command-injection-v3.pdf

Command Injection
Talha Eroglu, Okan Yildiz
Abstract
OS Command Injection is a type of cyber attack that involves injecting mali-
cious code into a legitimate system command. This allows the attacker to gain
unauthorized access to sensitive information or to manipulate system functions.
The attack is possible when an application does not properly validate user in-
put, allowing the attacker to insert arbitrary commands that are executed by
the operating system.
OS Command Injection attack can include data loss, system compromise,
and loss of trust in the affected organization. In some cases, the attack can
even lead to financial losses or legal repercussions.
This article discusses Command Injection attacks, what vulnerable appli-
cations are and how to exploit the vulnerability, and finally, how to prevent
attacks by writing safe codes.

Contents
1 What is Command Injection ? 3
2 Possible consequences of command injection 4
3 How to exploit example vulnerabilities ? 5
3.1 Detecting vulnerability in first form . . . . . . . . . . . . . . . . 6
3.2 Detecting vulnerability in alternative implementation of first form 7
3.3 Detecting vulnerability in second form . . . . . . . . . . . . . . . 8
3.4 Detecting vulnerability in last form . . . . . . . . . . . . . . . . . 9
3.5 Burp Suite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.6 One step further, Reverse TCP Shell . . . . . . . . . . . . . . . . 15
4 Vulnerable code examples 16
5 How to prevent command injection ? 17
6 Attempt to exploit more secure code 21
7 References 24
2

1. What is Command Injection ?
Command injection is a security vulnerability that allows an attacker to
execute arbitrary operating system (OS) commands on a server that is hosting
an application. Shell injection or OS command injection are other names for
command injection.
Before deeply diving into ’Command Injection’, we can clarify the distinction
between Command Injection and ’Code Execution’ (Code injection) Any attack
involving the injection of code that an application interprets or executes is
called ”code execution.” The misuse of untrusted data inputs is exploited in this
kind of attack. The absence of adequate input/output data validation makes it
possible. Code execution attacks have the critical limitation of being bound to
the application or system they target. For example, if we ran a successful code
execution attack in the test environment, the permissions granted to Python on
the host machine would limit us. Let’s talk about command injection, which is
the subject of our article, Executing commands in a system shell or other areas
of the environment is a standard definition of command injection. Without
injecting malicious code, the attacker increases a vulnerable application’s default
functionality by passing commands to the system shell.
Insufficient input validation leads to OS Command Injection attacks; how-
ever, they are only possible if the application code includes operating system
calls that have user input combined with the invocation.
This vulnerability can occur in every computer program and on any platform.
Any language that executes a command through the system shell is susceptible
to injection, which can be orchestrated on Windows and Unix systems. For
instance, you can find command injection vulnerabilities in router-embedded
software, online applications and APIs, server-side scripts, mobile apps, and
even the core operating system software.
A command injection can compromise the infrastructure of that program,
its data, the entire system, associated servers, and other devices.
3

2. Possible consequences of command injection
• Network traffic from the target system can be redirected to another system
under the attacker’s control, allowing them to intercept and see private
information.
• Security measures like firewalls and intrusion detection/prevention sys-
tems (IDS/IPS) can be avoided using command injection.
• The target system may be completely compromised if an attacker can run
any code on it.
• Using command injection, an attacker can change direction and target dif-
ferent systems connected to the same network as the vulnerable machine.
• Attackers can alter data on the target system, which might result in lost
or inaccurate data.
• By injecting malicious commands that use up all the resources available,
an attacker can cause a Denial of Service (DoS) to the target system.
4

3. How to exploit example vulnerabilities ?
As seen in Figure 1, a web application runs on a virtual machine, accessed
from the localhost:8000 port. Functionalities in the application were imple-
mented using different libraries and methods. These buttons call OS commands
on the host server and provides output to the user.
First, we will discuss using suitable payloads for vulnerability detection in var-
ious implementations. Payloads vary by host operating system, but in this
article, we will only cover Linux and unix-like operating systems. However,
since the basic concept is the same, you can access different payloads over the
internet. Then, after ensuring the vulnerability exists for each sample, we will
see how we can practically test payloads using Burp Suite. Finally, we will
discuss Pseudo-terminal, Reverse TCP Shells and summarize the exploitation
part.
Figure 1: Web application developed in Django
5

