@ -1,56 +1,56 @@
# american fuzzy lop
[](https://travis-ci.org/google/AFL)
Originally developed by Michal Zalewski < lcamtuf @ google . com > .
See [QuickStartGuide.txt ](docs/QuickStartGuide.txt ) if you don't have time to read
this file.
[](https://travis-ci.org/google/AFL) # code
Originally developed by Michal Zalewski < lcamtuf @ google . com > . # code
``````
See [QuickStartGuide.txt ](docs/QuickStartGuide.txt ) if you don't have time to read # code
this file. ``````
# code
## 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
majority of remote code execution and privilege escalation bugs found to date # code
in security-critical software.
Unfortunately, fuzzing is also relatively shallow; blind, random mutations
Unfortunately, fuzzing is also relatively shallow; blind, random mutations # code
make it very unlikely to reach certain code paths in the tested code, leaving
some vulnerabilities firmly outside the reach of this technique.
# code
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
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.
useful for guiding the fuzzing effort in the long haul. # code
# code
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
## 2) The afl-fuzz approach # code
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.
program control flow. ``````
# code
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
3) Attempt to trim the test case to the smallest size that doesn't alter # code
the measured behavior of the program,
4) Repeatedly mutate the file using a balanced and well-researched variety
of traditional fuzzing strategies,
of traditional fuzzing strategies, # code
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
@ -58,9 +58,9 @@ Simplifying a bit, the overall algorithm can be summed up as:
6) Go to 2.
The discovered test cases are also periodically culled to eliminate ones that
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.
instrumentation-driven effort minimization steps. # code
As a side result of the fuzzing process, the tool creates a small,
self-contained corpus of interesting test cases. These are extremely useful
@ -69,10 +69,10 @@ 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.
superior to blind fuzzing or coverage-only tools. # code
## 3) Instrumenting programs for use with AFL
# code
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.
@ -89,28 +89,28 @@ $ 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++` .
For C++ programs, you'd would also want to set `CXX=/path/to/afl/afl-g++` . # code
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,
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
data from stdin or from a file and passes it to the tested library. In such a # code
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
runtime (usually by setting `LD_LIBRARY_PATH` ). The simplest option is a static # code
build, usually possible via:
# code
```shell
$ 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. Libdislocator, a helper library included with AFL (see
libdislocator/README.dislocator) can help uncover heap corruption issues, too.
simple memory bugs. Libdislocator, a helper library included with AFL (see # code
libdislocator/README.dislocator) can help uncover heap corruption issues, too. # code
# code
PS. ASAN users are advised to review [notes_for_asan.txt ](docs/notes_for_asan.txt ) file for important
caveats.
@ -118,36 +118,36 @@ caveats.
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.
with a version of QEMU running in the lesser-known "user space emulation" mode. # code
QEMU is a project separate from AFL, but you can conveniently build the
feature by doing:
feature by doing: ``````
# code
```shell
$ cd qemu_mode
$ ./build_qemu_support.sh
```
# code
For additional instructions and caveats, see qemu_mode/README.qemu.
The mode is approximately 2-5x slower than compile-time instrumentation, is
less conducive to parallelization, and may have some other quirks.
less conducive to parallelization, and may have some other quirks. # code
## 5) Choosing initial test cases
## 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
contains a good example of the input data normally expected by the targeted # code
application. There are two basic rules:
- Keep the files small. Under 1 kB is ideal, although not strictly necessary.
- Keep the files small. Under 1 kB is ideal, although not strictly necessary. # code
For a discussion of why size matters, see [perf_tips.txt ](docs/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.
You can find many good examples of starting files in the testcases/ subdirectory # code
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
@ -155,82 +155,82 @@ 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:
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 # code
store its findings, plus a path to the binary to test. # code
For target binaries that accept input directly from stdin, the usual syntax is: ``````
``````
```shell
$ ./afl-fuzz -i testcase_dir -o findings_dir /path/to/program [...params...]
