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american fuzzy lop

  Written and maintained by Michal Zalewski <>

  Copyright 2013, 2014, 2015 Google Inc. All rights reserved.
  Released under terms and conditions of Apache License, Version 2.0.

  For new versions and additional information, check out:

  To compare notes with other users or get notified about major new features,
  send a mail to <>.

  ** See QuickStartGuide.txt if you don't have time to read this file. **

1) Challenges of guided fuzzing

Fuzzing is one of the most powerful and proven strategies for identifying
security issues in real-world software; it is responsible for the vast
majority of remote code execution and privilege escalation bugs found to date
in security-critical software.

Unfortunately, fuzzing is also relatively shallow; blind, random mutations
make it very unlikely to reach certain code paths in the tested code, leaving
some vulnerabilities firmly outside the reach of this technique.

There have been numerous attempts to solve this problem. One of the early
approaches - pioneered by Tavis Ormandy - is corpus distillation. The method
relies on coverage signals to select a subset of interesting seeds from a
massive, high-quality corpus of candidate files, and then fuzz them by
traditional means. The approach works exceptionally well, but requires such
a corpus to be readily available. In addition, block coverage measurements
provide only a very simplistic understanding of program state, and are less
useful for guiding the fuzzing effort in the long haul.

Other, more sophisticated research has focused on techniques such as program
flow analysis ("concolic execution"), symbolic execution, or static analysis.
All these methods are extremely promising in experimental settings, but tend
to suffer from reliability and performance problems in practical uses - and
currently do not offer a viable alternative to "dumb" fuzzing techniques.

2) The afl-fuzz approach

American Fuzzy Lop is a brute-force fuzzer coupled with an exceedingly simple
but rock-solid instrumentation-guided genetic algorithm. It uses a modified
form of edge coverage to effortlessly pick up subtle, local-scale changes to
program control flow.

Simplifying a bit, the overall algorithm can be summed up as:

  1) Load user-supplied initial test cases into the queue,

  2) Take next input file from the queue,

  3) Attempt to trim the test case to the smallest size that doesn't alter
     the measured behavior of the program,

  4) Repeatedly mutate the file using a balanced and well-researched variety
     of traditional fuzzing strategies,

  5) If any of the generated mutations resulted in a new state transition
     recorded by the instrumentation, add mutated output as a new entry in the

  6) Go to 2.

The discovered test cases are also periodically culled to eliminate ones that
have been obsoleted by newer, higher-coverage finds; and undergo several other
instrumentation-driven effort minimization steps.

As a side result of the fuzzing process, the tool creates a small,
self-contained corpus of interesting test cases. These are extremely useful
for seeding other, labor- or resource-intensive testing regimes - for example,
for stress-testing browsers, office applications, graphics suites, or
closed-source tools.

The fuzzer is thoroughly tested to deliver out-of-the-box performance far
superior to blind fuzzing or coverage-only tools.

3) Instrumenting programs for use with AFL

When source code is available, instrumentation can be injected by a companion
tool that works as a drop-in replacement for gcc or clang in any standard build
process for third-party code.

The instrumentation has a fairly modest performance impact; in conjunction with
other optimizations implemented by afl-fuzz, most programs can be fuzzed as fast
or even faster than possible with traditional tools.

The correct way to recompile the target program may vary depending on the
specifics of the build process, but a nearly-universal approach would be:

$ CC=/path/to/afl/afl-gcc ./configure
$ make clean all

For C++ programs, you'd would also want to set CXX=/path/to/afl/afl-g++.

The clang wrappers (afl-clang and afl-clang++) can be used in the same way;
clang users may also opt to leverage a higher-performance instrumentation mode,
as described in llvm_mode/README.llvm.

When testing libraries, you need to find or write a simple program that reads
data from stdin or from a file and passes it to the tested library. In such a
case, it is essential to link this executable against a static version of the
instrumented library, or to make sure that the correct .so file is loaded at
runtime (usually by setting LD_LIBRARY_PATH). The simplest option is a static
build, usually possible via:

$ CC=/path/to/afl/afl-gcc ./configure --disable-shared

Setting AFL_HARDEN=1 when calling 'make' will cause the CC wrapper to
automatically enable code hardening options that make it easier to detect
simple memory bugs.

PS. ASAN users are advised to review notes_for_asan.txt file for important

4) Instrumenting binary-only apps

When source code is *NOT* available, the fuzzer offers experimental support for
fast, on-the-fly instrumentation of black-box binaries. This is accomplished
with a version of QEMU running in the lesser-known "user space emulation" mode.

