Nyx fuzzer was developed at Ruhr University Bochum as a research project by Sergej Schumilo and Cornelius Aschermann and many co-oauthors: Thorsten Holz, Ali Abbasi, Simon Woerner, Moritz Schloengel, Robert Gawlik, Sebastian Schinzel, Tim Blazytko, Ahmad-Reza Sadeghi, Daniel Teuchert, Patrick Jauernig, and Tommaso Frassetto. The technology powering Nyx was documented and published by multiple papers.

kAFL: Hardware-Assisted Feedback Fuzzing for OS Kernels

Paper, Slides, Talk,

Many kinds of memory safety vulnerabilities have been endangering software systems for decades. Amongst other approaches, fuzzing is a promising technique to unveil various software faults. Recently, feedback-guided fuzzing demonstrated its power, producing a steady stream of security-critical software bugs. Most fuzzing efforts—especially feedback fuzzing—are limited to user space components of an operating system (OS), although bugs in kernel components are more severe, because they allow an attacker to gain access to a system with full privileges. Unfortunately, kernel components are difficult to fuzz as feedback mechanisms (i.e., guided code coverage) cannot be easily applied. Additionally, non-determinism due to interrupts, kernel threads, statefulness, and similar mechanisms poses problems. Furthermore, if a process fuzzes its own kernel, a kernel crash highly impacts the performance of the fuzzer as the OS needs to reboot.

In this paper, we approach the problem of coverage-guided kernel fuzzing in an OS-independent and hardware-assisted way: We utilize a hypervisor and Intel’s Processor Trace (PT) technology. This allows us to remain independent of the target OS as we just require a small user space component that interacts with the targeted OS. As a result, our approach introduces almost no performance overhead, even in cases where the OS crashes, and performs up to 17,000 executions per second on an off-the-shelf laptop. We developed a framework called kernel-AFL (kAFL) to assess the security of Linux, macOS, and Windows kernel components. Among many crashes, we uncovered several flaws in the ext4 driver for Linux, the HFS and APFS file system of macOS, and the NTFS driver of Windows.

    author = {Schumilo, Sergej and Aschermann, Cornelius and Gawlik, Robert and Schinzel, Sebastian and Holz, Thorsten},
    title = {{kAFL: Hardware-Assisted Feedback Fuzzing for OS Kernels}},
    year = {2017},
    booktitle = {USENIX Security Symposium}

REDQUEEN: Fuzzing with Input-to-State Correspondence

Paper, Slides, Talk,

Automated software testing based on fuzzing has experienced a revival in recent years. Especially feedback-driven fuzzing has become well-known for its ability to efficiently perform randomized testing with limited input corpora. Despite a lot of progress, two common problems are magic numbers and (nested) checksums. Computationally expensive methods such as taint tracking and symbolic execution are typically used to overcome such roadblocks. Unfortunately, such methods often require access to source code, a rather precise description of the environment (e.g., behavior of library calls or the underlying OS), or the exact semantics of the platform’s instruction set.

In this paper, we introduce a lightweight, yet very effective alternative to taint tracking and symbolic execution to facilitate and optimize state-of-the-art feedback fuzzing that easily scales to large binary applications and unknown environments. We observe that during the execution of a given program, parts of the input often end up directly (i.e., nearly unmodified) in the program state. This input-to-state correspondence can be exploited to create a robust method to overcome common fuzzing roadblocks in a highly effective and efficient manner. Our prototype implementation, called REDQUEEN, is able to solve magic bytes and (nested) checksum tests automatically for a given binary executable. Additionally, we show that our techniques outperform various state-of-the-art tools on a wide variety of targets across different privilege levels (kernel-space and userland) with no platform-specific code. REDQUEEN is the first method to find more than 100% of the bugs planted in LAVA-M across all targets. Furthermore, we were able to discover 65 new bugs and obtained 16 CVEs in multiple programs and OS kernel drivers. Finally, our evaluation demonstrates that REDQUEEN is fast, widely applicable and outperforms concurrent approaches by up to three orders of magnitude.

  title={REDQUEEN: Fuzzing with Input-to-State Correspondence},
  author={Aschermann, Cornelius and Schumilo, Sergej and Blazytko, Tim and Gawlik, Robert and Holz, Thorsten},
  booktitle={Symposium on Network and Distributed System Security (NDSS)},

NAUTILUS: Fishing for Deep Bugs with Grammars

Paper, Slides, Talk,

Fuzzing is a well-known method for efficiently identifying bugs in programs. Unfortunately, when fuzzing targets that require highly-structured inputs such as interpreters, many fuzzing methods struggle to pass the syntax checks. More specifically, interpreters often process inputs in multiple stages: first syntactic, then semantic correctness is checked. Only if these checks are passed, the interpreted code gets executed. This prevents fuzzers from executing “deeper” - and hence potentially more interesting - code. Typically two valid inputs that lead to the execution of different features in the target application require too many mutations for simple mutation-based fuzzers to discover: making small changes like bit flips usually only leads to the execution of error paths in the parsing engine. So-called grammar fuzzers are able to pass the syntax checks by using Context-Free Grammars. Using feedback can significantly increase the efficiency of fuzzing engines. Hence, it is commonly used in state-of-the-art mutational fuzzers that do not use grammars. Yet, grammar fuzzers do not make use of code coverage, i.e., they do not know whether any input triggers new functionality or not.

