This document discusses the environment variables used by AFL++ to expose various exotic functions that may be (rarely) useful for power users or for some types of custom fuzzing setups. For general information about AFL++, see README.md.
Note: Most tools will warn on any unknown AFL++ environment variables; for
example, because of typos. If you want to disable this check, then set the
AFL_IGNORE_UNKNOWN_ENVS environment variable.
1) Settings for all compilers
Starting with AFL++ 3.0, there is only one compiler: afl-cc.
To select the different instrumentation modes, use one of the following options:
Pass the –afl-MODE command-line option to the compiler. Only this option accepts further AFL-specific command-line options.
Use a symlink to afl-cc: afl-clang, afl-clang++, afl-clang-fast, afl-clang-fast++, afl-clang-lto, afl-clang-lto++, afl-g++, afl-g++-fast, afl-gcc, afl-gcc-fast. This option does not accept AFL-specific command-line options. Instead, use environment variables.
AFL_CC_COMPILERenvironment variable with
MODE. To select
MODE, use one of the following values:
The compile-time tools do not accept AFL-specific command-line options. The –afl-MODE command line option is the only exception. The other options make fairly broad use of environment variables instead:
Some build/configure scripts break with AFL++ compilers. To be able to pass them, do:
export CC=afl-cc export CXX=afl-c++ export AFL_NOOPT=1 ./configure --disable-shared --disabler-werror unset AFL_NOOPT make
AFL_CXXlets you use alternate downstream compilation tools, rather than the default ‘as’, ‘clang’, or ‘gcc’ binaries in your
If you are a weird person that wants to compile and instrument asm text files, then use the
AFL_AS_FORCE_INSTRUMENT=1 afl-gcc foo.s -o foo
Most AFL tools do not print any output if stdout/stderr are redirected. If you want to get the output into a file, then set the
AFL_DEBUGenvironment variable. This is sadly necessary for various build processes which fail otherwise.
By default, the wrapper appends
-O3to optimize builds. Very rarely, this will cause problems in programs built with -Werror, because
-O3enables more thorough code analysis and can spew out additional warnings. To disable optimizations, set
AFL_DONT_OPTIMIZE. However, if
-fno-unroll-loopsare set, these are not overridden.
AFL_HARDENautomatically adds code hardening options when invoking the downstream compiler. This currently includes
-fstack-protector-all. The setting is useful for catching non-crashing memory bugs at the expense of a very slight (sub-5%) performance loss.
AFL_INST_RATIOto a percentage between 0 and 100 controls the probability of instrumenting every branch. This is (very rarely) useful when dealing with exceptionally complex programs that saturate the output bitmap. Examples include ffmpeg, perl, and v8.
(If this ever happens, afl-fuzz will warn you ahead of the time by displaying the “bitmap density” field in fiery red.)
AFL_INST_RATIOto 0 is a valid choice. This will instrument only the transitions between function entry points, but not individual branches.
Note that this is an outdated variable. A few instances (e.g., afl-gcc) still support these, but state-of-the-art (e.g., LLVM LTO and LLVM PCGUARD) do not need this.
AFL_NO_BUILTINcauses the compiler to generate code suitable for use with libtokencap.so (but perhaps running a bit slower than without the flag).
AFL_PATHcan be used to point afl-gcc to an alternate location of afl-as. One possible use of this is utils/clang_asm_normalize/, which lets you instrument hand-written assembly when compiling clang code by plugging a normalizer into the chain. (There is no equivalent feature for GCC.)
AFL_QUIETwill prevent afl-as and afl-cc banners from being displayed during compilation, in case you find them distracting.
AFL_USE_...automatically enables supported sanitizers - provided that your compiler supports it. Available are:
AFL_USE_ASAN=1- activates the address sanitizer (memory corruption detection)
AFL_USE_CFISAN=1- activates the Control Flow Integrity sanitizer (e.g. type confusion vulnerabilities)
AFL_USE_LSAN- activates the leak sanitizer. To perform a leak check within your program at a certain point (such as at the end of an
__AFL_LOOP()), you can run the macro
__AFL_LEAK_CHECK();which will cause an abort if any memory is leaked (you can combine this with the
__AFL_LSAN_ON();macros to avoid checking for memory leaks from memory allocated between these two calls.
AFL_USE_MSAN=1- activates the memory sanitizer (uninitialized memory)
AFL_USE_TSAN=1- activates the thread sanitizer to find thread race conditions
AFL_USE_UBSAN=1- activates the undefined behavior sanitizer
TMPDIRis used by afl-as for temporary files; if this variable is not set, the tool defaults to /tmp.