QEMU is a project separate from AFL, but you can conveniently build the
feature by doing:

$ cd qemu_mode
$ ./

For additional instructions and caveats, see qemu_mode/README.qemu.

The mode is approximately 2-5x slower than compile-time instrumentation, is
less conductive to parallelization, and may have some other quirks.

5) Choosing initial test cases

To operate correctly, the fuzzer requires one or more starting file that
contains a good example of the input data normally expected by the targeted
application. There are two basic rules:

  - Keep the files small. Under 1 kB is ideal, although not strictly necessary.
    For a discussion of why size matters, see perf_tips.txt.

  - Use multiple test cases only if they are functionally different from
    each other. There is no point in using fifty different vacation photos
    to fuzz an image library.

You can find many good examples of starting files in the testcases/ subdirectory
that comes with this tool.

PS. If a large corpus of data is available for screening, you may want to use
the afl-cmin utility to identify a subset of functionally distinct files that
exercise different code paths in the target binary.

6) Fuzzing binaries

The fuzzing process itself is carried out by the afl-fuzz utility. This program
requires a read-only directory with initial test cases, a separate place to
store its findings, plus a path to the binary to test.

For target binaries that accept input directly from stdin, the usual syntax is:

$ ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program [...params...]

For programs that take input from a file, use '@@' to mark the location in
the target's command line where the input file name should be placed. The
fuzzer will substitute this for you:

$ ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program @@

You can also use the -f option to have the mutated data written to a specific
file. This is useful if the program expects a particular file extension or so.

For programs that accept input via a network, use:

$ ./afl-fuzz -i testcase_dir -o findings_dir [-D delay_before_write] \
             [-t timeout_delay] [-L ] -N network_specification /path/to/program \

where -L is specified only if the target program acts as a client to a network
server (sends data to the server before receiving input in return).  Otherwise,
afl-fuzz assumes the target program acts as a server or daemon and expects to
receive data from network clients and respond.  The network_specification
and the delay parameters are discussed below in (12).

Non-instrumented binaries can be fuzzed in the QEMU mode (add -Q in the command
line) or in a traditional, blind-fuzzer mode (specify -n).

You can use -t and -m to override the default timeout and memory limit for the
executed process; rare examples of targets that may need these settings touched
include compilers and video decoders.

Tips for optimizing fuzzing performance are discussed in perf_tips.txt.

Note that afl-fuzz starts by performing an array of deterministic fuzzing
steps, which can take several days. If you want quick & dirty results right
away, akin to zzuf or honggfuzz, add the -d option to the command line.

7) Interpreting output

See the status_screen.txt file for information on how to interpret the
displayed stats and monitor the health of the process. Be sure to consult this
file especially if any UI elements are highlighted in red.

The fuzzing process will continue until you press Ctrl-C. At minimum, you want
to allow the fuzzer to complete one queue cycle, which may take anywhere from a
couple of hours to a week or so.

There are three subdirectories created within the output directory and updated
in real time:

  - queue/   - test cases for every distinctive execution path, plus all the
               starting files given by the user. This is the synthesized corpus
               mentioned in section 2.

               Before using this corpus for any other purposes, you can shrink
               it to a smaller size using the afl-cmin tool. The tool will find
               a smaller subset of files offering equivalent edge coverage.

  - crashes/ - unique test cases that cause the tested program to receive a
               fatal signal (e.g., SIGSEGV, SIGILL, SIGABRT). The entries are 
               grouped by the received signal.

  - hangs/   - unique test cases that cause the tested program to time out. Note
               that when default (aggressive) timeout settings are in effect,
               this can be slightly noisy due to latency spikes and other
               natural phenomena.

Crashes and hangs are considered "unique" if the associated execution paths
involve any state transitions not seen in previously-recorded faults. If a
single bug can be reached in multiple ways, there will be some count inflation
early in the process, but this should quickly taper off.

The file names for crashes and hangs are correlated with parent, non-faulting
queue entries. This should help with debugging.

When you can't reproduce a crash found by afl-fuzz, the most likely cause is
that you are not setting the same memory limit as used by the tool. Try:

$ ( ulimit -Sv $[LIMIT_MB << 10]; /path/to/tested_binary ... )

Change LIMIT_MB to match the -m parameter passed to afl-fuzz. On OpenBSD,
also change -Sv to -Sd.