```
``` ``````
# code
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:
```shell
```shell # code
$ ./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.
You can also use the -f option to have the mutated data written to a specific # code
file. This is useful if the program expects a particular file extension or so. ``````
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.
include compilers and video decoders. # code
Tips for optimizing fuzzing performance are discussed in [perf_tips.txt ](docs/perf_tips.txt ).
Note that afl-fuzz starts by performing an array of deterministic fuzzing
Note that afl-fuzz starts by performing an array of deterministic fuzzing ``````
steps, which can take several days, but tend to produce neat test cases. If you
want quick & dirty results right away - akin to zzuf and other traditional
fuzzers - add the -d option to the command line.
fuzzers - add the -d option to the command line. # code
``````
## 7) Interpreting output
See the [status_screen.txt ](docs/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.
# code
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
# code
There are three subdirectories created within the output directory and updated # code
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
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
Before using this corpus for any other purposes, you can shrink # code
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
fatal signal (e.g., SIGSEGV, SIGILL, SIGABRT). The entries are # code
grouped by the received signal.
- hangs/ - unique test cases that cause the tested program to time out. The
default time limit before something is classified as a hang is
the larger of 1 second and the value of the -t parameter.
the larger of 1 second and the value of the -t parameter. # code
The value can be fine-tuned by setting AFL_HANG_TMOUT, but this
is rarely necessary.
is rarely necessary. # code
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.
early in the process, but this should quickly taper off. # code
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:
When you can't reproduce a crash found by afl-fuzz, the most likely cause is # code
that you are not setting the same memory limit as used by the tool. Try: # code
```shell
$ LIMIT_MB=50
@ -243,28 +243,28 @@ also change -Sv to -Sd.
Any existing output directory can be also used to resume aborted jobs; try:
```shell
$ ./afl-fuzz -i- -o existing_output_dir [...etc...]
```
$ ./afl-fuzz -i- -o existing_output_dir [...etc...] # code
``` # code
If you have gnuplot installed, you can also generate some pretty graphs for any
If you have gnuplot installed, you can also generate some pretty graphs for any # code
active fuzzing task using afl-plot. For an example of how this looks like,
see [http://lcamtuf.coredump.cx/afl/plot/ ](http://lcamtuf.coredump.cx/afl/plot/ ).
## 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.
multi-core systems, parallelization is necessary to fully utilize the hardware. # code
For tips on how to fuzz a common target on multiple cores or multiple networked
machines, please refer to [parallel_fuzzing.txt ](docs/parallel_fuzzing.txt ).
The parallel fuzzing mode also offers a simple way for interfacing AFL to other
fuzzers, to symbolic or concolic execution engines, and so forth; again, see the
fuzzers, to symbolic or concolic execution engines, and so forth; again, see the # code
last section of [parallel_fuzzing.txt ](docs/parallel_fuzzing.txt ) for tips.
## 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
say, images, multimedia, compressed data, regular expression syntax, or shell # code
scripts. It is somewhat less suited for languages with particularly verbose and
redundant verbiage - notably including HTML, SQL, or JavaScript.
@ -277,48 +277,48 @@ magic headers, or other special tokens associated with the targeted data type
To use this feature, you first need to create a dictionary in one of the two
formats discussed in dictionaries/README.dictionaries; and then point the fuzzer
to it via the -x option in the command line.
to it via the -x option in the command line. # code
``````
(Several common dictionaries are already provided in that subdirectory, too.)
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:
[http://lcamtuf.blogspot.com/2015/04/finding-bugs-in-sqlite-easy-way.html ](http://lcamtuf.blogspot.com/2015/04/finding-bugs-in-sqlite-easy-way.html )
PS. Even when no explicit dictionary is given, afl-fuzz will try to extract
[http://lcamtuf.blogspot.com/2015/04/finding-bugs-in-sqlite-easy-way.html ](http://lcamtuf.blogspot.com/2015/04/finding-bugs-in-sqlite-easy-way.html ) ``````
``````
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.
