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Secure programming habits in PHP

Posted on Saturday, November 4, 2006

The goal of this article is to show common threats and challenges of programming secure PHP applications. The wonderful thing about PHP is that people with little or even no programming experience are able to achieve simple goals very quickly. The problem, on the other hand, is that many programmers are not really conscious about what is going behind the curtains. Security and convenience do not often go hand in hand — but they can.

PHP has some very flexible file handling functions. The include(), require() and fopen() functions accept local path names as well as remote files using URLs. A lot of vulnerabilities I have seen are due to incorrect handling of dynamic file or path names.

On a site I will not mention in this article (because the problem still has not been solved) has one script which includes various HTML files and displays them in the proper layout. Have a look at the following URL:

The variable $i obviously contains the file name to be included. When you see a URL like this, a lot of questions should come to your mind:

– Has the programmer considered directory traversals like i=../../../etc/passwd?
– Does he check for the .html extension?
– Does he use fopen() to include the files?
– Has he thought about not allowing remote files?

In this case, every answer was negative. Time to play! Of course, it is now possible to read all the files the httpd user has read access for. But what is even more exciting is the fact that the include() function is used to include the HTML file. Consider this:

Where exec.html contains a couple of lines of code:

passthru ('cat /etc/passwd');  
passthru ('useradd myuser -p password');  
passthru ('echo another hacked server! | mail');  

I am sure you get the idea. A lot of bad things can be done from here.

Per default, PHP writes most of the variables into the global scope. Of course, this is very convenient. On the other hand, you can get lost in large scripts very quickly. Where did that variable come from? If it is not set, where could it come from? All EGPCS (Environment, GET, POST, Cookie, and Server) variables are put into the global scope.

The global associative arrays $HTTP_ENV_VARS, $HTTP_GET_VARS, $HTTP_POST_VARS, $HTTP_COOKIE_VARS, $HTTP_SERVER_VARS and $HTTP_SESSION_VARS will be created when the configuration directive trackvars is set. This allows you to look for a variable only in the place you expect it to come from. Note: As of PHP 4.0.3, trackvars is always turned on.

This security hole was reported to the Bugtraq mailing list by Ismael Peinado Palomo on July 25th, 2001. Mambo Site Server 3.0.x, a dynamic portal engine and content management tool based on PHP and MySQL, is vulnerable to a typical global scope exploit. The code has been modified and simplified.

Under the ‘admin/’ directory, index.php checks whether the password matches the one in the database after posting the form:

if ($row['pass'] == $postedpass) {  
  header("Location: index2.php");  

When the passwords match, the variables $name, $fullname and $id are registered as session variables. The user then gets redirected to index2.php. Let us see what happens there:

if (!$PHPSESSID) {  
  header("Location: index.php");  
} else {  
  if (!$name) session_register("name");  
  if (!$fullname) session_register("fullname");  
  if (!$id) session_register("id");  

|If the session ID has not been set, the user will be directed back to the login screen. If there is a session ID, though, the script will resume the session and will put the previously set session variables into the global scope. Nice. Let us see how we can exploit this. Consider the following URL: &fullname=brian&id=admin

The GET variables $PHPSESSID, $name, $fullname and $id are created as global variables per default. So when you look at the if-else-structure above, you will notice that the script figures $PHPSESSID is set and that the three variables dedicated to authorize and identify the user can be set to anything you want. The database has not even been queried. A quick fix for this problem — by far not the perfect one — would be to check for $HTTP_SESSION_VARS['id'] or $_SESSION['id'] (PHP => v4.1.0) instead of $id.

Programming in PHP would be boring without a decent SQL database connected to the web server. However, assembling SQL queries with unchecked variables is a dangerous thing to do.

The following bug in PHP-Nuke 5.x has been reported to the Bugtraq mailing on August 3, 2001. It is actually a combination of exploiting global variables and an unchecked SQL query variable.

The PHP-Nuke developers decided to add the “nuke” prefix to all tables in order to avoid conflicts with other scripts. The prefix can be changed when multiple Nuke sites are run using the same database. Per default, $prefix = "nuke"; is defined in the configuration file config.php.

Let us now look at a few lines from the script article.php.

if (!isset($mainfile)) {  

if (!isset($sid) && !isset($tid)) {  

And a bit further down: the SQL query.

mysql_query("UPDATE $prefix"._stories." SET counter=counter+1 where sid=$sid");  

To change the SQL query, we need to make sure $prefix is not set to its default value so we can set an arbitrary value via GET. The configuration file config.php is included in mainfile.php. As we know from the last chapter, we can set the variables $mainfile, $sid and $tid to any value using GET parameters. By doing so, the script will think mainfile.php has been included and $prefix has been set accordingly. Now, we are in a position to execute any SQL query starting with UPDATE. So the following query will set all admin passwords to '1’: &prefix=nuke.authors%20set%20pwd=1%23

The query now looks like this:

UPDATE nuke.nuke_authors set pwd=1#_stories SET counter=counter+1 where sid=$sid

Of course, anything after # will be considered as a comment and will be ignored.

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