3.1. Detecting vulnerability in first form
When we enter an IP address using the first button for its intended purpose,
we encounter an expected output, as in Figure 2.
Figure 2: Submitting IP with Ping button
Then we write ’; whoami’ to the search query to check if we can run extra
OS Commands, suspecting that user-supplied input is not sanitized. In the
red framed output section in Figure 3, we see a new field consisting of a ’dev.’
This is because we bypassed the IP field with ’;’ and executed the ’whoami’
instruction in the host machine.
Figure 3: IP
6

3.2. Detecting vulnerability in alternative implementation of first form
Now we have another implementation of ping functionality in our web appli-
cation. When we try to use ’; unixcommand ’ we confront with error message,
’Invalid IP address form’.In Figure 4.
We may be dealing with some input sanitizing here. Therefore, we need to
Figure 4: Error message when trying to inject commands
bypass this control by changing the inputs accepted by the system. So, first,
we try the valid input form; after changing the inputs step by step, we see that
the commands added to the end of the proper IP addresses are not stuck in this
control. We successfully printed out working directory with ’pwd’ command.
In Figure 5.
Figure 5: Succesfully bypassing input check
7

3.3. Detecting vulnerability in second form
Figure 6: Save form
First, we can understand the save form and what it does in the background.
With two inputs, it gives the output as ”Your alias saved successfully.” Nothing
changes if we try to give OS commands to the Username field (Figure 7).
Figure 7: Consistent output
But when we use the Alias field for the commands, we also get ’test’ in addition
to the success message. We tried to read the passwords in the etc/host/ direc-
tory, and although we did not have a decent output, the change in the output
gave us a clue that something was wrong here. (Figure 8)
Figure 8: Changed output
Based on the previous tip, a string manipulation might exist. When we try
to execute our command between semi-colons, we see that we can inject the
command in Figure 9. We can also get working directory and search for other
user inputs. Figure 10
8

Figure 9: Successfully injecting ls command
Figure 10: Accessing saved user information with cat command
3.4. Detecting vulnerability in last form
We can also execute OS commands by manipulating the URL area in the
same way as in other examples.
Figure 11: Executing OS commands on DNS Lookup form
3.5. Burp Suite
We will use the Ping form while showing the Burp Suite usage step by step.
• Open web page and ensure under the Proxy tab, Intercept is on
• Give an IP address as input in our web page and submit.
• Burp Suite will intercept our request, right click on the Intercepted Raw
request and Send To Intruder.
9

Figure 12: Intercepted request
10

• Clear unrelated payload positions with buttons on the right such that we
will only have our IP field as payload position.
Figure 13: Clearing Payload Positions
11

• Go to Payloads, under Payload Options, load your payload and start the
attack.
Figure 14: Loading payload file
12

Figure 15: Preview of the payload file.
13

• As seen in the Figure 16, it can be determined which command is working,
and you can examine the responses.
Figure 16: Attack requests and responses
14

3.6. One step further, Reverse TCP Shell
Using Commix (Command Injection Exploiter), we can open a pseudo-shell
and then configure a reverse TCP shell on the target machine with the raw
intercept file we got from Burp Suite. A reverse TCP shell is a type of shell
in which the target machine opens a connection back to the attacking machine.
This allows the attacker to control the target machine and access its files, execute
commands, and more. It is called ”reverse” because it is the opposite of the more
common ”forward” shell, in which the attacker connects to the target machine
from their own. It provides the attacker with a wide range of capabilities,
including the ability to run arbitrary code, collect system information, and
escalate privileges.
Figure 17: Pseudo-Terminal in our test environment
15

4. Vulnerable code examples
In this part, we will discuss vulnerable code examples and codes we ex-
ploited in the previous section. Like other injection attacks, unsanitized user
input makes command injection possible. And this is irrespective of the pro-
gramming language used. Code examples will be written in Python.
The figures below contain the vulnerable codes we exploited in the previous
section.
Figure 18: First Ping
Figure 19: Second Ping
16