If a dictionary is really hard to come by, another option is to let AFL run
for a while, and then use the token capture library that comes as a companion
utility with AFL. For that, see libtokencap/README.tokencap.
utility with AFL. For that, see libtokencap/README.tokencap. # code
# code
## 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.
queue, making it easier to diagnose faults. # code
Having said that, it's important to acknowledge that some fuzzing crashes can be
difficult to quickly evaluate for exploitability without a lot of debugging and
Having said that, it's important to acknowledge that some fuzzing crashes can be # code
difficult to quickly evaluate for exploitability without a lot of debugging and # code
code analysis work. To assist with this task, afl-fuzz supports a very unique
"crash exploration" mode enabled with the -C flag.
"crash exploration" mode enabled with the -C flag. # code
# code
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.
crashing state. ``````
Mutations that do not result in a crash are rejected; so are any changes that
do not affect the execution path.
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
The output is a small corpus of files that can be very rapidly examined to see # code
what degree of control the attacker has over the faulting address, or whether # code
it is possible to get past an initial out-of-bounds read - and see what lies # code
beneath.
Oh, one more thing: for test case minimization, give afl-tmin a try. The tool
@ -329,19 +329,19 @@ $ ./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
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 file simpler without altering the execution path. # code
The minimizer accepts the -m, -t, -f and @@ syntax in a manner compatible with
afl-fuzz.
# code
Another recent addition to AFL is the afl-analyze tool. It takes an input
file, attempts to sequentially flip bytes, and observes the behavior of the
file, attempts to sequentially flip bytes, and observes the behavior of the # code
tested program. It then color-codes the input based on which sections appear to
be critical, and which are not; while not bulletproof, it can often offer quick
insights into complex file formats. More info about its operation can be found
near the end of [technical_details.txt ](docs/technical_details.txt ).
near the end of [technical_details.txt ](docs/technical_details.txt ). # code
## 11) Going beyond crashes
@ -352,21 +352,21 @@ found by modifying the target programs to call abort() when, say:
- Two bignum libraries produce different outputs when given the same
fuzzer-generated input,
- An image library produces different outputs when asked to decode the same
- An image library produces different outputs when asked to decode the same # code
input image several times in a row,
- A serialization / deserialization library fails to produce stable outputs
- A serialization / deserialization library fails to produce stable outputs ``````
when iteratively serializing and deserializing fuzzer-supplied data,
- A compression library produces an output inconsistent with the input file
when asked to compress and then decompress a particular blob.
Implementing these or similar sanity checks usually takes very little time;
if you are the maintainer of a particular package, you can make this code
if you are the maintainer of a particular package, you can make this code # code
conditional with `#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION` (a flag also
shared with libfuzzer) or `#ifdef __AFL_COMPILER` (this one is just for AFL).
## 12) Common-sense risks
## 12) 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:
@ -379,19 +379,19 @@ tasks, fuzzing may put strain on your hardware and on the OS. In particular:
- 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.
limits, but can't prevent each and every possible mishap. The bottom line # code
is that you shouldn't be fuzzing on systems where the prospect of data loss # code
is not an acceptable risk. # code
- 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.
systems, this will be usually heavily cached, resulting in fairly modest # code
"physical" I/O - but there are many factors that may alter this equation. # code
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:
A good way to monitor disk I/O on Linux is the 'iostat' command: # code
```shell
```shell # code
$ iostat -d 3 -x -k [...optional disk ID...]
```
@ -403,34 +403,34 @@ Here are some of the most important caveats for AFL:
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.
detection unless you manually add some code to catch that. # code
- 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
To work around this, you can comment out the relevant checks (see # code
experimental/libpng_no_checksum/ for inspiration); if this is not possible,
you can also write a postprocessor, as explained in
you can also write a postprocessor, as explained in # code
experimental/post_library/.