2) Settings for LLVM and LTO: afl-clang-fast / afl-clang-fast++ / afl-clang-lto / afl-clang-lto++
The native instrumentation helpers (instrumentation and gcc_plugin) accept a subset of the settings discussed in section 1, with the exception of:
AFL_AS, since this toolchain does not directly invoke GNU
AFL_INST_RATIO, as we use collision free instrumentation by default. Not all passes support this option though as it is an outdated feature.
LLVM modes support
AFL_LLVM_DICT2FILE=/absolute/path/file.txtwhich will write all constant string comparisons to this file to be used later with afl-fuzz’
AFL_KEEP_ASSEMBLY, since no temporary assembly files are created.
Then there are a few specific features that are only available in instrumentation mode:
Select the instrumentation mode
AFL_LLVM_INSTRUMENT - this configures the instrumentation mode.
CLANG - outdated clang instrumentation
CLASSIC - classic AFL (map[cur_loc ^ prev_loc » 1]++) (default)
You can also specify CTX and/or NGRAM, separate the options with a comma “,” then, e.g.:
Note: It is actually not a good idea to use both CTX and NGRAM. :)
CTX - context sensitive instrumentation
GCC - outdated gcc instrumentation
LTO - LTO instrumentation
NATIVE - clang’s original pcguard based instrumentation
NGRAM-x - deeper previous location coverage (from NGRAM-2 up to NGRAM-16)
PCGUARD - our own pcgard based instrumentation (default)
AFL_LLVM_CMPLOG=1 during compilation will tell afl-clang-fast to
produce a CmpLog binary.
For afl-gcc-fast, set
For more information, see instrumentation/README.cmplog.md.
AFL_LLVM_INSTRUMENT=CTX activates context sensitive
branch coverage - meaning that each edge is additionally combined with its
caller. It is highly recommended to increase the
MAP_SIZE_POW2 definition in
config.h to at least 18 and maybe up to 20 for this as otherwise too many map
For more information, see instrumentation/README.llvm.md#6) AFL++ Context Sensitive Branch Coverage.
INSTRUMENT LIST (selectively instrument files and functions)
This feature allows selective instrumentation of the source.
AFL_LLVM_DENYLIST with a file name and/or
function will only instrument (or skip) those files that match the names listed
in the specified file.
For more information, see instrumentation/README.instrument_list.md.
This great feature will split compares into series of single byte comparisons to allow afl-fuzz to find otherwise rather impossible paths. It is not restricted to Intel CPUs. ;-)
AFL_LLVM_LAF_TRANSFORM_COMPARESwill split string compare functions.
AFL_LLVM_LAF_SPLIT_COMPARESwill split all floating point and 64, 32 and 16 bit integer CMP instructions.
AFL_LLVM_LAF_SPLIT_FLOATSwill split floating points, needs
AFL_LLVM_LAF_SPLIT_COMPARESto be set.
AFL_LLVM_LAF_SPLIT_SWITCHESwill split all
AFL_LLVM_LAF_ALLsets all of the above.
For more information, see instrumentation/README.laf-intel.md.
This is a different way of instrumentation: first it compiles all code in LTO (link time optimization) and then performs an edge inserting instrumentation which is 100% collision free (collisions are a big issue in AFL and AFL-like instrumentations). This is performed by using afl-clang-lto/afl-clang-lto++ instead of afl-clang-fast, but is only built if LLVM 11 or newer is used.
AFL_LLVM_INSTRUMENT=CFG will use Control Flow Graph instrumentation. (Not
recommended for afl-clang-fast, default for afl-clang-lto as there it is a
different and better kind of instrumentation.)
None of the following options are necessary to be used and are rather for manual use (which only ever the author of this LTO implementation will use). These are used if several separated instrumentations are performed which are then later combined.
AFL_LLVM_DOCUMENT_IDS=filewill document to a file which edge ID was given to which function. This helps to identify functions with variable bytes or which functions were touched by an input.