Any existing output directory can be also used to resume aborted jobs; try:

$ ./afl-fuzz -i- -o existing_output_dir [...etc...]

If you have gnuplot installed, you can also generate some pretty graphs for any
active fuzzing task using afl-plot. For an example of how this looks like,

8) Parallelized fuzzing

Every instance of afl-fuzz takes up roughly one core. This means that on
multi-core systems, parallelization is necessary to fully utilize the hardware.
For tips on how to fuzz a common target on multiple cores or multiple networked
machines, please refer to parallel_fuzzing.txt.

Note the limitations to multi-core execution for fuzzing network services under

9) Fuzzer dictionaries

By default, afl-fuzz mutation engine is optimized for compact data formats -
say, images, multimedia, compressed data, regular expression syntax, or shell
scripts. It is somewhat less suited for languages with particularly verbose and
redundant verbiage - notably including HTML, SQL, or JavaScript.

To avoid the hassle of building syntax-aware tools, afl-fuzz provides a way to
seed the fuzzing process with an optional dictionary of language keywords,
magic headers, or other special tokens associated with the targeted data type
- and use that to reconstruct the underlying grammar on the go:

To use this feature, you first need to create a dictionary in one of the two
formats discussed in testcases/README.testcases; and then point the fuzzer to
it via the -x option in the command line.

There is no way to provide more structured descriptions of the underlying
syntax, but the fuzzer will likely figure out some of this based on the
instrumentation feedback alone. This actually works in practice, say:

PS. Even when no explicit dictionary is given, afl-fuzz will try to extract
existing syntax tokens in the input corpus by watching the instrumentation
very closely during deterministic byte flips. This works for some types of
parsers and grammars, but isn't nearly as good as the -x mode.

10) Crash triage

The coverage-based grouping of crashes usually produces a small data set that
can be quickly triaged manually or with a very simple GDB or Valgrind script.
Every crash is also traceable to its parent non-crashing test case in the
queue, making it easier to diagnose faults.

Having said that, it's important to acknowledge that some fuzzing crashes can be
difficult quickly evaluate for exploitability without a lot of debugging and
code analysis work. To assist with this task, afl-fuzz supports a very unique
"crash exploration" mode enabled with the -C flag.

In this mode, the fuzzer takes one or more crashing test cases as the input,
and uses its feedback-driven fuzzing strategies to very quickly enumerate all
code paths that can be reached in the program while keeping it in the
crashing state.

Mutations that do not result in a crash are rejected; so are any changes that
do not affect the execution path.

The output is a small corpus of files that can be very rapidly examined to see
what degree of control the attacker has over the faulting address, or whether
it is possible to get past an initial out-of-bounds read - and see what lies

Oh, one more thing: for test case minimization, give afl-tmin a try. The tool
can be operated in a very simple way:

$ ./afl-tmin -i test_case -o minimized_result -- /path/to/program [...]

The tool works with crashing and non-crashing test cases alike. In the crash
mode, it will happily accept instrumented and non-instrumented binaries. In the
non-crashing mode, the minimizer relies on standard AFL instrumentation to make
the file simpler without altering the execution path.

The minimizer accepts the -m, -t, -f and @@ syntax in a manner compatible with

11) Common-sense risks

Please keep in mind that, similarly to many other computationally-intensive
tasks, fuzzing may put strain on your hardware and on the OS. In particular:

  - Your CPU will run hot and will need adequate cooling. In most cases, if
    cooling is insufficient or stops working properly, CPU speeds will be
    automatically throttled. That said, especially when fuzzing on less
    suitable hardware (laptops, smartphones, etc), it's not entirely impossible
    for something to blow up.

  - Targeted programs may end up erratically grabbing gigabytes of memory or
    filling up disk space with junk files. AFL tries to enforce basic memory
    limits, but can't prevent each and every possible mishap. The bottom line
    is that you shouldn't be fuzzing on systems where the prospect of data loss
    is not an acceptable risk.

  - Fuzzing involves billions of reads and writes to the filesystem. On modern
    systems, this will be usually heavily cached, resulting in fairly modest
    "physical" I/O - but there are many factors that may alter this equation.
    It is your responsibility to monitor for potential trouble; with very heavy
    I/O, the lifespan of many HDDs and SSDs may be reduced.

    A good way to monitor disk I/O on Linux is the 'iostat' command:

    $ iostat -d 3 -x -k [...optional disk ID...]

