Thursday, August 1, 2013

Introducing Binfuzz.js

Tomorrow morning I will be giving a demonstration of Binfuzz.js at Blackhat Arsenal 2013. Please stop by the Arsenal area from 10:00 - 12:30. The slides are already available on the Blackhat website.

The Binfuzz.js page on is now live, and all the code is uploaded to Github.

What is Binfuzz.js?

Binfuzz.js is a library for fuzzing structured binary data in JavaScript. Structured binary data is data that can be easily represented by one or more C structures. Binfuzz.js uses the definition of a structure to create instances of the structure with invalid or edge-case values. Supported structure features include nested structures, counted arrays, file offset fields, and length fields. The live example uses Binfuzz.js to generate Windows ICO files (a surprisingly complex format) to stress your browser's icon parsing and display code.


Binfuzz.js includes support for:

  • Several predefined elementary types: Int8, Int16, Int32 and Blob.
  • Nested structures
  • Arrays
  • Counter Fields (e.g. field A = number of elements in Array B)
  • Length Fields (e.g. field A = length of Blob B)
  • File Offsets (e.g. field A = how far from the start of the file is Blob B?
  • Custom population functions (e.g. field A = fieldB.length + fieldC.length)

The ICO fuzzing example includes uses of all of these because I needed them to implement ICO file generation.


Binfuzz.js calculates the total number of combinations based on how many possible combinations there are for each individual field. It is then possible to generate a structured data instance corresponding to a specific combination number. It is not necessary to generate prior combinations. This way random combinations can be selected when fuzzing run time is limited.


The best way to learn is by doing, and I wanted to learn JavaScript. So I decided to create an ICO file fuzzer in JavaScript. I chose ICO files because of favicon.ico, a file browsers automatically request when navigating to a new page. After starting the project, I realized I got a lot more than I bargained for. Icons are a surprisingly complex format that has evolved over time. There are several images in one file, each image has corresponding metadata, there are internal fields that refer to offsets in the file, and the size of the icon data for each image depends the metadata. All of these interlinked reationships need to be described and processed by Binfuzz.js.