In this paper, we propose NAUTILUS, a method to efficiently fuzz programs that require highly-structured inputs by combining the use of grammars with the use of code coverage feedback. This allows us to recombine aspects of interesting inputs that were learned individually, and to dramatically increase the probability that any generated input will be accepted by the parser. We implemented a proof-of-concept fuzzer that we tested on multiple targets, including ChakraCore (the JavaScript engine of Microsoft Edge), PHP, mruby, and Lua. NAUTILUS identified multiple bugs in all of the targets: Seven in mruby, three in PHP, two in ChakraCore, and one in Lua. Reporting these bugs was awarded with a sum of 2600 USD and 6 CVEs were assigned. Our experiments show that combining context-free grammars and feedback-driven fuzzing significantly outperforms state-of-the-art approaches like American Fuzzy Lop (AFL) by an order of magnitude and grammar fuzzers by more than a factor of two when measuring code coverage.

  title={{Nautilus:  Fishing for Deep Bugs with Grammars}},
  author={Aschermann, Cornelius and Frassetto, Tommaso and Holz, Thorsten and Jauernig, Patrick and Sadeghi, Ahmad-Reza and Teuchert, Daniel },
  booktitle={Symposium on Network and Distributed System Security (NDSS)},

GRIMOIRE: Synthesizing Structure while Fuzzing

Paper, Slides, Talk,

In the past few years, fuzzing has received significant attention from the research community. However, most of this attention was directed towards programs without a dedicated parsing stage. In such cases, fuzzers which leverage the input structure of a program can achieve a significantly higher code coverage compared to traditional fuzzing approaches. This advancement in coverage is achieved by applying large-scale mutations in the application’s input space. However, this improvement comes at the cost of requiring expert domain knowledge, as these fuzzers depend on structure input specifications (e.g., grammars). Grammar inference, a technique which can automatically generate such grammars for a given program, can be used to address this shortcoming. Such techniques usually infer a program’s grammar in a pre-processing step and can miss important structures that are uncovered only later during normal fuzzing.

In this paper, we present the design and implementation of GRIMOIRE, a fully automated coverage-guided fuzzer which works without any form of human interaction or pre-configuration; yet, it is still able to efficiently test programs that expect highly structured inputs. We achieve this by performing large-scale mutations in the program input space using grammar-like combinations to synthesize new highly structured inputs without any pre-processing step. Our evaluation shows that GRIMOIRE outperforms other coverage-guided fuzzers when fuzzing programs with highly structured inputs. Furthermore, it improves upon existing grammar-based coverage-guided fuzzers. Using GRIMOIRE, we identified 19 distinct memory corruption bugs in real-world programs and obtained 11 new CVEs.

    author = {Tim Blazytko and Cornelius Aschermann and Moritz Schl{\"o}gel and Ali Abbasi and Sergej Schumilo and Simon W{\"o}rner and Thorsten Holz},
    title =  {{GRIMOIRE}: Synthesizing Structure while Fuzzing},,
    year = {2019},
    booktitle = {USENIX Security Symposium}

IJON: Exploring Deep State Spaces via Fuzzing

Paper, Slides, Talk,

Although current fuzz testing (fuzzing) methods are highly effective, there are still many situations such as complex state machines where fully automated approaches fail. State-of- the-art fuzzing methods offer very limited ability for a human to interact and aid the fuzzer in such cases. More specifically, most current approaches are limited to adding a dictionary or new seed inputs to guide the fuzzer. When dealing with complex programs, these mechanisms are unable to uncover new parts of the code base. In this paper, we propose IJON, an annotation mechanism that a human analyst can use to guide the fuzzer. In contrast to the two aforementioned techniques, this approach allows a more systematic exploration of the program’s behavior based on the data representing the internal state of the program. As a consequence, using only a small (usually one line) annotation, a user can help the fuzzer to solve previously unsolvable challenges. We extended various AFL-based fuzzers with the ability to annotate the source code of the target application with guidance hints. Our evaluation demonstrates that such simple annotations are able to solve problems that—to the best of our knowledge— no other current fuzzer or symbolic execution based tool can overcome. For example, with our extension, a fuzzer is able to play and solve games such as Super Mario Bros. or resolve more complex patterns such as hash map lookups. To further demonstrate the capabilities of our annotations, we use AFL combined with IJON to uncover both novel security issues and issues that previously required a custom and comprehensive grammar to be uncovered. Lastly, we show that using IJON and AFL, one can solve many challenges from the CGC data set that resisted all fully automated and human guided attempts so far.

  author    = {Cornelius Aschermann and Sergej Schumilo and Ali Abbasi and Thorsten Holz},
  title     = {Ijon: Exploring Deep State Spaces via Fuzzing},
  booktitle = {IEEE Symposium on Security and Privacy},
  year      = {2020},

HYPER-CUBE: High-Dimensional Hypervisor Fuzzing

Paper, Slides, Talk,

Applying modern fuzzers to novel targets is often a very lucrative venture. Hypervisors are part of a very critical code base: compromising them could allow an attacker to compromise the whole cloud infrastructure of any cloud provider. In this paper, we build a novel fuzzer that aims explicitly at testing modern hypervisors.

