Writing a Simple Fuzzer

Writing a Simple Fuzzer in Metasploit

What is a Fuzzer?

A Fuzzer is a tool used by security professionals to provide invalid and unexpected data to the inputs of a program. A typical Fuzzer tests an application for buffer overflow, invalid format strings, directory traversal attacks, command execution vulnerabilities, SQL Injection, XSS and more.

Because the Metasploit Framework provides a very complete set of libraries to security professionals for many network protocols and data manipulations, the Framework is a good candidate for quick development of a simple Fuzzer.

Metasploit’s Rex  library

The Rex::Text module provides lots of handy methods for dealing with text like:

  • Buffer conversion
  • Encoding (html, url, etc)
  • Checksumming
  • Random string generation

The last point is obviously extremely helpful in writing a simple Fuzzer. This will help you writing Fuzzer tools such as a simple URL Fuzzer or full Network Fuzzer.

For more information about Rex, please refer to the Rex API documentation.


Here are some of the functions that you can find in Rex::Text :

root@kali:~# grep "def self.rand" /usr/share/metasploit-framework/lib/rex/text.rb
def self.rand_char(bad, chars = AllChars)
def self.rand_base(len, bad, *foo)
def self.rand_text(len, bad='', chars = AllChars)
def self.rand_text_alpha(len, bad='')
def self.rand_text_alpha_lower(len, bad='')
def self.rand_text_alpha_upper(len, bad='')
def self.rand_text_alphanumeric(len, bad='')
def self.rand_text_numeric(len, bad='')
def self.rand_text_english(len, bad='')
def self.rand_text_highascii(len, bad='')
def self.randomize_space(str)
def self.rand_hostname
def self.rand_state()