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diff --git a/README_new.md b/README_new.md new file mode 100644 index 00000000..7e817fd3 --- /dev/null +++ b/README_new.md @@ -0,0 +1,1054 @@ +# american fuzzy lop plus plus (afl++) + + <img align="right" src="https://raw.githubusercontent.com/andreafioraldi/AFLplusplus-website/master/static/logo_256x256.png" alt="AFL++ Logo"> + +  + + Release Version: [2.66c](https://github.com/AFLplusplus/AFLplusplus/releases) + + Github Version: 2.66d + + Repository: [https://github.com/AFLplusplus/AFLplusplus](https://github.com/AFLplusplus/AFLplusplus) + + afl++ is maintained by: + * Marc "van Hauser" Heuse <mh@mh-sec.de>, + * Heiko "hexcoder-" Eißfeldt <heiko.eissfeldt@hexco.de>, + * Andrea Fioraldi <andreafioraldi@gmail.com> and + * Dominik Maier <mail@dmnk.co>. + + Originally developed by Michal "lcamtuf" Zalewski. + + Note that although afl now has a Google afl repository [https://github.com/Google/afl](https://github.com/Google/afl), + it is unlikely to receive any notable enhancements: [https://twitter.com/Dor3s/status/1154737061787660288](https://twitter.com/Dor3s/status/1154737061787660288) + includes all necessary/interesting changes from Google's afl 2.57b + +## The enhancements compared to the original stock afl + + Among other changes afl++ has a more performant llvm_mode, supports + llvm up to version 12, QEMU 3.1 with more speed and features, + better *BSD, Solaris and Android support and much, much, much more. + + | Feature/Instrumentation | afl-gcc | llvm_mode | gcc_plugin | qemu_mode | unicorn_mode | + | ----------------------- |:-------:|:---------:|:----------:|:----------------:|:------------:| + | NeverZero | x | x(1) | (2) | x | x | + | Persistent mode | | x | x | x86[_64]/arm[64] | x | + | LAF-Intel / CompCov | | x | | x86[_64]/arm[64] | x86[_64]/arm | + | CmpLog | | x | | x86[_64]/arm[64] | | + | Instrument file list | | x | x | (x)(3) | | + | Non-colliding coverage | | x(4) | | (x)(5) | | + | InsTrim | | x | | | | + | Ngram prev_loc coverage | | x(6) | | | | + | Context coverage | | x | | | | + | Auto dictionary | | x(7) | | | | + | Snapshot LKM support | | x | | (x)(5) | | + + neverZero: + + (1) default for LLVM >= 9.0, env var for older version due an efficiency bug in llvm <= 8 + + (2) GCC creates non-performant code, hence it is disabled in gcc_plugin + + (3) partially via AFL_CODE_START/AFL_CODE_END + + (4) with pcguard mode and LTO mode for LLVM >= 11 + + (5) upcoming, development in the branch + + (6) not compatible with LTO instrumentation and needs at least LLVM >= 4.1 + + (7) only in LTO mode with LLVM >= 11 + + Among others, the following features and patches have been integrated: + + * NeverZero patch for afl-gcc, llvm_mode, qemu_mode and unicorn_mode which prevents a wrapping map value to zero, increases coverage + + * Persistent mode and deferred forkserver for qemu_mode + + * Unicorn mode which allows fuzzing of binaries from completely different platforms (integration provided by domenukk) + + * The new CmpLog instrumentation for LLVM and QEMU inspired by [Redqueen](https://www.syssec.ruhr-uni-bochum.de/media/emma/veroeffentlichungen/2018/12/17/NDSS19-Redqueen.pdf) + + * Win32 PE binary-only fuzzing with QEMU and Wine + + * AFLfast's power schedules by Marcel Böhme: [https://github.com/mboehme/aflfast](https://github.com/mboehme/aflfast) + + * The MOpt mutator: [https://github.com/puppet-meteor/MOpt-AFL](https://github.com/puppet-meteor/MOpt-AFL) + + * LLVM mode Ngram coverage by Adrian Herrera [https://github.com/adrianherrera/afl-ngram-pass](https://github.com/adrianherrera/afl-ngram-pass) + + * InsTrim, an effective CFG llvm_mode instrumentation implementation for large targets: [https://github.com/csienslab/instrim](https://github.com/csienslab/instrim) + + * C. Holler's afl-fuzz Python mutator module and llvm_mode instrument file support: [https://github.com/choller/afl](https://github.com/choller/afl) + + * Custom mutator by a library (instead of Python) by kyakdan + + * LAF-Intel/CompCov support for llvm_mode, qemu_mode and unicorn_mode (with enhanced capabilities) + + * Radamsa and hongfuzz mutators (as custom mutators). + + * QBDI mode to fuzz android native libraries via QBDI framework + + A more thorough list is available in the [PATCHES](docs/PATCHES.md) file. + + So all in all this is the best-of afl that is currently out there :-) + + For new versions and additional