all of the global variables for the script and runs with the same privileges as the script running the code. It’s not hard to see why this is a problem—type this into the form:

include('/etc/passwd');

Unfortunately, there’s no easy way to ensure that a script like this can ever be secure.

You can globally disable particular function calls by listing them, separated by commas, in the disable_functions configuration option in php.ini. For example, you may never have need for the system( ) function, so you can disable it entirely with:

disable_functions = system

This doesn’t make eval( ) any safer, though, as there’s no way to prevent important variables from being changed or built-in constructs such as echo( ) from being called.

Note that the preg_replace( ) function with the /e option also calls eval( ) on PHP code, so don’t use user-supplied data ...