12) Fuzzing network services

Both client and server (daemon) programs that communicate using an 
IP network (IPv4 or IPv6) can be fuzzed using the command line

$ ./afl-fuzz -i testcase_dir -o findings_dir [-D delay_before_write] \
             [-t timeout_delay] [-L] -N network_specification /path/to/program \

where the network_specification has a form similar to a URL:


Afl-fuzz has two network fuzzing modes, where it acts as a client to a
network server that expects input via a socket (no -L option), and where it
acts as a server to a network client that sends data to afl-fuzz before
receiving input (using the -L option).  In the first case, afl-fuzz sends data
to the port specified by the URL.  In the second case, afl-fuzz receives data
on that port and sends a (fuzzed) response to the port the target (client) used
to send its data.

Note that when afl-fuzz acts as a server (the -L option), there is no control
over how the target client manages its use of sockets - and in particular, a
client that uses ephemeral sockets (the usual case) will rapidly consume the
network stack's pool of available sockets.  Some operating systems are able to
reclaim used ephemeral sockets to keep the pool from being exhausted; others
may experience difficulties.

While the -D and -t command line arguments are optional, they are almost
always necessary when fuzzing a program using network protocols, as
described below.

Case is irrelevant in the network specification.  For programs that
use a stream (connection-based) protocol, use TCP, and for programs
that use a datagram (connectionless) protocol, use UDP.  The hostspec
must be one of ::1 (forcing IPv6 networking), (forcing IPv4
networking), and localhost (which is typically configured as IPv4 but
may suppport IPv6 on some systems).  Only loopback networking (local
to the host) is supported.  The port must be either a port number in
the range 1..65535 or a service name known to the system being used.
You can test programs that use privileged ports, but you then have to
provide afl-fuzz with those additional privileges (e.g., root).  It is
usually better to reconfigure the program being tested so that it will
use a non-privileged port during fuzzing.

Programs that implement network services, also called daemons, are
typically transaction-based: They wait for a request and send a
response, and some expect a sequence of request/response transactions.
Afl-fuzz implements fuzzing only for the first write to the target
program and ignores all responses from the target.  Most network
services expect to run as background processes and process requests
from many processes -- they do not normally exit.  A timeout delay is
required in order to terminate these processes, and the default
timeout delay used in afl-fuzz is usually too long.  The user needs to
experimentally determine a timeout delay (in milliseconds) that
produces a sufficiently low percentage of hangs (exits forced by
expiration of the delay) while allowing the input to the target from
afl-fuzz to be completely processed.  (Note that afl-fuzz will usually
count these hangs as a single unique hang.) Since a network service
does not normally exit, the initial timing performed by afl-fuzz will
fail unless a '+' character is appended to the timeout_delay
parameter, indicating that afl-fuzz is to ignore these timeouts.

Network services programs also require some time to perform start-up
processing, create and bind a socket to an address and port, and begin
listening for traffic on that socket.  Connection requests (TCP) and
sends (UDP) generated by afl-fuzz will fail if made before the network
service is ready.  Afl-fuzz implements a delay and retry procedure to
avoid this problem, where the delay is specified by the
delay_before_write parameter (in milliseconds).  The first connection
attempt (for TCP) or write (for UDP) is not made until after this
delay, and the delay also specifies the wait time before each
subsequent attempt.  Afl-fuzz will attempt to connect or send to the
same each target process a maximum of three times.

The delay_before_write parameter, in particular, and to a lesser
extent the timeout_delay parameter limit the maximum achievable rate
of target program executions and therefore need to be small. A rule of
thumb is the timeout_delay value should be slightly longer than three
times the delay_before_write value, and the delay_before_write value
should be as small as possible while consistent with an acceptable
fraction of target process executions that time out (for example,
around 0.1%).

Network client program have similar characteristics that require the use of
the delay parameter, but they write to their expected server (afl-fuzz in this
case) before reading from their network socket.  This makes coordination between
afl-fuzz and (client) target somewhat more challenging.  While the delay and
timeout parameters can usually be adjusted to obtain execution rates similar
to those for server programs, when afl-fuzz exits (due to a ^C interrupt) it
may hang.  In this case, use (on UNIX or Linux) the ps command to find the
process id (PID) of the afl-fuzz process, and use the kill command to terminate
it (typically, "kill -9 PID").  This will also terminate (or scavenge) the
network client program's process, which may be in a zombie state that can not
otherwise be removed (without rebooting the system).