information, check out: + [https://github.com/AFLplusplus/AFLplusplus](https://github.com/AFLplusplus/AFLplusplus) + + To compare notes with other users or get notified about major new features, + send a mail to <afl-users+subscribe@googlegroups.com>. + + See [docs/QuickStartGuide.md](docs/QuickStartGuide.md) if you don't have time to + read this file. + +## Branches + + The following branches exist: + + * [master/trunk](https://github.com/AFLplusplus/AFLplusplus/) : stable state of afl++ - it is synced from dev from time to + time when we are satisfied with it's stability + * [dev](https://github.com/AFLplusplus/AFLplusplus/tree/dev) : development state of afl++ - bleeding edge and you might catch a + checkout which does not compile or has a bug. *We only accept PRs in dev!!* + * (any other) : experimental branches to work on specific features or testing + new functionality or changes. + + For releases, please see the [Releases](https://github.com/AFLplusplus/AFLplusplus/releases) tab. + +## Google Summer of Code 2020 (and any other students and enthusiast developers) + +We are happy to be part of [Google Summer of Code 2020](https://summerofcode.withgoogle.com/organizations/5100744400699392/)! :-) + +We have several ideas we would like to see in AFL++ to make it even better. +However, we already work on so many things that we do not have the time for +all the big ideas. + +This can be your way to support and contribute to AFL++ - extend it to +something cool. + +We have an idea list in [docs/ideas.md](docs/ideas.md). + +For everyone who wants to contribute (and send pull requests) please read +[CONTRIBUTING.md](CONTRIBUTING.md) before your submit. + +## Building and installing afl++ + +An easy way to install afl++ with everything compiled is available via docker: +You can use the [Dockerfile](Dockerfile) (which has gcc-10 and clang-11 - +hence afl-clang-lto is available!) or just pull directly from the docker hub: +```shell +docker pull aflplusplus/aflplusplus +docker run -ti -v /location/of/your/target:/src aflplusplus/aflplusplus +``` +This image is automatically generated when a push to master happens. +You will find your target source code in /src in the container. + +If you want to build afl++ yourself you have many options. +The easiest is to build and install everything: + +```shell +sudo apt install build-essential libtool-bin python3-dev automake flex bison libglib2.0-dev libpixman-1-dev clang python3-setuptools llvm +make distrib +sudo make install +``` +It is recommended to install the newest available gcc, clang and llvm-dev +possible in your distribution! + +Note that "make distrib" also builds llvm_mode, qemu_mode, unicorn_mode and +more. If you just want plain afl then do "make all", however compiling and +using at least llvm_mode is highly recommended for much better results - +hence in this case + +```shell +make source-only +``` +is what you should choose. + +These build targets exist: + +* all: just the main afl++ binaries +* binary-only: everything for binary-only fuzzing: qemu_mode, unicorn_mode, libdislocator, libtokencap +* source-only: everything for source code fuzzing: llvm_mode, libdislocator, libtokencap +* distrib: everything (for both binary-only and source code fuzzing) +* man: creates simple man pages from the help option of the programs +* install: installs everything you have compiled with the build options above +* clean: cleans everything compiled, not downloads (unless not on a checkout) +* deepclean: cleans everything including downloads +* code-format: format the code, do this before you commit and send a PR please! +* tests: runs test cases to ensure that all features are still working as they should +* unit: perform unit tests (based on cmocka) +* help: shows these build options + +[Unless you are on Mac OS X](https://developer.apple.com/library/archive/qa/qa1118/_index.html) you can also build statically linked versions of the +afl++ binaries by passing the STATIC=1 argument to make: + +```shell +make all STATIC=1 +``` + +These build options exist: + +* STATIC - compile AFL++ static +* ASAN_BUILD - compiles with memory sanitizer for debug purposes +* PROFILING - compile with profiling information (gprof) +* NO_PYTHON - disable python support +* AFL_NO_X86 - if compiling on non-intel/amd platforms +* LLVM_CONFIG - if your distro doesn't use the standard name for llvm-config (e.g. Debian) + +e.g.: make ASAN_BUILD=1 + +## Good examples and