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Environment variables

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

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.

  • Use the AFL_CC_COMPILER environment variable with MODE. To select MODE, use one of the following values:

    • GCC (afl-gcc/afl-g++)
    • GCC_PLUGIN (afl-g*-fast)
    • LLVM (afl-clang-fast*)
    • LTO (afl-clang-lto*).

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
  • Setting AFL_AS, AFL_CC, and AFL_CXX lets you use alternate downstream compilation tools, rather than the default 'as', 'clang', or 'gcc' binaries in your $PATH.

  • If you are a weird person that wants to compile and instrument asm text files, then use the AFL_AS_FORCE_INSTRUMENT variable: 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_DEBUG environment variable. This is sadly necessary for various build processes which fail otherwise.

  • By default, the wrapper appends -O3 to optimize builds. Very rarely, this will cause problems in programs built with -Werror, because -O3 enables more thorough code analysis and can spew out additional warnings. To disable optimizations, set AFL_DONT_OPTIMIZE. However, if -O... and/or -fno-unroll-loops are set, these are not overridden.

  • Setting AFL_HARDEN automatically adds code hardening options when invoking the downstream compiler. This currently includes -D_FORTIFY_SOURCE=2 and -fstack-protector-all. The setting is useful for catching non-crashing memory bugs at the expense of a very slight (sub-5%) performance loss.

  • Setting AFL_INST_RATIO to 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.)

    Setting AFL_INST_RATIO to 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_BUILTIN causes the compiler to generate code suitable for use with (but perhaps running a bit slower than without the flag).

  • AFL_PATH can 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.)

  • Setting AFL_QUIET will prevent afl-as and afl-cc banners from being displayed during compilation, in case you find them distracting.

  • Setting 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_OFF(); and __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
  • TMPDIR is 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 as.

  • 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.txt which will write all constant string comparisons to this file to be used later with afl-fuzz' -x option.

  • An option to AFL_LLVM_DICT2FILE is AFL_LLVM_DICT2FILE_NO_MAIN=1 which skill not parse main().

  • TMPDIR and AFL_KEEP_ASSEMBLY, since no temporary assembly files are created.

  • LLVM modes compiling C++ will normally set rpath in the binary if LLVM is not in a usual location (/usr or /lib). Setting AFL_LLVM_NO_RPATH=1 disables this behaviour in case it isn't desired. For example, the compiling toolchain might be in a custom location, but the target machine has LLVM runtime libs in the search path.

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.

Available options:

  • 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.: AFL_LLVM_INSTRUMENT=CLASSIC,CTX,NGRAM-4

    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 pcguard based instrumentation (default)


Setting AFL_LLVM_CMPLOG=1 during compilation will tell afl-clang-fast to produce a CmpLog binary.

For afl-gcc-fast, set AFL_GCC_CMPLOG=1 instead.

For more information, see instrumentation/


Setting AFL_LLVM_CTX or 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 collisions occur.

For more information, see instrumentation/ AFL++ Context Sensitive Branch Coverage.

INSTRUMENT LIST (selectively instrument files and functions)

This feature allows selective instrumentation of the source.

Setting AFL_LLVM_ALLOWLIST or 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/


This feature is able to find simple injection vulnerabilities in insecure calls to mysql/mariadb/nosql/postgresql/ldap and XSS in libxml2.

  • Setting AFL_LLVM_INJECTIONS_ALL will enable all injection hooking

  • Setting AFL_LLVM_INJECTIONS_SQL will enable SQL injection hooking

  • Setting AFL_LLVM_INJECTIONS_LDAP will enable LDAP injection hooking

  • Setting AFL_LLVM_INJECTIONS_XSS will enable XSS injection hooking


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. ;-)

  • Setting AFL_LLVM_LAF_TRANSFORM_COMPARES will split string compare functions.

  • Setting AFL_LLVM_LAF_SPLIT_COMPARES will split all floating point and 64, 32 and 16 bit integer CMP instructions.

  • Setting AFL_LLVM_LAF_SPLIT_FLOATS will split floating points, needs AFL_LLVM_LAF_SPLIT_COMPARES to be set.

  • Setting AFL_LLVM_LAF_SPLIT_SWITCHES will split all switch constructs.

  • Setting AFL_LLVM_LAF_ALL sets all of the above.

For more information, see instrumentation/


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_LTO_CALLER activates collision free CALLER instrumentation
  • AFL_LLVM_LTO_CALLER sets the maximum mumber of single block functions to dig deeper into a real function. Default 0.
  • AFL_LLVM_DOCUMENT_IDS=file will 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_DONTWRITEID prevents that the highest location ID written into the instrumentation is set in a global variable.
  • AFL_LLVM_LTO_STARTID sets the starting location ID for the instrumentation. This defaults to 1.
  • AFL_LLVM_MAP_ADDR sets 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_DYNAMIC sets the shared memory address to be dynamic.
  • AFL_LLVM_LTO_SKIPINIT skips adding initialization code. Some global vars (e.g. the highest location ID) are not injected. Needed to instrument with WAFL. For more information, see instrumentation/


Setting AFL_LLVM_INSTRUMENT=NGRAM-{value} or AFL_LLVM_NGRAM_SIZE activates 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 MAP_SIZE_POW2 definition in config.h to at least 18 and maybe up to 20 for this as otherwise too many map collisions occur.