AFL_LLVM_LTO_DONTWRITEIDprevents that the highest location ID written into the instrumentation is set in a global variable.
AFL_LLVM_LTO_STARTIDsets the starting location ID for the instrumentation. This defaults to 1.
AFL_LLVM_MAP_ADDRsets the fixed map address to a different address than the default
0x10000. A value of 0 or empty sets the map address to be dynamic (the original AFL way, which is slower).
AFL_LLVM_MAP_DYNAMICsets the shared memory address to be dynamic.
For more information, see instrumentation/README.lto.md.
ngram prev_loc coverage. Good values are 2, 4, or 8 (any value between 2 and 16
is valid). It is highly recommended to increase the
in config.h to at least 18 and maybe up to 20 for this as otherwise too many map
For more information, see instrumentation/README.llvm.md#7) AFL++ N-Gram Branch Coverage.
AFL_LLVM_NOT_ZERO=1during compilation will use counters that skip zero on overflow. This is the default for llvm >= 9, however, for llvm versions below that this will increase an unnecessary slowdown due a performance issue that is only fixed in llvm 9+. This feature increases path discovery by a little bit.
AFL_LLVM_SKIP_NEVERZERO=1will not implement the skip zero test. If the target performs only a few loops, then this will give a small performance boost.
Thread safe instrumentation counters (in all modes)
AFL_LLVM_THREADSAFE_INST will inject code that implements thread safe
counters. The overhead is a little bit higher compared to the older non-thread
safe case. Note that this disables neverzero (see NOT_ZERO).
3) Settings for GCC / GCC_PLUGIN modes
There are a few specific features that are only available in GCC and GCC_PLUGIN mode.
GCC mode only: Setting
AFL_KEEP_ASSEMBLYprevents afl-as from deleting instrumented assembly files. Useful for troubleshooting problems or understanding how the tool works.
To get them in a predictable place, try something like:
mkdir assembly_here TMPDIR=$PWD/assembly_here AFL_KEEP_ASSEMBLY=1 make clean all
GCC_PLUGIN mode only: Setting
AFL_GCC_ALLOWLISTwith a filename will only instrument those files that match the names listed in this file (one filename per line).
AFL_GCC_BLOCKLISTwith a file name and/or function will only skip those files that match the names listed in the specified file. See instrumentation/README.instrument_list.md for more information.
AFL_GCC_OUT_OF_LINE=1will instruct afl-gcc-fast to instrument the code with calls to an injected subroutine instead of the much more efficient inline instrumentation.
AFL_GCC_SKIP_NEVERZERO=1will not implement the skip zero test. If the target performs only a few loops, then this will give a small performance boost.
4) Settings for afl-fuzz
The main fuzzer binary accepts several options that disable a couple of sanity checks or alter some of the more exotic semantics of the tool:
AFL_AUTORESUMEwill resume a fuzz run (same as providing
-i -) for an existing out folder, even if a different
-iwas provided. Without this setting, afl-fuzz will refuse execution for a long-fuzzed out dir.
AFL_BENCH_JUST_ONEcauses the fuzzer to exit after processing the first queue entry; and
AFL_BENCH_UNTIL_CRASHcauses it to exit soon after the first crash is found.
AFL_CMPLOG_ONLY_NEWwill only perform the expensive cmplog feature for newly found test cases and not for test cases that are loaded on startup (
-i in). This is an important feature to set when resuming a fuzzing session.
AFL_CRASH_EXITCODEsets the exit code AFL++ treats as crash. For example, if
AFL_CRASH_EXITCODE='-1'is set, each input resulting in a
-1return code (i.e.
exit(-1)got called), will be treated as if a crash had occurred. This may be beneficial if you look for higher-level faulty conditions in which your target still exits gracefully.
AFL_CUSTOM_MUTATOR_LIBRARYto a shared library with afl_custom_fuzz() creates additional mutations through this library. If afl-fuzz is compiled with Python (which is autodetected during building afl-fuzz), setting
AFL_PYTHON_MODULEto a Python module can also provide additional mutations. If
AFL_CUSTOM_MUTATOR_ONLYis also set, all mutations will solely be performed with the custom mutator. This feature allows to configure custom mutators which can be very helpful, e.g., fuzzing XML or other highly flexible structured input. For details, see /docs/custom_mutators/.