writeups + +Here are some good writeups to show how to effectively use AFL++: + + * [https://aflplus.plus/docs/tutorials/libxml2_tutorial/](https://aflplus.plus/docs/tutorials/libxml2_tutorial/) + * [https://bananamafia.dev/post/gb-fuzz/](https://bananamafia.dev/post/gb-fuzz/) + * [https://securitylab.github.com/research/fuzzing-challenges-solutions-1](https://securitylab.github.com/research/fuzzing-challenges-solutions-1) + * [https://securitylab.github.com/research/fuzzing-sockets-FTP](https://securitylab.github.com/research/fuzzing-sockets-FTP) + +If you are interested in fuzzing structured data (where you define what the +structure is), these links have you covered: + * Superion for afl++: [https://github.com/adrian-rt/superion-mutator](https://github.com/adrian-rt/superion-mutator) + * libprotobuf raw: [https://github.com/bruce30262/libprotobuf-mutator_fuzzing_learning/tree/master/4_libprotobuf_aflpp_custom_mutator](https://github.com/bruce30262/libprotobuf-mutator_fuzzing_learning/tree/master/4_libprotobuf_aflpp_custom_mutator) + * libprotobuf for old afl++ API: [https://github.com/thebabush/afl-libprotobuf-mutator](https://github.com/thebabush/afl-libprotobuf-mutator) + +If you find other good ones, please send them to us :-) + +## How to fuzz with afl++ + +The following describes how to fuzz with a target if source code is available. +If you have a binary-only target please skip to [#Instrumenting binary-only apps](#Instrumenting binary-only apps) + +Fuzzing source code is a two step process. + +1. compile the target with a special compiler that prepares the target to be + fuzzed efficiently. This step is called "instrumenting a target". +2. Prepare the fuzzing by selecting and optimizing the input corpus for the + target. +3. perform the fuzzing of the target by randomly mutating input and assessing + if a generated input was processed in a new path in the target binary + +### 1. Instrumenting that target + +#### a) Selecting the best afl++ compiler for instrumenting the target + +afl++ comes with different compilers and instrumentation options. +The following evaluation flow will help you to select the best possible. + +It is highly recommended to have the newest llvm version possible installed, +anything below 9 is not recommended. + +``` ++--------------------------------+ +| clang/clang++ 11+ is available | --> use afl-clang-lto and afl-clang-lto++ ++--------------------------------+ see [llvm/README.lto.md](llvm/README.lto.md) + | + | if not, or if the target fails with with afl-clang-lto/++ + | + v ++---------------------------------+ +| clang/clang++ 3.3+ is available | --> use afl-clang-fast and afl-clang-fast++ ++---------------------------------+ see [llvm/README.md](llvm/README.md) + | + | if not, or if the target fails with afl-clang-fast/++ + | + v + +--------------------------------+ + | if you want to instrument only | -> use afl-gcc-fast and afl-gcc-fast++ + | parts of the target | see [gcc_plugin/README.md](gcc_plugin/README.md) and + +--------------------------------+ [gcc_plugin/README.instrument_file.md](gcc_plugin/README.instrument_file.md) + | + | if not, or if you do not have a gcc with plugin support + | + v + use afl-gcc and afl-g++ +``` + +#### b) Selecting instrumentation options + +The following options are available when you instrument with afl-clang-fast or +afl-clang-lto: + + * Splitting integer, string, float and switch compares so afl++ can easier + solve these. This is an important option if you do not have a very good + good and large input corpus. This technique is called laf-intel or COMPCOV. + To use this set the following environment variable before compiling the + target: `export AFL_LLVM_LAF_ALL=1` + You can read more about this in [llvm/README.laf-intel.md](llvm/README.laf-intel.md) + * A different technique is to instrument the target so that any compare values + in the target are sent to afl++ which then tries to put this value into the + fuzzing data at different locations. This technique is very fast and good - + if the target does not transform input data before comparison. Therefore + technique is called `input to state` or `redqueen`. + If you want to use this technique, then you have to compile the target + twice, once specifically with/for this mode. + You can read more about this in [llvm_mode/README.cmplog.md](llvm_mode/README.cmplog.md) + +If you use afl-clang-fast, afl-clang-lto or afl-gcc-fast you have