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Program Environment Fuzzing (EFuzz)

EFuzz (εFuzz) is a new type of fuzzer that can fuzz just about anything, including:

  • Network servers/clients
  • Graphical User Interface (GUI) applications
  • Editors
  • Compilers
  • Databases
  • any other Linux user-mode software, etc.

For example, the following demo shows how to fuzz a GUI application using EFuzz:

EFuzz demo

[Demo of EFuzz fuzzing a GUI application binary (gnome-calculator). First, the app is run normally, and all environmental interactions (including user inputs) are recorded to a file. Next, the app is fuzzed, this time by repeatedly replaying the original recording but with one or mutations applied. EFuzz quickly finds several crashes, as highlighted in red.]

Unlike existing fuzzers, which only fuzz a specific input source (like AFL), EFuzz fuzzes the entire interaction between the subject and its environment (ε)---including all files (configuration, cache, resource, fonts, etc.), sockets (session manager, accessibility service, name service, etc.), user interactions via the windowing system, special files, devices, and standard streams, as illustrated below:

Full calculator app environment

In other words, EFuzz considers the entire environment (ε) itself is the fuzz target---a.k.a. program environment fuzzing. This even includes user interactions via the windowing system (e.g., mouse movements, button clicks, etc.), which are treated as just another input source, no different than any other socket or file. This makes EFuzz a very comprehensive fuzzer, as it will automatically fuzz all inputs (and input combinations), even obscure inputs that are normally overlooked in a typical fuzz campaign.

EFuzz is also very general, and is designed to fuzz off-the-shelf user-mode Linux binaries with zero configuration or set-up. See the demo above.

How EFuzz Works?

EFuzz is based on an underlying full environmental Record and Replay (rr) and fuzzing infrastructure (RR+Fuzzing=RRFuzz). The infrastructure is based on the insight that, for typical user-mode programs, all environmental interactions pass through the kernel-user mode interface. Thus, by recording this interface, we also implicitly record the program's interaction with the environment, which then can be subject to fuzzing. Basically, EFuzz works in two phases:

  1. A Recording phase that executes the program normally, and records all interactions (e.g., system calls, signals, thread switches, etc.) between the subject and its environment into a file.
  2. Next, a Replay-with-Fuzzing phase that repeatedly re-executes the program in a fuzzing loop. However, this time the original environmental interactions are replayed from the recording, but with one or more possible mutations applied. These mutations can induce new program behaviours and possible crashes.

Since EFuzz works at the abstraction of environmental interactions, it can fuzz a very diverse range of subjects without any special-case handling. This makes EFuzz powerful and easy-to-use.

An example of EFuzz usage is shown in the simple demo above:

  1. The first phase records the gnome-calculator app using the command:

     $ ./EFuzz record gnome-calculator

    This creates an out/ sub-directory, and the recording is stored in a out/RECORD.pcap.gz file.

  2. The second phase fuzzes the gnome-calculator app using the command:

     $ ./EFuzz fuzz

    The fuzzer uses the original recording from step (1.) as the initial seed, and repeatedly re-executes the program. Rather than replaying the original recording exactly, the fuzzer will apply one (or more) mutations, inducing new program behaviours and possible crashes.

Discovered crashes are stored in the out/crash/ sub-directory, and can be replayed using the command:

    $ ./EFuzz replay out/crash/SIGSEGV_XXXX_m0YYYY.patch

We applied EFuzz to a diverse range of programs and discovered several new bugs, including those that are difficult or impossible to find with traditional fuzzers. For more information, please see the preprint listed below.


To build EFuzz, simply run the script.

    $ ./

This has only been tested on Ubuntu-based systems.