AFL_CYCLE_SCHEDULESwill switch to a different schedule every time a cycle is finished.
AFL_DEBUG_CHILDwill not suppress the child output. This lets you see all output of the child, making setup issues obvious. For example, in an unicornafl harness, you might see python stacktraces. You may also see other logs that way, indicating why the forkserver won’t start. Not pretty but good for debugging purposes. Note that
AFL_DISABLE_TRIMtells afl-fuzz not to trim test cases. This is usually a bad idea!
AFL_KEEP_TIMEOUTSwill keep longer running inputs if they reach new coverage
AFL_EXIT_ON_SEED_ISSUESwill restore the vanilla afl-fuzz behavior which does not allow crashes or timeout seeds in the initial -i corpus.
AFL_EXIT_ON_TIMEcauses afl-fuzz to terminate if no new paths were found within a specified period of time (in seconds). May be convenient for some types of automated jobs.
AFL_EXIT_WHEN_DONEcauses afl-fuzz to terminate when all existing paths have been fuzzed and there were no new finds for a while. This would be normally indicated by the cycle counter in the UI turning green. May be convenient for some types of automated jobs.
AFL_EXPAND_HAVOC_NOWwill start in the extended havoc mode that includes costly mutations. afl-fuzz automatically enables this mode when deemed useful otherwise.
AFL_FAST_CALkeeps the calibration stage about 2.5x faster (albeit less precise), which can help when starting a session against a slow target.
AFL_FORCE_UIwill force painting the UI on the screen even if no valid terminal was detected (for virtual consoles).
AFL_FORKSRV_INIT_TMOUTallows you to specify a different timeout to wait for the forkserver to spin up. The default is the
config.h(usually 10), so for a
-t 100, the default would wait for
1000milliseconds. Setting a different time here is useful if the target has a very slow startup time, for example, when doing full-system fuzzing or emulation, but you don’t want the actual runs to wait too long for timeouts.
AFL_HANG_TMOUTallows you to specify a different timeout for deciding if a particular test case is a “hang”. The default is 1 second or the value of the
-tparameter, whichever is larger. Dialing the value down can be useful if you are very concerned about slow inputs, or if you don’t want AFL++ to spend too much time classifying that stuff and just rapidly put all timeouts in that bin.
If you are Jakub, you may need
AFL_I_DONT_CARE_ABOUT_MISSING_CRASHES. Others need not apply, unless they also want to disable the
If afl-fuzz encounters an incorrect fuzzing setup during a fuzzing session (not at startup), it will terminate. If you do not want this, then you can set
When running in the
AFL_IMPORT_FIRSTcauses the fuzzer to import test cases from other instances before doing anything else. This makes the “own finds” counter in the UI more accurate. Beyond counter aesthetics, not much else should change.
AFL_INPUT_LEN_MAXare an alternative to the afl-fuzz -g/-G command line option to control the minimum/maximum of fuzzing input generated.
AFL_KILL_SIGNAL: Set the signal ID to be delivered to child processes on timeout. Unless you implement your own targets or instrumentation, you likely don’t have to set it. By default, on timeout and on exit,
AFL_KILL_SIGNAL=9) will be delivered to the child.
AFL_FORK_SERVER_KILL_SIGNAL: Set the signal ID to be delivered to the fork server when AFL++ is terminated. Unless you implement your fork server, you likely do not have to set it. By default,
AFL_FORK_SERVER_KILL_SIGNAL=15) will be delivered to the fork server. If only
AFL_FORK_SERVER_KILL_SIGNALwill be set to same value as
AFL_KILL_SIGNALto provide backward compatibility. If
AFL_FORK_SERVER_KILL_SIGNALis also set, it takes precedence.
NOTE: Uncatchable signals, such as
SIGKILL, cause child processes of the fork server to be orphaned and leaves them in a zombie state.
AFL_MAP_SIZEsets the size of the shared map that afl-analyze, afl-fuzz, afl-showmap, and afl-tmin create to gather instrumentation data from the target. This must be equal or larger than the size the target was compiled with.