the option to +selectivly only instrument parts of the target that you are interested in: + + * To instrument only those parts of the target that you are interested in + create a file with all the filenames of the source code that should be + instrumented. + For afl-clang-lto and afl-gcc-fast - or afl-clang-fast if either the clang + version is < 7 or the CLASSIC instrumentation is used - just put one + filename per line, no directory information necessary, and set + `export AFL_LLVM_INSTRUMENT_FILE=yourfile.txt` + see [llvm_mode/README.instrument_file.md](llvm_mode/README.instrument_file.md) + For afl-clang-fast > 6.0 or if PCGUARD instrumentation is used then use the + llvm sancov allow-list feature: [http://clang.llvm.org/docs/SanitizerCoverage.html](http://clang.llvm.org/docs/SanitizerCoverage.html) + +There are many more options and modes available however these are most of the +time less effective. See: + * [llvm_mode/README.ctx.md](llvm_mode/README.ctx.md) + * [llvm_mode/README.ngram.md](llvm_mode/README.ngram.md) + * [llvm_mode/README.instrim.md](llvm_mode/README.instrim.md) + * [llvm_mode/README.neverzero.md](llvm_mode/README.neverzero.md) + +#### c) Modify the target + +If the target has features that makes fuzzing more difficult, e.g. +checksums, HMAC etc. then modify the source code so that this is +removed. +This can even be done for productional source code be eliminating +these checks within this specific defines: + +``` +#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION + // say that the checksum or HMAC was fine - or whatever is required + // to eliminate the need for the fuzzer to guess the right checksum + return 0; +#endif +``` + +#### d) Instrument the target + +In this step the target source code is compiled so that it can be fuzzed. + +Basically you have to tell the target build system that the selected afl++ +compiler is used. Also - if possible - you should always configure the +build system that the target is compiled statically and not dynamically. +How to do this is described below. + +Then build the target. (Usually with `make`) + +##### configure + +For `configure` build systems this is usually done by: +`CC=afl-clang-fast CXX=afl-clang-fast++ ./configure --disable-shared` + +Note that if you using the (better) afl-clang-lto compiler you also have to +AR to llvm-ar[-VERSION] and RANLIB to llvm-ranlib[-VERSION] - as it is +described in [llvm/README.lto.md](llvm/README.lto.md) + +##### cmake + +For `configure` build systems this is usually done by: +`mkdir build; cd build; CC=afl-clang-fast CXX=afl-clang-fast++ cmake ..` + +Note that if you using the (better) afl-clang-lto compiler you also have to +AR to llvm-ar[-VERSION] and RANLIB to llvm-ranlib[-VERSION] - as it is +described in [llvm/README.lto.md](llvm/README.lto.md) + +##### other build systems or if configure/cmake didn't work + +Sometimes cmake and configure do not pick up the afl compiler, or the ranlib/ar +that is needed - because this was just not foreseen by the developer of the +target. Or they have non-standard options. Figure out if there is a +non-standard way to set this, otherwise set the build normally and edit the +generated build environment afterwards by hand to point to the right compiler +(and/or ranlib and ar). + +#### d) Better instrumentation + +If you just fuzz a target program as-is you are wasting a great opportunity for +much more fuzzing speed. + +This requires the usage of afl-clang-lto or afl-clang-fast + +This is the so-called `persistent mode`, which is much, much faster but +requires that you code a source file that is specifically calling the target +functions that you want to fuzz, plus a few specific afl++ functions around +it. See [llvm_mode/README.persistent_mode.md](llvm_mode/README.persistent_mode.md) for details. + +Basically if you do not fuzz a target in persistent mode then you are just +doing it for a hobby and not professionally :-) + +### 2. Preparing the fuzzing + +As you fuzz the target with mutated input, having as diverse inputs for the +target as possible improves the efficiency a lot. + +#### a) Collect inputs +Try to gather valid inputs for the target from wherever you can. E.g. if it +the PNG picture format try to find as many png files as possible, e.g. from +reported bugs, test suites, random downloads from the internet, unit test +case data - from all kind of PNG software. + +If the input is not known files, you can also