| ================== |
| american fuzzy lop |
| ================== |
| |
| Written and maintained by Michal Zalewski <lcamtuf@google.com> |
| |
| Copyright 2013, 2014, 2015, 2016 Google Inc. All rights reserved. |
| Released under terms and conditions of Apache License, Version 2.0. |
| |
| For new versions and additional information, check out: |
| http://lcamtuf.coredump.cx/afl/ |
| |
| To compare notes with other users or get notified about major new features, |
| send a mail to <afl-users+subscribe@googlegroups.com>. |
| |
| ** 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 |
| 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. |
| |
| 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. Libdislocator, a helper library included with AFL (see |
| libdislocator/README.dislocator) can help uncover heap corruption issues, too. |
| |
| PS. ASAN users are advised to review notes_for_asan.txt file for important |
| caveats. |
| |
| 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 |
| $ ./build_qemu_support.sh |
| |
| 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. |
| |
| 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, 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. |
| |
| 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: |
| |
| $ 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: |
| |
| $ ./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/. |
| |
| 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. |
| |
| 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 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 |
| 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 |
| |
| 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. |
| |
| (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 |
| |
| 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. |
| |
| 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 |
| beneath. |
| |
| 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 |
| 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 technical_details.txt. |
| |
| 11) 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). |
| |
| 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: |
| |
| - 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...] |
| |
| 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 |
| 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 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 |
| |
| 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 |
| |
| - 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 |
| happens to you, please consult http://lcamtuf.coredump.cx/prep/. |
| |
| 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 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 Bohme |
| Van-Thuan Pham Abhik Roychoudhury |
| |
| Thank you! |
| |
| 15) Contact |
| ----------- |
| |
| Questions? Concerns? Bug reports? The author can be usually reached at |
| <lcamtuf@google.com>. |
| |
| 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 |
| archives first, try: |
| |
| https://groups.google.com/group/afl-users |
| |
| 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: |
| |
| https://cla.developers.google.com/clas |
| |
| Sorry about the hassle. Of course, no CLA is required for feature requests or |
| bug reports. |