For more information, see instrumentation/ AFL++ N-Gram Branch Coverage.


  • Setting AFL_LLVM_NOT_ZERO=1 during 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.

  • Setting AFL_LLVM_SKIP_NEVERZERO=1 will 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)

Setting 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_ASSEMBLY prevents 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_INSTRUMENT_FILE or AFL_GCC_ALLOWLIST with a filename will only instrument those files that match the names listed in this file (one filename per line).

    Setting AFL_GCC_DENYLIST or AFL_GCC_BLOCKLIST with a file name and/or function will only skip those files that match the names listed in the specified file. See instrumentation/ for more information.

    Setting AFL_GCC_OUT_OF_LINE=1 will instruct afl-gcc-fast to instrument the code with calls to an injected subroutine instead of the much more efficient inline instrumentation.

    Setting AFL_GCC_SKIP_NEVERZERO=1 will not implement the skip zero test. If the target performs only a few loops, then this will give a small performance boost.

4) Runtime settings

The following environment variables are for a compiled AFL++ target.

  • Setting AFL_DUMP_MAP_SIZE when executing the target directly will dump the map size of the target and exit.

  • Setting AFL_OLD_FORKSERVER will use the old AFL vanilla forkserver. This makes only sense when you a) compile in a classic colliding coverage mode (e.g. AFL_LLVM_INSTRUMENT=CLASSIC) or if the map size of the target is below MAP_SIZE (65536 by default), AND b) you want to use this compiled AFL++ target with a different tool that expects vanilla AFL behaviour, e.g. symcc, symqemu, nautilus, etc. You would use this option together with the target fuzzing application.

  • Setting AFL_DISABLE_LLVM_INSTRUMENTATION will disable collecting instrumentation. (More of an internal option.)

5) 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:

  • Setting AFL_AUTORESUME will resume a fuzz run (same as providing -i -) for an existing out folder, even if a different -i was provided. Without this setting, afl-fuzz will refuse execution for a long-fuzzed out dir.

  • Benchmarking only: AFL_BENCH_JUST_ONE causes the fuzzer to exit after processing the first queue entry; and AFL_BENCH_UNTIL_CRASH causes it to exit soon after the first crash is found.

  • AFL_CMPLOG_ONLY_NEW will 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_IGNORE_SEED_PROBLEMS will skip over crashes and timeouts in the seeds instead of exiting.

  • Setting AFL_CRASH_EXITCODE sets the exit code AFL++ treats as crash. For example, if AFL_CRASH_EXITCODE='-1' is set, each input resulting in a -1 return 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.

  • Setting AFL_CUSTOM_MUTATOR_LIBRARY to 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_MODULE to a Python module can also provide additional mutations. If AFL_CUSTOM_MUTATOR_ONLY is 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

  • Setting AFL_CUSTOM_MUTATOR_LATE_SEND will call the afl_custom_fuzz_send() function after the target has been restarted. (This is needed for e.g. TCP services.)

  • Setting AFL_CYCLE_SCHEDULES will switch to a different schedule every time a cycle is finished.

  • Setting AFL_DEBUG_CHILD will 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_DEBUG_CHILD_OUTPUT is deprecated.

  • Setting AFL_DISABLE_TRIM tells afl-fuzz not to trim test cases. This is usually a bad idea!

  • Setting AFL_DISABLE_REDUNDANT disables any queue items that are redundant. This can be useful with huge queues.

  • Setting AFL_KEEP_TIMEOUTS will keep longer running inputs if they reach new coverage

  • On the contrary, if you are not interested in any timeouts, you can set AFL_IGNORE_TIMEOUTS to get a bit of speed instead.

  • AFL_EXIT_ON_SEED_ISSUES will restore the vanilla afl-fuzz behavior which does not allow crashes or timeout seeds in the initial -i corpus.

  • AFL_CRASHING_SEEDS_AS_NEW_CRASH will treat crashing seeds as new crash. these crashes will be written to crashes folder as op:dry_run, and orig:<seed_file_name>.

  • AFL_EXIT_ON_TIME causes 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_DONE causes 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.

  • Setting AFL_EXPAND_HAVOC_NOW will start in the extended havoc mode that includes costly mutations. afl-fuzz automatically enables this mode when deemed useful otherwise.

  • AFL_FAST_CAL keeps the calibration stage about 2.5x faster (albeit less precise), which can help when starting a session against a slow target. AFL_CAL_FAST works too.