modify a target program to write +away normal data it receives and processes to a file and use these. + +#### b) Making the input corpus unique + +Use the afl++ tool `afl-cmin` to remove inputs from the corpus that do not +use a different paths in the target. +Put all files from step a) into one directory, e.g. INPUTS. + +Put all the files from step a) + +If the target program is to be called by fuzzing as `bin/target -d INPUTFILE` +the run afl-cmin like this: +`afl-cmin -i INPUTS -o INPUTS_UNIQUE -- bin/target -d @@` +Note that the INPUTFILE that the target program would read has to be set as `@@`. + +If the target reads from stdin instead, just omit the `@@` as this is the +default. + +#### b) Minimizing all corpus files + +The shorter the input files are so that they still traverse the same path +within the target, the better the fuzzing will be. This is done with `afl-tmin` +however it is a long processes as this has to be done for every file: + +``` +mkdir input +cd INPUTS_UNIQUE +for i in *; do + afl-tmin -i "$i" -o "../input/$i" -- bin/target -d @@ +done +``` + +This can also be parallelized, e.g. with `parallel` + +#### c) done! + +The INPUTS_UNIQUE/ directory from step a) - or even better if you minimized the +corpus in step b) then the files in input/ is then the input corpus directory +to be used in fuzzing! :-) + +### Fuzzing the target + +In this final step we fuzz the target. +There are not that many useful options to run the target - unless you want to +use many CPU cores for the fuzzing, which will make the fuzzing much more useful. + +If you just use one CPU for fuzzing, then you are fuzzing just for fun and not +seriously :-) + +#### a) running afl-fuzz + +If you have an input corpus from step 2 then specify this directory with the `-i` +option. Otherwise create a new directory and create a file with any content +in there. + +If you do not want anything special, the defaults are already the usual best, +hence all you need (from the example in 2a): +`afl-fuzz -i input -o output -- bin/target -d @@` +Note that the directory specified with -o will be created if it does not exist. + +If you need to stop and re-start the fuzzing, use the same command line option +and switch the input directory with a dash (`-`): +`afl-fuzz -i - -o output -- bin/target -d @@` + +afl-fuzz never stops fuzzing. To terminate afl++ simply press Control-C. + +When you start afl-fuzz you will see a user interface that shows what the status +is: +[docs/screenshot.png](docs/screenshot.png) +All the entries are explained in [docs/status_screen.md](docs/status_screen.md) + +#### b) Using multiple cores + +If you want to seriously fuzz then use as many cores as possible to fuzz your +target. + +On the same machine - due to the nature how afl++ works - there is a maximum +number of CPU cores that are useful, more and the overall performance degrades +instead. This value depends on the target and the limit is between 24 and 64 +cores per machine. + +There should be one main fuzzer (`-M main` option) and as many secondary +fuzzers (eg `-S variant1`) as you cores that you use. +Every -M/-S entry needs a unique name (that can be whatever), however the same +-o output directory location has to be used for all. + +For every secondary there should be a variation, e.g.: + * one should fuzz the target that was compiled differently: with sanitizers + activated (`export AFL_USE_ASAN=1 ; export AFL_USE_UBSAN=1 ; + export AFL_USE_CFISAN=1 ; ` + * one should fuzz the target with CMPLOG/redqueen (see above) + * At 1-2 should fuzz a target compiled with laf-intel/COMPCOV (see above). + +All other secondaries should be: + * 1/2 with MOpt option enabled: `-L 0` + * run with a different power schedule, available are: + `explore (default), fast, coe, lin, quad, exploit, mmopt, rare, seek` + which you can set with e.g. `-p seek` + +You can also use different fuzzers. +If you are afl-spinoffs or afl conforming, then just use the same -o directory +and give it a unique `-S` name. +Examples are e.g.: + * + * + * + +However you can also sync afl++ with honggfuzz, libfuzzer, entropic, etc. +Just show the main fuzzer (-M) with the `-F` option where the queue +directory of these other fuzzers are, e.g. `-F /src/target/honggfuzz` + +#### c) How long to fuzz a target? + +This is a difficult question. +Basically if no new path is found for a long time (e.g. for a day or a week) +then you can expect that your fuzzing won't