Fuzz testing is most commonly classified based on its analysis approach. The two main categories of fuzz testing solutions are white-box and black-box fuzzers. In this blog, we explain the difference between approaches as well as their main features.
Contents
- Black-box and protocol fuzz testing
- White-box fuzz testing
- Fuzz testing market overview 2025
- Fuzz testing solutions comparison
Black-box and protocol fuzz testing
Black-box fuzzers were invented in 1980 to test the robustness of applications by sending malformed or unexpected inputs. Black-box fuzzers do not require access to the application’s source code. The main drawback of such an approach is that you don’t know what you have tested and what parts of your software were reached. As a result, many bugs remain undetected.
One of the main use cases that follow the black-box approach is protocol fuzz testing, which focuses on testing specific protocols. In this paper, we’ll focus on protocol fuzzers as the primary representative of the black-box approach, as it’s widely used in automotive, embedded systems, and medical industries.
How black-box fuzz testing works
White-box fuzz testing
White-box fuzzers emerged as software development became more complex and security-critical. The first source code fuzzer, American Fuzzy Lop (AFL), developed in 2013, introduced the concept of instrumenting source code to gain feedback on code coverage achieved by each generated input. This provided developers with insights into how deeply fuzz tests could penetrate the code.
The state-of-the-art approach to applying white-box fuzzing is to write fuzz tests tailored to specific APIs, functions, and methods. This approach excels in uncovering subtle bugs and vulnerabilities in specific parts of a codebase by harnessing feedback loops. They use techniques like coverage-guided fuzzing to generate inputs that maximize code coverage, ensuring that even edge cases are explored.
How modern white-box fuzzers work
Fuzz testing market overview 2025
The fuzz testing market currently consists of open-source and commercial white-box fuzzers, as well as commercial black-box fuzz testing tools. The main features of each group of tools with examples of players are represented below.
| White-box fuzzers | Black-box fuzzers | |
| Commercial |
Example: CI Fuzz by Code Intelligence |
Example: Defensics |
| Open-source |
Example: AFL++ |
Fuzz testing landscape
.webp?width=300&height=255&name=White%20paper%20-%20Fuzz%20testing%20solutions%20comparison%20(mockup).webp)
Fuzz testing solutions comparison
For security and development teams exploring the fuzz testing landscape, understanding the available tools, their differences, and their suitability for specific use cases can be challenging.
Discover the fuzz testing landscape and find out how CI Fuzz by Code Intelligence, one of the market leaders, compares to other solutions—download the free comparison guide.
Inside this guide, you’ll find:
- An overview of fuzz testing tools available on the market
- The two main categories of fuzz testing and their key features
- A detailed comparison of CI Fuzz—one of the market leaders—with white-box and protocol fuzzers.