Securing PHP in Shared Hosting Environment

One of the problems inherent to using an Apache Module in a shared web hosting environment is the fact Apache modules gain all the permissions of Apache. Hence, using something like PHP as an Apache module can create serious security vulnerabilities. Since the PHP scripts run in the Apache context, users can easily cause problems (e.g. peeking in other users’ directories, overwritting other peoples files, etc). The easy way to solve this issue is to run PHP as CGI only. This means a user’s scripts will run in the context of the user’s permissions, rather than Apache’s.

Running as a CGI presents other problems. For example, the majority of 3rdparty PHP Applications assume one is running PHP as an Apache module. To use a script via CGI, one has to use something called a she-bang. The first line in the script is a special line that points to the binary that actually executes the script. Thus, if the PHP CGI binary is /usr/bin/php, then the first line in the script must be:

#!/usr/bin/php

Another (mainly superficial) problem is often CGI scripts must be placed in a cgi-bin directory. This means the URI to your blog won’t be www.example.com or blog.example.com but www.example.com/cgi-bin/blog.php or blog.example.com/cgi-bin/ Not exactly pretty. There are ways around that particular problem. There are still bigger problems, even running PHP as a CGI binary: how to lock down the user? You are essentially giving the user a lot of system access (the same is true when running Perl, bash, Ruby and other Scripting languages), how can we prevent a user from stomping all over the system? Even as CGI, the scripts are still executed as the Apache user.

Two common ways are PHPSuExec(*) and SuPHP. Both provide means of changing the UID/GID of the executing process to that of the end user, and many, many other safeguards. However, they both fail on one common problem with PHP: by default, PHP allows users to place their own php.ini file in their Document Root, which will be used to the exclusion of the system php.ini Thus if you have disabled various functions (e.g. dl, system, passthru, etc), a user can easily override it by dropping a blank php.ini file in his Document Root (typically ~/public_html)

Disabling user php.ini files

There are several ways to disable user php.ini files.

• Configure php with –with-config-file-path=/usr/local/lib –with-config-file-scan-dir=/usr/local/lib
• ( Won’t work with PHP 4.4.x) Install the Zend Optimizer
• ( Won’t work with PHP 4.4.x) Create a wrapper script

As noted, only the first option works for all modern versions of PHP. The directory path provided should match where your system php.ini file is located. Once compiled with these options, PHP is essentially “jailed” into using a php.ini file where configure stipulated. One odd thing to note with this, phpinfo() will lie. Try the following in a user account:

• In your user Document Root directory, create an empty file named php.ini\
• In a separate file, place <?php phpinfo(); ?>
• Access the phpinfo file via your web browser
• Note that phpinfo() reports it is using the php.ini file located in your Document Root

An easy way to verify which is being used is to disable a function in the system php.ini file (such as ini_get). In the user php.ini, override that (disable_functions=). Create a short script that tries to use the disabled function. You’ll receive an error stating the function is disabled for security purposes.

What about the other methods? In PHP 5.2.x, installing the Zend Optimizer forces PHP to use the one stipulated in the Zend configuration, thus overriding the user php.ini file. And the wrapper script?

Locate your PHP CGI binary (we’ll use /usr/bin/php as an example) and rename it to php-cgi (thus /usr/bin/php-cgi). In the same directory, create a file named php. Give it the same permissions as php-cgi and insert the following contents:

#!/bin/sh/usr/bin/php-cgi -c /usr/local/lib/php.ini

The -c parameter instructs PHP to use the configuration file stipulated and ignore all others. Astute readers/users will notice this doesn’t filter out the possiblility of a user passing -c/path/to/user/php.ini If you are concerned with that, a script similar to the following will work (as contents of /usr/bin/php):

#!/usr/bin/perl --use strict;use warnings;die '-c not allowed' if grep { $_ } map { $_ eq '-c' } @ARGV;exec( qw(/usr/bin/php-cgi -c /usr/local/lib/php.ini), @ARGV );

Now, if a user attemps to pass -c/path/to/user/php.ini to the script, it will die. If done via a web-accessible script, Apache will return a 500 error and the die message will be in the error_log The use of Perl is just an example, you could do something in BASH, Python, C or whatever. Unfortunately, PHP 4.4.x ignores -c when compiled as a CGI binary, hence this trick only works on PHP 5.

Can you spot the bug

Found in bBlog code:

function maketimestamp($day,$month,$year,$hour,$minute)
{
// make timestamp format of YYYYMMDDHHMMSS
$string = $year.$month.$day.$hour.$minute.'00';
$timestamp = mktime(substr($string,8,2), substr($string,10,2), substr($string,12,2), substr($string,4,2), substr($string,6,2), substr($string,0,4));
return $timestamp;
}


Where in there is the bug? Ok, let me help you. Imagine that somewhere in the bBlog code, maketimestamp function is called with the following parameters:

maketimestamp(1, 6, 2005, 6, 32);

Let’s trace the function step by step. First, the arguments are concatentated in to a string:

$string = $year.$month.$day.$hour.$minute.'00';

With our example $string now contains the following:

'20056163200';

Next, this string is passed to the internal mktime function, where the parameters are supplied by substring our concatenated string. With our example, here are the values passed to mktime:

$timestamp = mktime(20, 0, null, 61, 63, , 2005);

Examining the PHP Documentation for mktime we find the parameters are:

mktime ( [int hour [, int minute [, int second [, int month [, int day [, int year [, int is_dst]]]]]]] )

Because of the very unnecessary string concatenation in this user function, we pass invalid month and day values, and the values for hour, minute and second do not match the user supplied values.

When examining the function in response to a bug report, my first though was “what in the world was the developer thinking?” What need required the string creation? Why do we need a wrapper around a PHP internal function?

The fix is rather easy. Since I did not know the reason for the wrapper, I left it, but changed the body to:

return mktime($hour, $minute, 00, $month, $day, $year);


Now the function works, returning the timestamp requested.