be fruitful anymore. +However often this just means that you should switch out secondaries for +others, e.g. custom mutator modules, sync to very different fuzzers, etc. + +### The End + +This is basically all you need to know to professionally run fuzzing campaigns. +If you want to know more, the rest of this README and the tons of texts in +[docs/](docs/) will have you covered. + +## 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 the 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. + + +## 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 the 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 + queue. + + 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. + + +## Instrumenting programs for use with AFL + +PLEASE NOTE: llvm_mode compilation with afl-clang-fast/afl-clang-fast++ +instead of afl-gcc/afl-g++ is much faster and has many cool features. +See llvm_mode/ - however few code does not compile with llvm. +We support llvm versions 3.4 to 12. + +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: + +```shell +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.md](llvm_mode/README.md). +Clang/LLVM has a much better performance and works with LLVM version 3.4 to 12. + +Using the LAF Intel performance enhancements are also recommended, see +[llvm_mode/README.laf-intel.md](llvm_mode/README.laf-intel.md) + +Using partial instrumentation is also recommended, see +[llvm_mode/README.instrument_file.md](llvm_mode/README.instrument_file.md) + +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: + +```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.md](libdislocator/README.md)) can help uncover heap corruption issues, too. + +PS. ASAN users are advised to review [docs/notes_for_asan.md](docs/notes_for_asan.md) +file for important caveats. + + +## 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: + +```shell +cd qemu_mode +./build_qemu_support.sh +``` + +For additional instructions and caveats, see [qemu_mode/README.md](qemu_mode/README.md). + +If possible you should use the persistent mode, see [qemu_mode/README.persistent.md](qemu_mode/README.persistent.md). + +The mode is approximately 2-5x slower than compile-time instrumentation, is +less conducive to parallelization, and may have some other quirks. + +If [afl-dyninst](https://github.com/vanhauser-thc/afl-dyninst) works for +your binary, then you can use afl-fuzz normally and it will have twice +the speed compared to qemu_mode. + +A more comprehensive description of these and other options can be found in +[docs/binaryonly_fuzzing.md](docs/binaryonly_fuzzing.md) + +## Power schedules + +The power schedules were copied from Marcel Böhme's AFLfast implementation and +measure differently which queue entries to prefer and therefore may find +different paths faster for large queues. + +The available schedules are: + + - explore (default, original AFL) + - exploit (original AFL) + - fast (AFLfast) + - coe (AFLfast) + - quad (AFLfast) + - lin (AFLfast) + - rare (afl++ experimental) + - mmopt (afl++ experimental) + - seek (afl++ experimental) + +In parallel mode (-M/-S, several instances with the shared queue), we suggest +to run the main node using the default explore schedule (`-p explore`) and the +secondary nodes with different schedules. If a schedule does not perform well +for a target, restart the secondary nodes with a different schedule. + +More details can be found in the paper published at the 23rd ACM Conference on +Computer and Communications Security [CCS'16](https://www.sigsac.org/ccs/CCS2016/accepted-papers/) + +## 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.md](docs/perf_tips.md). + + - 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. + + +## 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: + +```shell +./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: + +```shell +./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. + +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.md](docs/perf_tips.md). + +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. + +## Interpreting output + +See the [docs/status_screen.md](docs/status_screen.md) 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 a 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. 