AFL_MAX_DET_EXTRASwill change the threshold at what number of elements in the
-xdictionary and LTO autodict (combined) the probabilistic mode will kick off. In probabilistic mode, not all dictionary entries will be used all of the time for fuzzing mutations to not slow down fuzzing. The default count is
200elements. So for the 200 + 1st element, there is a 1 in 201 chance, that one of the dictionary entries will not be used directly.
AFL_NO_AFFINITYdisables attempts to bind to a specific CPU core on Linux systems. This slows things down, but lets you run more instances of afl-fuzz than would be prudent (if you really want to).
AFL_NO_ARITHcauses AFL++ to skip most of the deterministic arithmetics. This can be useful to speed up the fuzzing of text-based file formats.
AFL_NO_AUTODICTwill not load an LTO generated auto dictionary that is compiled into the target.
AFL_NO_COLOURwill omit control sequences for coloring console output when configured with USE_COLOR and not ALWAYS_COLORED.
The CPU widget shown at the bottom of the screen is fairly simplistic and may complain of high load prematurely, especially on systems with low core counts. To avoid the alarming red color for very high CPU usages, you can set
AFL_NO_FORKSRVdisables the forkserver optimization, reverting to fork + execve() call for every tested input. This is useful mostly when working with unruly libraries that create threads or do other crazy things when initializing (before the instrumentation has a chance to run).
Note that this setting inhibits some of the user-friendly diagnostics normally done when starting up the forkserver and causes a pretty significant performance drop.
AFL_NO_SNAPSHOTwill advise afl-fuzz not to use the snapshot feature if the snapshot lkm is loaded.
AFL_NO_UIinhibits the UI altogether and just periodically prints some basic stats. This behavior is also automatically triggered when the output from afl-fuzz is redirected to a file or to a pipe.
AFL_NO_STARTUP_CALIBRATIONwill skip the initial calibration of all starting seeds, and start fuzzing at once.
In QEMU mode (-Q) and FRIDA mode (-O),
AFL_PATHwill be searched for afl-qemu-trace and afl-frida-trace.so.
If you are using persistent mode (you should, see instrumentation/README.persistent_mode.md), some targets keep inherent state due which a detected crash test case does not crash the target again when the test case is given. To be able to still re-trigger these crashes, you can use the
AFL_PERSISTENT_RECORDvariable with a value of how many previous fuzz cases to keep prior a crash. If set to e.g., 10, then the 9 previous inputs are written to out/default/crashes as RECORD:000000,cnt:000000 to RECORD:000000,cnt:000008 and RECORD:000000,cnt:000009 being the crash case. NOTE: This option needs to be enabled in config.h first!
AFL_POST_LIBRARYis deprecated, use
AFL_PRELOADcauses AFL++ to set
LD_PRELOADfor the target binary without disrupting the afl-fuzz process itself. This is useful, among other things, for bootstrapping libdislocator.so.
In QEMU mode (-Q), setting
AFL_QEMU_CUSTOM_BINwill cause afl-fuzz to skip prepending
afl-qemu-traceto your command line. Use this if you wish to use a custom afl-qemu-trace or if you need to modify the afl-qemu-trace arguments.
AFL_SHUFFLE_QUEUErandomly reorders the input queue on startup. Requested by some users for unorthodox parallelized fuzzing setups, but not advisable otherwise.
When developing custom instrumentation on top of afl-fuzz, you can use
AFL_SKIP_BIN_CHECKto inhibit the checks for non-instrumented binaries and shell scripts; and
AFL_DUMB_FORKSRVin conjunction with the
-nsetting to instruct afl-fuzz to still follow the fork server protocol without expecting any instrumentation data in return. Note that this also turns off auto map size detection.
AFL_SKIP_CPUFREQskips the check for CPU scaling policy. This is useful if you can’t change the defaults (e.g., no root access to the system) and are OK with some performance loss.
AFL_STATSDenables StatsD metrics collection. By default, AFL++ will send these metrics over UDP to 127.0.0.1:8125. The host and port are configurable with
AFL_STATSD_PORTrespectively. To enable tags (banner and afl_version), you should provide
AFL_STATSD_TAGS_FLAVORthat matches your StatsD server (see
AFL_STATSD_TAGS_FLAVORto one of
signalfxallows you to add tags to your fuzzing instances. This is especially useful when running multiple instances (
-M/-Sfor example). Applied tags are
bannercorresponds to the name of the fuzzer provided through
afl_versioncorresponds to the currently running AFL++ version (e.g.,
++3.0c). Default (empty/non present) will add no tags to the metrics. For more information, see /docs/rpc_statsd/.
