9.8. PHP

SecureReality has put out a very interesting paper titled ``A Study In Scarlet - Exploiting Common Vulnerabilities in PHP'' [Clowes 2001], which discusses some of the problems in writing secure programs in PHP. Clowes concludes that ``it is very hard to write a secure PHP application (in the default configuration of PHP), even if you try''.

Granted, there are security issues in any language, but one particular issue stands out in PHP that arguably makes PHP less secure than most languages: the way it loads data into its namespace. By default, all environment variables and values sent to PHP over the web are automatically loaded into the same namespace (global variables) that normal variables are loaded into - so attackers can set arbitrary variables to arbitrary values, which keep their values unless explicitly reset by a PHP program. In addition, PHP automatically creates variables with a default value when they're first requested, so it's common for PHP programs to not initialize variables (and if you forget to set a variable, PHP won't complain). Thus, by default PHP allows an attacker to completely control the values of all variables in a program unless the program takes special care to override the attacker. Once the program takes over, it can reset these variables, but failing to reset any variable (even one not obvious) might open a vulnerability in the PHP program.

For example, the following PHP program (an example from Clowes) intends to only let those who know the password to get some important information, but an attacker can set ``auth'' in their web browser and subvert the authorization check:

  if ($pass == "hello")
   $auth = 1;
  if ($auth == 1)
   echo "some important information";

If you've decided to use PHP, here are some of my recommendations (many of these recommendations are based on ways to counter the issues that Clowes raises):

PHP is widely used, and hopefully future versions of PHP will modified so that it's easier to write secure programs in PHP. I think it's sad that a language that's intended to support ease-of-use turns out to be so hard to use securely; in my mind, the current defaults aren't easy to use at all! It wouldn't be hard to overcome these problems. For example, perhaps PHP could support a different file extension (such as .php6 or .sphp for ``secure PHP'') that had more secure defaults. The secure default should include setting ``register_globals'' to ``off'', and also including several functions to make it much easier to for users to specify and limit the input they'll accept from external sources. Then web servers (such as Apache) could separately configure this secure PHP installation. Routines could be placed in the PHP library to make it easy for users to list the input variables they want to accept; some functions could check the patterns these variables must have and/or the type that the variable must be coerced to. In my opinion, PHP is currently a bad choice for secure web development (register_globals is on), but PHP could be trivially modified to become a reasonable choice.