- 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 ](docs/notes_for_asan.txt )
for tips.
for tips. # code
- There is no direct support for fuzzing network services, background
- There is no direct support for fuzzing network services, background ``````
daemons, or interactive apps that require UI interaction to work. 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:
way. Preeny may offer a relatively simple option, too - see: ``````
https://github.com/zardus/preeny
Some useful tips for modifying network-based services can be also found at:
Some useful tips for modifying network-based services can be also found at: # code
https://www.fastly.com/blog/how-to-fuzz-server-american-fuzzy-lop
- AFL doesn't output human-readable coverage data. If you want to monitor
coverage, use afl-cov from Michael Rash: https://github.com/mrash/afl-cov
- Occasionally, sentient machines rise against their creators. If this
# code
- Occasionally, sentient machines rise against their creators. If this ``````
happens to you, please consult http://lcamtuf.coredump.cx/prep/.
Beyond this, see INSTALL for platform-specific tips.
@ -438,56 +438,56 @@ 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:
bug reports, or patches from: # code
```
Jann Horn Hanno Boeck
Felix Groebert Jakub Wilk
Felix Groebert Jakub Wilk # code
Richard W. M. Jones Alexander Cherepanov
Tom Ritter Hovik Manucharyan
Sebastian Roschke Eberhard Mattes
Padraig Brady Ben Laurie
@dronesec Luca Barbato
Sebastian Roschke Eberhard Mattes ``````
Padraig Brady Ben Laurie # code
@dronesec Luca Barbato # code
Tobias Ospelt Thomas Jarosch
Martin Carpenter Mudge Zatko
Joe Zbiciak Ryan Govostes
Michael Rash William Robinet
Michael Rash William Robinet # code
Jonathan Gray Filipe Cabecinhas
Nico Weber Jodie Cunningham
Andrew Griffiths Parker Thompson
Jonathan Neuschfer Tyler Nighswander
Ben Nagy Samir Aguiar
Aidan Thornton Aleksandar Nikolich
Ben Nagy Samir Aguiar ``````
Aidan Thornton Aleksandar Nikolich # code
Sam Hakim Laszlo Szekeres
David A. Wheeler Turo Lamminen
Andreas Stieger Richard Godbee
Louis Dassy teor2345
Louis Dassy teor2345 ``````
Alex Moneger Dmitry Vyukov
Keegan McAllister Kostya Serebryany
Keegan McAllister Kostya Serebryany # code
Richo Healey Martijn Bogaard
rc0r Jonathan Foote
Christian Holler Dominique Pelle
Jacek Wielemborek Leo Barnes
Jacek Wielemborek Leo Barnes # code
Jeremy Barnes Jeff Trull
Guillaume Endignoux ilovezfs
Daniel Godas-Lopez Franjo Ivancic
Austin Seipp Daniel Komaromy
Daniel Godas-Lopez Franjo Ivancic # code
Austin Seipp Daniel Komaromy # code
Daniel Binderman Jonathan Metzman
Vegard Nossum Jan Kneschke
Kurt Roeckx Marcel Bohme
Van-Thuan Pham Abhik Roychoudhury
Joshua J. Drake Toby Hutton
Rene Freingruber Sergey Davidoff
Joshua J. Drake Toby Hutton ``````
Rene Freingruber Sergey Davidoff ``````
Sami Liedes Craig Young
Andrzej Jackowski Daniel Hodson
```
Thank you!
Thank you! ``````
## 15) Contact
Questions? Concerns? Bug reports? Please use GitHub.
# code
Questions? Concerns? Bug reports? Please use GitHub. ``````
There is also a mailing list for the project; to join, send a mail to
< afl-users + subscribe @ googlegroups . com > . Or, if you prefer to browse
< afl-users + subscribe @ googlegroups . com > . Or, if you prefer to browse ``````
archives first, try: [https://groups.google.com/group/afl-users ](https://groups.google.com/group/afl-users ).
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