AFL_SYNC_TIMEallows you to specify a different minimal time (in minutes) between fuzzing instances synchronization. Default sync time is 30 minutes, note that time is halved for -M main nodes.
AFL_TARGET_ENVcauses AFL++ to set extra environment variables for the target binary. Example:
AFL_TARGET_ENV="VAR1=1 VAR2='a b c'" afl-fuzz .... This exists mostly for things like
LD_LIBRARY_PATHbut it would theoretically allow fuzzing of AFL++ itself (with ’target’ AFL++ using some AFL_ vars that would disrupt work of ‘fuzzer’ AFL++). Note that when using QEMU mode, the
AFL_TARGET_ENVenvironment variables will apply to QEMU, as well as the target binary. Therefore, in this case, you might want to use QEMU’s
QEMU_SET_ENVenvironment variable (see QEMU’s documentation because the format is different from
AFL_TARGET_ENV) to apply the environment variables to the target and not QEMU.
AFL_TESTCACHE_SIZEallows you to override the size of
#define TESTCASE_CACHEin config.h. Recommended values are 50-250MB - or more if your fuzzing finds a huge amount of paths for large inputs.
AFL_TMPDIRis used to write the
.cur_inputfile to if it exists, and in the normal output directory otherwise. You would use this to point to a ramdisk/tmpfs. This increases the speed by a small value but also reduces the stress on SSDs.
AFL_TRY_AFFINITYtries to attempt binding to a specific CPU core on Linux systems, but will not terminate if that fails.
The following environment variables are only needed if you implemented your own forkserver or persistent mode, or if __AFL_LOOP or __AFL_INIT are in a shared library and not the main binary:
AFL_DEFER_FORKSRVenforces a deferred forkserver even if none was detected in the target binary
AFL_PERSISTENTenforces persistent mode even if none was detected in the target binary
If you need an early forkserver in your target because of early constructors in your target, you can set
AFL_EARLY_FORKSERVER. Note that this is not a compile time option but a runtime option :-)
AFL_PIZZA_MODEto 1 to enable the April 1st stats menu, set to 0 to disable although it is 1st of April.
5) Settings for afl-qemu-trace
The QEMU wrapper used to instrument binary-only code supports several settings:
AFL_COMPCOV_LEVELenables the CompareCoverage tracing of all cmp and sub in x86 and x86_64 and memory comparison functions (e.g., strcmp, memcmp, …) when libcompcov is preloaded using
AFL_PRELOAD. More info at qemu_mode/libcompcov/README.md.
There are two levels at the moment,
AFL_COMPCOV_LEVEL=1that instruments only comparisons with immediate values / read-only memory and
AFL_COMPCOV_LEVEL=2that instruments all the comparisons. Level 2 is more accurate but may need a larger shared memory.
AFL_DEBUGwill print the found entry point for the binary to stderr. Use this if you are unsure if the entry point might be wrong - but use it directly, e.g.,
AFL_ENTRYPOINTallows you to specify a specific entry point into the binary (this can be very good for the performance!). The entry point is specified as hex address, e.g.,
0x4004110. Note that the address must be the address of a basic block.
AFL_INST_LIBScauses the translator to also instrument the code inside any dynamically linked libraries (notably including glibc).
It is possible to set
AFL_INST_RATIOto skip the instrumentation on some of the basic blocks, which can be useful when dealing with very complex binaries.
AFL_QEMU_COMPCOVenables the CompareCoverage tracing of all cmp and sub in x86 and x86_64. This is an alias of
AFL_COMPCOV_LEVELis not specified.
AFL_QEMU_FORCE_DFL, you force QEMU to ignore the registered signal handlers of the target.
When the target is i386/x86_64, you can specify the address of the function that has to be the body of the persistent loop using
AFL_QEMU_PERSISTENT_GPR=1, QEMU will save the original value of general purpose registers and restore them in each persistent cycle.
Another modality to execute the persistent loop is to specify also the
AFL_QEMU_PERSISTENT_RET=end addrenvironment variable. With this variable assigned, instead of patching the return address, the specified instruction is transformed to a jump towards
AFL_QEMU_PERSISTENT_RETADDR_OFFSET, you can specify the offset from the stack pointer in which QEMU can find the return address when
start addris hit.