Our high throughput fuzzer design for long running interactive targets allows us to fuzz a large number of hypervisors, both open source, and proprietary. In contrast to one-dimensional fuzzers such as AFL, HYPER-CUBE can interact with any number of interfaces in any order.

Our evaluation shows that we can find more bugs (over 2x) and coverage (as much as 2x) than state of the art hypervisor fuzzers. Additionally, in most cases, we were able to do so using multiple orders of magnitude less time than comparable fuzzers. HYPER-CUBE was also able to rediscover a set of well-known vulnerabilities for hypervisors, such as VENOM, in less than five minutes. In total, HYPER-CUBE found 54 novel bugs, and so far we obtained 37 CVEs.

Our evaluation results demonstrates that next generation coverage-guided fuzzers should incorporate a higher-throughput design for long running targets such as hypervisors.

  title={{HYPER-CUBE: High-Dimensional Hypervisor Fuzzing}},
  author={Schumilo, Sergej and Aschermann, Cornelius and Abbasi, Ali and W{\"o}rner, Simon and Holz, Thorsten},

Nyx: Greybox Hypervisor Fuzzing using Fast Snapshots and Affine Types

Paper, Slides, Talk,

A hypervisor (also know as virtual machine monitor, VMM) enforces the security boundaries between different virtual machines (VMs) running on the same physical machine. A malicious user who is able to run her own kernel on a cloud VM can interact with a large variety of attack surfaces. Exploiting a software fault in any of these surfaces leads to full access to all other VMs that are co-located on the same host. Hence, the efficient detection of hypervisor vulnerabilities is crucial for the security of the modern cloud infrastructure. Recent work showed that blind fuzzing is the most efficient approach to identify security issues in hypervisors, mainly due to an outstandingly high test throughput.

In this paper we present the design and implementation of NYX, a highly optimized, coverage-guided hypervisor fuzzer. We show how a fast snapshot restoration mechanism that allows us to reload the system under test thousands of times per second is key to performance. Furthermore, we introduce a novel mutation engine based on custom bytecode programs, encoded as directed acyclic graphs (DAG), and affine types, that enables the required flexibility to express complex interactions. Our evaluation shows that, while NYX has a lower throughput than the state-of-the-art hypervisor fuzzer, it performs competitively on simple targets: NYX typically requires only a few minutes longer to achieve the same test coverage. On complex devices, however, our approach is able to significantly outperform existing works. Moreover, we are able to uncover substantially more bugs: in total, we uncovered 44 new bugs with 22 CVEs requested. Our results demonstrate that coverage guidance is highly valuable, even if a blind fuzzer can be significantly faster.

@inproceedings {nyx,
author = {Sergej Schumilo and Cornelius Aschermann and Ali Abbasi and Simon W{\"o}r-ner and Thorsten Holz},
title = {Nyx: Greybox Hypervisor Fuzzing using Fast Snapshots and Affine Types},
booktitle = {30th {USENIX} Security Symposium ({USENIX} Security 21)},
year = {2021},
url = {},

Nyx-Net: Network Fuzzing with Incremental Snapshots


Coverage-guided fuzz testing (“fuzzing”) has become mainstream and we have observed lots of progress in this research area recently. However, it is still challenging to efficiently test network services with existing coverage-guided fuzzing methods. In this paper, we introduce the design and implementation of Nyx-Net, a novel snapshot-based fuzzing approach that can successfully fuzz a wide range of targets spanning servers, clients, games, and even Firefox’s Inter-Process Communication (IPC) interface. Compared to state-of-the-art methods, Nyx-Net improves test throughput by up to 300x and coverage found by up to 70%. Additionally, Nyx-Net is able to find crashes in two of ProFuzzBench’s targets that no other fuzzer found previously. When using Nyx-Net to play the game Super Mario, Nyx-Net shows speedups of 10-30x compared to existing work. Under some circumstances, Nyx-Net is even able play “faster than light”: solving the level takes less wall-clock time than playing the level perfectly even once. Nyx-Net is able to find previously unknown bugs in servers such as Lighttpd, clients such as MySQL client, and even Firefox’s IPC mechanism - demonstrating the strength and versatility of the proposed approach. Lastly, our prototype implementation was awarded a $20.000 bug bounty for enabling fuzzing on previously unfuzzable code in Firefox and solving a long-standing problem at Mozilla.

      title={Nyx-Net: Network Fuzzing with Incremental Snapshots}, 
      author={Sergej Schumilo and Cornelius Aschermann and Andrea Jemmett and Ali Abbasi and Thorsten Holz},


We also gave presentations on the work behind Nyx at various other conferences and occasions: