HOWTO: Write a fuzzer using Fusil

Identify your target

Before writing a fuzzer, you have to analyze your target: identify input vectors and list methods to watch target activity.

Identify input vectors

Example of input vectors:

  • the command line
  • environment variables
  • files
  • sockets (TCP, UDP, UNIX, etc.)
  • etc.

Some tools help in this job:

  • strace to watch syscalls (eg. “strace -e open program ...”)
  • ltrace to watch library function calls (eg. “ltrace -e getenv program ...”)
  • netstat to watch network connections (sockets)

Watch target activity

Example target probes:

  • standard output: stdout and stderr
  • process exit status (exit code or signal used to kill the process)
  • logging file (eg. /var/log/clamav/clamav.log)
  • network ping
  • run the process in a debugger to trace faults (signals)
  • ...

Hello world fuzzer

First draft

Let’s start with the most simple fuzzer. Create a file “” with this code:

#!/usr/bin/env python
from fusil.application import Application
from fusil.process.create import ProjectProcess

class Fuzzer(Application):
    def setupProject(self):
        ProjectProcess(self.project, ['echo', 'Hello World!'])

if __name__ == "__main__":

Source code details:

  • it’s a Python (executable) program
  • you have to inherit Application class and define the setupProject() method
  • setupProject() creates the project “agents”
  • the process agent is not stored in a variable: it’s transparently register in Fusil agent list
  • main() method starts the fuzzer, activate agents, execute the different sessions, etc.

Let’s try this amazing fuzzer! Stop it using CTRL+c:

$ chmod +x
$ ./
Fusil version 0.9.1 -- GNU GPL v2
Use directory: /home/haypo/prog/SVN/fusil/trunk/run-1
[0][session 1] Start session
[0][session 1] Project execution interrupted!
[0][session 1] Project done: total 5.6 seconds, aggresssivity: 1.0%
[0][session 1] Total: 0 success
[0][session 1] Process 8210 exited normally

Working example

This fuzzer is useless because it doesn’t know if the process is still active or not. We have to add some probes: WatchProcess to watch the exit status and WatchStdout to watch the standard output:

from import WatchProcess
from fusil.process.stdout import WatchStdout

class Fuzzer(Application):
    def setupProject(self):
        process = ProjectProcess(self.project, ['echo', '"Hello World!"'])

Let’s retry with a maximum of 1 session:

$ ./  --sessions=1
Fusil version 0.9.1 -- GNU GPL v2
Use directory: /home/haypo/prog/SVN/fusil/trunk/run-1
[0][session 1] Start session
[0][session 1] Process 8222 exited normally
Stop! Limited to 1 sessions, use --session option for more
Project done: 1 sessions in 0.1 seconds (63.8 ms per session), total 1.1 seconds, aggresssivity: 2.0%
Total: 0

See “Process 8222 exited normally” line: the session stopped because the process is done.

Real fuzzer

Execute the echo program with a constant argument is not so funny, we have to add some random arguments. ProjectProcess can’t be used because it creates automatically the process on session start. So we will use CreateProcess which allows to do some operations before the process start. The patch:

from fusil.process.create import CreateProcess
from fusil.bytes_generator import BytesGenerator, ASCII0
from random import randint, choice

class EchoProcess(CreateProcess):
    OPTIONS = ("-e", "-E", "-n")

    def __init__(self, project):
        CreateProcess.__init__(self, project, ["echo"])
        self.datagen = BytesGenerator(1, 10, ASCII0)

    def createCmdline(self):
        arguments = ['echo']
        for index in xrange(randint(3, 6)):
            if randint(1, 5) == 1:
                option = choice(self.OPTIONS)
                data = self.datagen.createValue()
        return arguments

    def on_session_start(self):
        self.cmdline.arguments = self.createCmdline()


  • The most important method is on_session_start(): it’s called when the event “session_start” is emitted. This method recreates the command line arguments and create the process.
  • createCmdline() creates random arguments for the echo program: 25% of options (-e, -E or -n) and 75% of random strings (ASCII characters in range 1..255)
  • __init__() is overwrite to create the datagen attribute
  • fusil.bytes_generator module contains tools (classes) to generate random byte strings

Test examples

See examples/ directory. It contains hello-world (first version of Hello World) and good-bye-world (working echo fuzzer).

Inject data to your target

Command line and environment variable

See process.rst documentation.


See mangle.rst documentation.

To write a C program, see c_tools.rst documentation


See network.rst documentation.

Watch target activity

Standard output and logging files

Use WatchStdout and FileWatch. See also file_watch.rst documentation.

Process exit status

Use WatchProcess and read process.rst documentation.