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 value can be fine-tuned by setting AFL_HANG_TMOUT, but this + is rarely necessary. + +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 the 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: + +```shell +LIMIT_MB=50 +( 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: + +```shell +./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, +see [http://lcamtuf.coredump.cx/afl/plot/](http://lcamtuf.coredump.cx/afl/plot/). + +## 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 [docs/parallel_fuzzing.md](docs/parallel_fuzzing.md). + +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 +last section of [docs/parallel_fuzzing.md](docs/parallel_fuzzing.md) for tips. + +## 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: + + [http://lcamtuf.blogspot.com/2015/01/afl-fuzz-making-up-grammar-with.html](http://lcamtuf.blogspot.com/2015/01/afl-fuzz-making-up-grammar-with.html) + +To use this feature, you first need to create a dictionary in one of the two +formats discussed in [dictionaries/README.md](dictionaries/README.md); +and then point the fuzzer to it via the -x option in the command line. + +(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 +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.md](libtokencap/README.tokencap.md). + +## 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 to 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 +beneath. + +Oh, one more thing: for test case minimization, give afl-tmin a try. The tool +can be operated in a very simple way: + +```shell +./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 +afl-fuzz. + +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 +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 [docs/technical_details.md](docs/technical_details.md). + +## Going beyond crashes + +Fuzzing is a wonderful and underutilized technique for discovering non-crashing +design and implementation errors, too. Quite a few interesting bugs have been +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 + input image several times in a row, + + - 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 +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). + +## Common-sense risks + +Please keep in mind that, similarly to many other computationally-intensive +tasks, fuzzing may put a 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: + +```shell + $ iostat -d 3 -x -k [...optional disk ID...] +``` + +## 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 processes 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 + examples/libpng_no_checksum/ for inspiration); if this is not possible, + you can also write a postprocessor, one of the hooks of custom mutators. + See [docs/custom_mutators.md](docs/custom_mutators.md) on how to use + `AFL_CUSTOM_MUTATOR_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 [docs/notes_for_asan.md](docs/notes_for_asan.md) + for tips. + + - 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: + [https://github.com/zardus/preeny](https://github.com/zardus/preeny) + + Some useful tips for modifying network-based services can be also found at: + [https://www.fastly.com/blog/how-to-fuzz-server-american-fuzzy-lop](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](https://github.com/mrash/afl-cov) + + - Occasionally, sentient machines rise against their creators. If this + happens to you, please consult [http://lcamtuf.coredump.cx/prep/](http://lcamtuf.coredump.cx/prep/). + +Beyond this, see INSTALL for platform-specific tips. + +## Special thanks + +Many of the improvements to the original afl and afl++ 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 Neuschaefer 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 Jeff Trull + Guillaume Endignoux ilovezfs + Daniel Godas-Lopez Franjo Ivancic + Austin Seipp Daniel Komaromy + Daniel Binderman Jonathan Metzman + Vegard Nossum Jan Kneschke + Kurt Roeckx Marcel Boehme + Van-Thuan Pham Abhik Roychoudhury + Joshua J. Drake Toby Hutton + Rene Freingruber Sergey Davidoff + Sami Liedes Craig Young + Andrzej Jackowski Daniel Hodson + Nathan Voss Dominik Maier + Andrea Biondo Vincent Le Garrec + Khaled Yakdan Kuang-che Wu + Josephine Calliotte Konrad Welc +``` + +Thank you! +(For people sending pull requests - please add yourself to this list :-) + +## Contact + +Questions? Concerns? Bug reports? The contributors can be reached via +[https://github.com/AFLplusplus/AFLplusplus](https://github.com/AFLplusplus/AFLplusplus) + +There is also a mailing list for the afl project; to join, send a mail to +<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) |