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Disable Dangerous Functions in PHP November 30, 2012

Posted by Tournas Dimitrios in PHP.

PHP is often characterized as an “easy to learn programming language  and is used as a footstep into web programming . Most of this misunderstanding is due to authors that write tutorials about PHP and often concentrate only on how to present an achievement ( a specific  programs task in PHP ) , while forgetting to mention that their script wasn’t meant for production . Beginners then go on to use these “easy” scripts on production servers and find themselves subject to many forms of attacks . The following chart is just  a representation of the most common PHP-vulnerabilities (listed by type)  .


As shown on the chart , the champion is the “Execute Code” vulnerability . The number represents the reported different ways this vulnerability was used to attack a PHP-application .

PHP  has build-in functionality to execute underlying shell commands on the system itself  (were PHP is installed on) . It provides a number of functions for executing external commands , among them shell_execexecpassthru , the backtricks (` `)  and system . It’s important to note that pretty much whatever you can do on the UNIX command line or in a shell script is allowed here . For example , you can use pipes to string together commands . Each of these commands spawns a child process to run the command or script you designate , and each captures the output of your command as it’s written to standard output (stdout) .

Many common attacks on PHP applications involve attackers uploading so-called “Web Shells”, which are scripts that give the attacker access to system functions for the purpose of taking over the web server . These Shells typically use the dangerous PHP functions for access to system commands . Disabling dangerous PHP functions makes using Web Shells more difficult , thus adding an extra layer of protection  .

How to disable these dangerous commands :

Very simple , open PHP’s ini-file and append the following command (don’t forget to restart Appache so that these configuration changes can be activated) :

disable_functions = shell_exec , show_shource , system , exec,  popen, proc_open , parse_ini_file,  passthru ,  symlink

Most likely , a shared host will have disabled shell access . This article only applies to PHP-code hosted on Cloud servers (like Amazon’s EC2) , VPS or on dedicated-servers  .

Final thoughts : 

I remember the words of my professor “great power must be accompanied with equal responsibilities” . My golden rule is to find alternative ways to accomplish a specific task that was meant to be run through the command shell . For instance , PHP has a “bunch” of build-in file-system function that can be used instead of shell-commands . Another example , I have seen in the past fellow coders using shell commands  to “ping” a remote server (echo exec(“ping “.$ip);) , with a little bit of code the same functionality can be done with PHP’s build-in functionality (fsockopen function) .  Which commands should you use and when ? This is entirely up to you and the needs you have . If you’re accepting user input and passing that information along to the shell , you better sanitize that user input . Strip out any commands that you think might be harmful , disallow users from sending in open requests and only allow them to choose from a list of possible alternatives . Make sure that each case of using a dangerous function is absolute necessary and is done in accordance with best practices . For code that passes arguments to shell commands, use escapeshellarg to prevent command injection vulnerabilities . Use absolute paths when executing external commands , don’t let users execute arbitrary commands . Be particularly careful with the backtick (`) operator (this operator is disabled if shell_exec is disabled) .



1. Joselyn - December 3, 2012

Hello Dimitrius:

I asking me if you could help me with an xml that I cannot load, can i send my code?, thanks in advance if you agree.

tournasdimitrios1 - December 4, 2012

Hi ,
As you might have discovered my Blog has over 700 articles , where some of those have many comments . I try to respond to all comments , on condition , these comments are in focus of the article’s subject and the reader has at least basic knowledge of web-programming (PHP , JQuery , AS3 ) . It’s my pleasure to help other developers , but I can’t make their “home work” (that would consume valuable energy , which I prefer to spend on writing new articles ) .
Kind regards .

2. Carolina - December 6, 2012

i just found this blog and have high hopes for it to continue. keep up the great work, its hard to find good ones. i have added to my favourites. thank you.

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