AFL_USE_QASAN, you can enable QEMU AddressSanitizer for dynamically linked binaries.
The underlying QEMU binary will recognize any standard “user space emulation” variables (e.g.,
QEMU_STACK_SIZE), but there should be no reason to touch them.
README.txtis written to the
crashes/directory when a first crash is found. Setting
AFL_NO_CRASH_READMEwill prevent this. Useful when counting crashes based on a file count in that directory.
7) Settings for afl-frida-trace
The FRIDA wrapper used to instrument binary-only code supports many of the same
afl-qemu-trace, but also has a number of additional advanced
options. These are listed in brief below (see
frida_mode/README.md for more details). These
settings are provided for compatibility with QEMU mode, the preferred way to
configure FRIDA mode is through its scripting
AFL_QEMU_DRIVER_NO_HOOK. When using the QEMU driver to provide a
mainloop for a user provided
LLVMFuzzerTestOneInput, this option configures the driver to read input from
stdinrather than using in-memory test cases.
AFL_FRIDA_INST_COVERAGE_FILE- File to write DynamoRio format coverage information (e.g., to be loaded within IDA lighthouse).
AFL_FRIDA_INST_DEBUG_FILE- File to write raw assembly of original blocks and their instrumented counterparts during block compilation.
AFL_FRIDA_INST_JIT- Enable the instrumentation of Just-In-Time compiled code. Code is considered to be JIT if the executable segment is not backed by a file.
AFL_FRIDA_INST_NO_OPTIMIZE- Don’t use optimized inline assembly coverage instrumentation (the default where available). Required to use
AFL_FRIDA_INST_NO_BACKPATCH- Disable backpatching. At the end of executing each block, control will return to FRIDA to identify the next block to execute.
AFL_FRIDA_INST_NO_PREFETCH- Disable prefetching. By default, the child will report instrumented blocks back to the parent so that it can also instrument them and they be inherited by the next child on fork, implies
AFL_FRIDA_INST_NO_PREFETCH_BACKPATCH- Disable prefetching of stalker backpatching information. By default, the child will report applied backpatches to the parent so that they can be applied and then be inherited by the next child on fork.
AFL_FRIDA_INST_SEED- Sets the initial seed for the hash function used to generate block (and hence edge) IDs. Setting this to a constant value may be useful for debugging purposes, e.g., investigating unstable edges.
AFL_FRIDA_INST_TRACE- Log to stdout the address of executed blocks, implies
AFL_FRIDA_INST_TRACE_UNIQUE- As per
AFL_FRIDA_INST_TRACE, but each edge is logged only once, requires
AFL_FRIDA_INST_UNSTABLE_COVERAGE_FILE- File to write DynamoRio format coverage information for unstable edges (e.g., to be loaded within IDA lighthouse).
AFL_FRIDA_JS_SCRIPT- Set the script to be loaded by the FRIDA scripting engine. See frida_mode/Scripting.md for details.
AFL_FRIDA_OUTPUT_STDOUT- Redirect the standard output of the target application to the named file (supersedes the setting of
AFL_FRIDA_OUTPUT_STDERR- Redirect the standard error of the target application to the named file (supersedes the setting of
AFL_FRIDA_PERSISTENT_DEBUG- Insert a Breakpoint into the instrumented code at
AFL_FRIDA_PERSISTENT_RETto allow the user to detect issues in the persistent loop using a debugger.
AFL_FRIDA_SECCOMP_FILE- Write a log of any syscalls made by the target to the specified file.
AFL_FRIDA_STALKER_ADJACENT_BLOCKS- Configure the number of adjacent blocks to fetch when generating instrumented code. By fetching blocks in the same order they appear in the original program, rather than the order of execution should help reduce locality and adjacency. This includes allowing us to vector between adjacent blocks using a NOP slide rather than an immediate branch.
AFL_FRIDA_STALKER_IC_ENTRIES- Configure the number of inline cache entries stored along-side branch instructions which provide a cache to avoid having to call back into FRIDA to find the next block. Default is 32.