EFuzz Usage


    $ ./EFuzz record -- vim hello.txt


    $ ./EFuzz replay


    $ ./EFuzz fuzz

Replay crash:

    $ ./EFuzz replay out/crash/SIGSEGV_XXXX_mYYYYY.patch

Debug crash:

    $ ./EFuzz replay out/crash/SIGSEGV_XXXX_mYYYYY.patch -d

EFuzz also supports (optional) AFL-style coverage instrumentation:

Instrument program binary:

    $ ./EFuzz instrument program

This will generate an instrumented program.rr binary, that can be substituted for the original.

It is also possible to instrument library code:

    $ ./EFuzz instrument /path/to/

This will place an instrumented version of the library into the lib/ sub-directory, which is included in the library search path.

EFuzz Files

During fuzzing, several files will be generated:

  • out/COMMAND.cmd: The command-line
  • out/RECORD.pcap.gz: The recording
  • out/crash/*.patch: Crashing patches
  • out/hang/*.patch: Hanging patches
  • out/abort/*.patch: Aborting patches
  • out/queue/m*/*.patch: Interesting patches
  • lib/: Location where the patched libc and instrumented libraries are stored.

Note that interactions stored in the out/RECORD.pcap.gz file can be viewed using Wireshark.

EFuzz Technical Information

EFuzz uses E9Patch to statically rewrite all syscall instructions in libc.

Thus, whenever the program invokes a system call (e.g., open, read, write, etc.), the call will be diverted to our record-and-replay infrastructure (RR+Fuzzing=RRFuzz). For recording, the infrastructure acts as a man-in-the-middle between user and kernel-mode, and all interactions are saved to a file. For replay (with fuzzing), the program is (repeatedly) executed again, but this time each system call is replayed from the original recording, possibly with one or more mutations applied. During replay, system calls are almost entirely serviced from the recording, and there is no interaction with the kernel and the real environment. In addition to libc system calls, EFuzz also special methods for handling the vDSO, signals, thread switches, and syscall instructions outside of libc. EFuzz does not use ptrace.

One problem is that program behaviour can (and will) diverge after a mutation is applied. Indeed, the main point of fuzzing is to induce new behaviours in the hope of finding new bugs. To handle this case, EFuzz resorts to "system call emulation" after mutation, in a "relaxed" replay-mode. This allows for diverse program behaviours to be explored using only the original recording. Please see the preprint below for more information.

EFuzz Status

Implementing a full environmental record and replay infrastructure is notoriously challenging. For example, the related rr-debug project estimates a total of five person-years total (source) was spent on development (as of 2017), and even then it cannot perfectly record and replay all programs. Our underlying RRFuzz framework has not nearly had as much development time, and the current implementation should be viewed as a "research prototype" that will not necessarily work for all programs. Nevertheless, we provide the system "as-is", since EFuzz has been proven effective at finding bugs that other fuzzers cannot detect, even if the implementation is not fully polished.

We will continue to develop and improve EFuzz depending on the level of interest from the community.

EFuzz Issues

Please keep in mind that EFuzz is alpha-quality software and a research prototype.

Please report obvious bugs here:

Issues relating to unexpected divergence during replay should generally not be reported. That said, pull requests that fix divergence issues are very welcome.

Similarly, issues relating to missing ioctls, fcntls, and prctls should not be reported. Instead, you should update the corresponding .tab file if possible, and consider making a pull request.


This research is supported by the National Research Foundation, Singapore, and Cyber Security Agency of Singapore under its National Cybersecurity R&D Programme (Fuzz Testing NRF-NCR25-Fuzz-0001). Any opinions, findings and conclusions, or recommendations expressed in this material are those of the author(s) and do not reflect the views of National Research Foundation, Singapore, and Cyber Security Agency of Singapore.


This software has been released under the GNU Public License (GPL) Version 3. See the LICENSE file for more information.


If you use this code in your scientific work, please cite the paper as follows:

        title={Program Environment Fuzzing},
        author={Meng, Ruijie and Duck, Gregory J. and Roychoudhury, Abhik},
        journal={arXiv preprint arXiv:2404.13951},


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