Figure 20: DNS Lookup
Figure 21: Nickname save
5. How to prevent command injection ?
In this part, we will discuss how to prevent command injection, discuss
general techniques, and finally share the fixed version of the code we exploit.
• Validate and sanitize user input: This includes evaluating all user input to
ensure it is in the desired format and free of potentially harmful characters
or commands. This can assist in avoiding dangerous code injection into
your applications by attackers.
• Avoid using OS Commands: Applications should not include codes that
utilize operating system commands fed with user-controllable data. There
are nearly always safer alternatives for using server-level instructions that
attackers can’t manipulate into doing anything other than what they were
programmed to do.
• Use an up-to-date, secure web application framework: Modern web ap-
plication frameworks have built-in protection against frequent injection
17

attacks. To stop these attacks, ensure you are utilizing an up-to-date and
secure framework.
• Use WAF: By filtering incoming traffic and preventing requests that con-
tain harmful code or commands, a web application firewall (WAF) can
help to safeguard your applications. This may add another level of de-
fense against command injection attacks.
• Lastly, informing your users about the dangers of injection attacks and
the necessity of only entering reliable information into your applications
is crucial. This can lessen the chance of attackers taking advantage of
unsuspecting users.
To avoid command injection, Use an internal API whenever possible (if one
exists) rather than an OS command. Avoid passing user-controlled input wher-
ever possible, and if you have to, use an array with a sequence of program
arguments rather than a single string. To do this in Python, we must set the
subprocess’s shell flag to false. (shell=False’). Because shell=True propagates
existing shell settings and variables, using it is risky. Executing OS commands
with shell=True makes it considerably more straightforward for a malicious ac-
tor to execute commands since variables, glob patterns, and other unique shell
features in the command string are processed before the command is launched.
On the other hand, when using particular shell features, such as word splitting
or argument expansion, shell=True can be useful. If such a feature is necessary,
use the various modules available to you instead, such as os.path.expandvars()
for parameter expansion or shlex for word splitting. More work is required, but
other issues are avoided.
In our code, for the ping function, first, we disabled the shell flag for the
subprocess.run command. After we added a function to use the ipaddress li-
brary, Our IP address is not actually being verified by the ipaddress module. It
attempts to generate a Python IP address object using the supplied string. We
must develop our IP validation function to apply this reasoning to an IP.
18

After implementing the validate ip function, we add a tested, secure regex to
ensure only if the input is a valid IP address.
Figure 22: More secure ping function
In DNS lookup function, we added nslookup library and avoided usage of
OS commands.
Figure 23: More secure DNS Lookup function
19

Also in nick function, we used write() and open() instead of OS commands.
Figure 24: More secure nick function
20

6. Attempt to exploit more secure code
Since we have provided more secure implementation, we can test using the
Burp suite as we did in the 3rd section by combining open-source payload ex-
amples and payloads that we used previously. Figure 25
Figure 25: Payloads
21

First, we turn intercept on and send our request from the web page. After
loading payloads, Figure 26, we start an attack on different forms in our web
application.
Figure 26: Starting attack with payloads
22

Finally, we can inspect the responses of different payloads and ensure that
we did manage to prevent injecting commands to the host OS.
Figure 27: Inspecting responses
23

7. References
• https://semgrep.dev/docs/cheat-sheets/python-command-injection/
• https://portswigger.net/support/using-burp-to-test-for-os-command-injection-vulnerabil
• https://knowledge-base.secureflag.com/vulnerabilities/code_injection/
os_command_injection_python.html
• https://thesecmaster.com/what-is-command-injection-vulnerability-and-how-to-prevent-it
• https://portswigger.net/web-security/os-command-injection
• https://github.com/commixproject/commix
• https://stackoverflow.com/questions/3172470/actual-meaning-of-shell-true-in-subprocess
• https://www.stackhawk.com/blog/command-injection-python/
• https://www.shiftleft.io/blog/find-command-injection-in-source-code/
• https://www.abstractapi.com/guides/validate-ip-address-python
• https://cheatsheetseries.owasp.org/cheatsheets/OS_Command_Injection_
Defense_Cheat_Sheet.html
• https://github.com/payloadbox/command-injection-payload-list
24

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