AFL_FRIDA_STATS_FILE- Write statistics information about the code being instrumented to the given file name. The statistics are written only for the child process when new block is instrumented (when the
AFL_FRIDA_STATS_INTERVALhas expired). Note that just because a new path is found does not mean a new block needs to be compiled. It could be that the existing blocks instrumented have been executed in a different order.
AFL_FRIDA_STATS_INTERVAL- The maximum frequency to output statistics information. Stats will be written whenever they are updated if the given interval has elapsed since last time they were written.
AFL_FRIDA_TRACEABLE- Set the child process to be traceable by any process to aid debugging and overcome the restrictions imposed by YAMA. Supported on Linux only. Permits a non-root user to use
gcoreor similar to collect a core dump of the instrumented target. Note that in order to capture the core dump you must set a sufficient timeout (using
-t) to avoid
afl-fuzzkilling the process whilst it is being dumped.
8) Settings for afl-cmin
The corpus minimization script offers very little customization:
AFL_ALLOW_TMPpermits this and some other scripts to run in /tmp. This is a modest security risk on multi-user systems with rogue users, but should be safe on dedicated fuzzing boxes.
AFL_KEEP_TRACESmakes the tool keep traces and other metadata used for minimization and normally deleted at exit. The files can be found in the
AFL_PATHoffers a way to specify the location of afl-showmap and afl-qemu-trace (the latter only in
AFL_PRINT_FILENAMESprints each filename to stdout, as it gets processed. This can help when embedding
afl-showmapin other scripts.
9) Settings for afl-tmin
Virtually nothing to play with. Well, in QEMU mode (
AFL_PATH will be
searched for afl-qemu-trace. In addition to this,
TMPDIR may be used if a
temporary file can’t be created in the current working directory.
You can specify
AFL_TMIN_EXACT if you want afl-tmin to require execution paths
to match when minimizing crashes. This will make minimization less useful, but
may prevent the tool from “jumping” from one crashing condition to another in
very buggy software. You probably want to combine it with the
10) Settings for afl-analyze
You can set
AFL_ANALYZE_HEX to get file offsets printed as hexadecimal instead
11) Settings for libdislocator
The library honors these environment variables:
AFL_ALIGNED_ALLOC=1will force the alignment of the allocation size to
max_align_tto be compliant with the C standard.
AFL_LD_HARD_FAILalters the behavior by calling
abort()on excessive allocations, thus causing what AFL++ would perceive as a crash. Useful for programs that are supposed to maintain a specific memory footprint.
AFL_LD_LIMIT_MBcaps the size of the maximum heap usage permitted by the library, in megabytes. The default value is 1 GB. Once this is exceeded, allocations will return NULL.
calloc()overflows. Most of the common allocators check for that internally and return NULL, so it’s a security risk only in more exotic setups.
AFL_LD_VERBOSEcauses the library to output some diagnostic messages that may be useful for pinpointing the cause of any observed issues.
11) Settings for libtokencap
This library accepts
AFL_TOKEN_FILE to indicate the location to which the
discovered tokens should be written.
12) Third-party variables set by afl-fuzz & other tools
Several variables are not directly interpreted by afl-fuzz, but are set to optimal values if not already present in the environment:
ASAN_OPTIONSare set to (among others):
abort_on_error=1 detect_leaks=0 malloc_context_size=0 symbolize=0 allocator_may_return_null=1
If you want to set your own options, be sure to include
abort_on_error=1- otherwise, the fuzzer will not be able to detect crashes in the tested app. Similarly, include
symbolize=0, since without it, AFL++ may have difficulty telling crashes and hangs apart.
Similarly, the default
LSAN_OPTIONSare set to:
exit_code=23 fast_unwind_on_malloc=0 symbolize=0 print_suppressions=0
Be sure to include the first ones for LSAN and MSAN when customizing anything, since some MSAN and LSAN versions don’t call
abort()on error, and we need a way to detect faults.
In the same vein, by default,
MSAN_OPTIONSare set to:
exit_code=86 (required for legacy reasons) abort_on_error=1 symbolize=0 msan_track_origins=0 allocator_may_return_null=1
LD_BIND_NOWis set to speed up fuzzing by forcing the linker to do all the work before the fork server kicks in. You can override this by setting
LD_BIND_LAZYbeforehand, but it is almost certainly pointless.