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Unleashing the Power of Scapy for Protocol Fuzzing

Updated: Feb 7

Network fuzzing is like a high-tech pillow fight: instead of feathers, it's throwing packets everywhere and hoping something breaks!!!

Cybersecurity is a critical aspect of any network or software system, and fuzzing is arguably one of the most potent techniques used to identify such security vulnerabilities. Fuzzing involves injecting unexpected or invalid data into the system, which can trigger unforeseen behaviours, potentially leading to security breaches or crashes. Scapy is one of the many tools that can be used for fuzzing, and it stands out as a versatile and efficient option.

Introduction to Scapy

Scapy is a Python module designed for packet manipulation, crafting, and network communication. Its comprehensive set of features enables the creation of customized network traffic, making it an ideal tool for fuzzing. Scapy's flexibility allows users to craft packets with precise control over their contents, enabling targeted fuzzing of specific protocols or applications.

Advantages of Scapy for Fuzzing

Scapy offers several advantages for fuzzing:

  • Python-based: Leveraging Python's power and simplicity, Scapy offers a user-friendly interface to create and send fuzzed packets.

  • Cross-platform compatibility: Scapy is compatible with Windows, Linux, and macOS, making it accessible to a wide range of users.

  • Extensive packet manipulation capabilities: Scapy's rich feature set enables the manipulation of various packet layers and protocols, including IP, TCP, UDP, and application-specific protocols.

  • Integration with other tools: Scapy can be integrated with other fuzzing frameworks and tools, enhancing its utility and flexibility.

Practical Applications of Scapy for Fuzzing

Scapy proves to be valuable in various fuzzing scenarios:

  • Protocol fuzzing: Scapy can be used to fuzz network protocols and uncover vulnerabilities or unexpected behaviours. For example, it can be employed to fuzz FTP protocol to identify weaknesses in error handling or protocol implementation.

  • Application fuzzing: Scapy can be used to fuzz application-specific protocols, such as HTTP, to uncover vulnerabilities in input validation or data processing. For example, it can be used to fuzz HTTP requests with invalid headers or unexpected data types.

  • Fuzzing for denial-of-service (DoS) attacks: Scapy can be utilized to simulate harmful network traffic, allowing organizations to test their systems' resilience against DoS attacks and address potential vulnerabilities before they can be exploited by attackers.

Installing Scapy

  • Since scapy is a Python library, it's commonly installed using the pip package manager. On your Kali Virtual machine, open a terminal and run the below command to install scapy:

pip3 install scapy

  • Verify the scapy installation using the commands below.

$ python -m scapy  # No error in output indicates the module was installed

Checking scapy installation on Kali Linux
Checking scapy installation on Kali Linux


$ python
import scapy
Checking Scapy installation on Kali
Checking Scapy installation on Kali

Fuzzing Vsftpd FTP Server Using Scapy

To illustrate Scapy's utility, let's consider fuzzing a vsftpd FTP server. The goal is to identify potential vulnerabilities by sending unexpected or invalid commands to the FTP server

vsftpd Lab Setup

Install docker on Kali Linux and host a vsftpd container.

  • Install docker using Kali terminal.

sudo apt-get update
sudo apt-get install
  • Create a group docker on Kali and add user Kali to the group. This enables the Kali user to work with the docker service.

sudo groupadd docker  
sudo usermod -aG docker $USER
newgrp docker
sudo service docker restart

  • Create a local network for the containers.

docker network create --subnet= localnetwork

  • Metasploitable2 is a vulnerable machine created for penetration testing and exploitation. You can install and run the metasploitable2 container in Kali by using the below Docker commands. Metasploitable2 has VSFTPd 2.3.4 installed and configured to run on FTP port 21.

docker pull tleemcjr/metasploitable2
docker run --name metasploitable2 -d --network localnetwork -P -it --rm tleemcjr/metasploitable2

  • Verify if metasploitable2 is running using the command below.

docker ps

Running docker on kali

  • Perform FTP enumeration using Nmap to check the FTP server details. We found that the FTP server version is vsftpd 2.3.4 and also anonymous login is enabled.

nmap -p 21 -A

Running nmap on kali

Python Scripting with Scapy

  • Python's Scapy library allows us to sniff, send, and receive network packets. The following script enables us to interact with and send modified packets to vsftpd 2.3.4. In the below script, is the IP address of the Metasploitable2 container. We are sending malformed packet (buffer of 1000 A's) via the USER command to the vsftpd FTP server on port 21.

  • Also in the below script, sport is the source port, dport is the destination port, and flags ("A" is for acknowledgement) are for TCP flags.

#! /usr/bin/env python
import scapy.all as scapy

syn_packet = scapy.IP(dst="") / scapy.TCP(dport=21, flags="S")
#send invalid inputs
user_command = "USER anonymous" + "A" * 1000
packet = syn_packet / scapy.TCP(sport=syn_packet[scapy.TCP].dport, dport=syn_packet[scapy.TCP].sport, flags="A") / scapy.Raw(load=user_command)

response = scapy.send(packet, verbose=True)
  • Save the above script as and run using the command below (from the same directory).

sudo python ./

Running fuzzftp python file

  • You can use tools such as Wireshark to view the packets sent to the vsftpd FTP server.

Wireshark packet capture


Scapy is a valuable tool for protocol fuzzing that provides a flexible and powerful platform for testing network and application security. Its Python-based interface, cross-platform compatibility, and extensive packet manipulation capabilities make it an attractive choice for security professionals and researchers. Scapy is versatile and effective in uncovering hidden security flaws, whether exploring protocol vulnerabilities, identifying application weaknesses, or testing against DoS attacks.


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