A note concerning network fuzzing on multi-core systems:

It is not possible to run two processes under a single operating
system kernel that bind to (listen to) the same port on the same
address. Thus, either a special wrapper (such as could be implemented
using LD_PRELOAD) can be used to remap each target's port to a
different value, or only one target process can be executed per kernel
(not per core). Parallel fuzzing of network services can be done using
several independent hosts (a cluster), or by reconfiguring the code
running on each core to use a different port.

13) Known limitations & areas for improvement

Here are some of the most important caveats for AFL:

  - AFL detects faults by checking for the first spawned process dying due to
    a signal (SIGSEGV, SIGABRT, etc). Programs that install custom handlers for
    these signals may need to have the relevant code commented out. In the same
    vein, faults in child processed spawned by the fuzzed target may evade
    detection unless you manually add some code to catch that.

  - As with any other brute-force tool, the fuzzer offers limited coverage if
    encryption, checksums, cryptographic signatures, or compression are used to
    wholly wrap the actual data format to be tested.

    To work around this, you can comment out the relevant checks (see
    experimental/libpng_no_checksum/ for inspiration); if this is not possible,
    you can also write a postprocessor, as explained in

  - There are some unfortunate trade-offs with ASAN and 64-bit binaries. This
    isn't due to any specific fault of afl-fuzz; see notes_for_asan.txt for

  - The supplied method for fuzzing network services or background daemons
    is limited to fuzzing the first packet received by the service or daemon.
    Since many of these processes are transactional, it may be desirable to
    provide a deterministic sequence of transactions with the target,
    followed by fuzzing. This is not supported and would be a nice topic
    for future work. Interactive apps that require UI interaction to work
    are also not supported. You may need to make simple code changes to
    make them behave in a more traditional way. Preeny may offer a relatively
    simple option, too - see:

    Some useful tips for modifying network-based services can be also found at:

  - AFL doesn't output human-readable coverage data. If you want to monitor
    coverage, use afl-cov from Michael Rash:

  - Only afl-fuzz implements network fuzzing capabilities at this time.  In
    particular, afl-cmin (a script), afl-tmin, and afl-showmap, all of which
    execute the target program, do not.

Beyond this, see INSTALL for platform-specific tips.

14) Special thanks

Many of the improvements to afl-fuzz wouldn't be possible without feedback,
bug reports, or patches from:

  Jann Horn                             Hanno Boeck
  Felix Groebert                        Jakub Wilk
  Richard W. M. Jones                   Alexander Cherepanov
  Tom Ritter                            Hovik Manucharyan
  Sebastian Roschke                     Eberhard Mattes
  Padraig Brady                         Ben Laurie
  @dronesec                             Luca Barbato
  Tobias Ospelt                         Thomas Jarosch
  Martin Carpenter                      Mudge Zatko
  Joe Zbiciak                           Ryan Govostes
  Michael Rash                          William Robinet
  Jonathan Gray                         Filipe Cabecinhas
  Nico Weber                            Jodie Cunningham
  Andrew Griffiths                      Parker Thompson
  Jonathan Neuschfer                    Tyler Nighswander
  Ben Nagy                              Samir Aguiar
  Aidan Thornton                        Aleksandar Nikolich
  Sam Hakim                             Laszlo Szekeres
  David A. Wheeler                      Turo Lamminen
  Andreas Stieger                       Richard Godbee
  Louis Dassy                           teor2345
  Alex Moneger                          Dmitry Vyukov
  Keegan McAllister                     Kostya Serebryany
  Richo Healey                          Martijn Bogaard
  rc0r                                  Jonathan Foote
  Christian Holler                      Dominique Pelle
  Jacek Wielemborek                     Leo Barnes
  Jeremy Barnes                         Doug Birdwell

Thank you!

15) Contact

Questions? Concerns? Bug reports? The author can be usually reached at

There is also a mailing list for the project; to join, send a mail to
<>. Or, if you prefer to browse
archives first, try:

PS. If you wish to submit raw code to be incorporated into the project, please
be aware that the copyright on most of AFL is claimed by Google. While you do
retain copyright on your contributions, they do ask people to agree to a simple
CLA first:

Sorry about the hassle. Of course, no CLA is required for feature requests or
bug reports.


american fuzzy lop for network fuzzing (unofficial) -- official afl site is



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