  • Setting AFL_FORCE_UI will force painting the UI on the screen even if no valid terminal was detected (for virtual consoles).

  • Setting AFL_FORKSRV_INIT_TMOUT allows you to specify a different timeout to wait for the forkserver to spin up. The specified value is the new timeout, in milliseconds. The default is the -t value times FORK_WAIT_MULT from config.h (usually 10), so for a -t 100, the default would wait for 1000 milliseconds. The AFL_FORKSRV_INIT_TMOUT value does not get multiplied. It overwrites the initial timeout afl-fuzz waits for the target to come up with a constant time. 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.

  • Setting AFL_HANG_TMOUT allows 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 -t parameter, 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 /proc/sys/kernel/core_pattern check.

  • 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 AFL_IGNORE_PROBLEMS. If you additionally want to also ignore coverage from late loaded libraries, you can set AFL_IGNORE_PROBLEMS_COVERAGE.

  • When running with multiple afl-fuzz or with -F, setting AFL_IMPORT_FIRST causes the fuzzer to import test cases from other instances before doing anything else. This makes the "own finds" counter in the UI more accurate.

  • When running with multiple afl-fuzz or with -F, setting AFL_FINAL_SYNC will cause the fuzzer to perform a final import of test cases when terminating. This is beneficial for -M main fuzzers to ensure it has all unique test cases and hence you only need to afl-cmin this single queue.

  • Setting AFL_INPUT_LEN_MIN and AFL_INPUT_LEN_MAX are 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, SIGKILL (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, SIGTERM (AFL_FORK_SERVER_KILL_SIGNAL=15) will be delivered to the fork server. If only AFL_KILL_SIGNAL is provided, AFL_FORK_SERVER_KILL_SIGNAL will be set to same value as AFL_KILL_SIGNAL to provide backward compatibility. If AFL_FORK_SERVER_KILL_SIGNAL is 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_SIZE sets 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.

  • Setting AFL_MAX_DET_EXTRAS will change the threshold at what number of elements in the -x dictionary 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 200 elements. So for the 200 + 1st element, there is a 1 in 201 chance, that one of the dictionary entries will not be used directly.

  • Setting AFL_NO_AFFINITY disables 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_ARITH causes AFL++ to skip most of the deterministic arithmetics. This can be useful to speed up the fuzzing of text-based file formats.

  • Setting AFL_NO_AUTODICT will not load an LTO generated auto dictionary that is compiled into the target.

  • Setting AFL_NO_COLOR or AFL_NO_COLOUR will 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_CPU_RED.

  • Setting AFL_NO_FORKSRV disables 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_SNAPSHOT will advise afl-fuzz not to use the snapshot feature if the snapshot lkm is loaded.

  • AFL_NO_FASTRESUME will not try to read or write a fast resume file.

  • Setting AFL_NO_UI inhibits 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.

  • Setting AFL_NO_STARTUP_CALIBRATION will skip the initial calibration of all starting seeds, and start fuzzing at once. Use with care, this degrades the fuzzing performance!

  • Setting AFL_NO_WARN_INSTABILITY will suppress instability warnings.

  • In QEMU mode (-Q) and FRIDA mode (-O), AFL_PATH will be searched for afl-qemu-trace and

  • If you are using persistent mode (you should, see instrumentation/, 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_RECORD variable 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!

  • Note that AFL_POST_LIBRARY is deprecated, use AFL_CUSTOM_MUTATOR_LIBRARY instead.

  • Setting AFL_PRELOAD causes AFL++ to set LD_PRELOAD for the target binary without disrupting the afl-fuzz process itself. This is useful, among other things, for bootstrapping

  • In QEMU mode (-Q), setting AFL_QEMU_CUSTOM_BIN will cause afl-fuzz to skip prepending afl-qemu-trace to 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_SHA1_FILENAMES causes AFL++ to generate files named by the SHA1 hash of their contents, rather than use the standard id:000000,... names.

  • AFL_SHUFFLE_QUEUE randomly 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_CHECK to inhibit the checks for non-instrumented binaries and shell scripts; and AFL_DUMB_FORKSRV in conjunction with the -n setting 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.

  • Setting AFL_SKIP_CPUFREQ skips 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.

  • Setting AFL_STATSD enables StatsD metrics collection. By default, AFL++ will send these metrics over UDP to The host and port are configurable with AFL_STATSD_HOST and AFL_STATSD_PORT respectively. To enable tags (banner and afl_version), you should provide AFL_STATSD_TAGS_FLAVOR that matches your StatsD server (see AFL_STATSD_TAGS_FLAVOR).

  • Setting AFL_STATSD_TAGS_FLAVOR to one of dogstatsd, influxdb, librato, or signalfx allows you to add tags to your fuzzing instances. This is especially useful when running multiple instances (-M/-S for example). Applied tags are banner and afl_version. banner corresponds to the name of the fuzzer provided through -M/-S. afl_version corresponds 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

  • AFL_SYNC_TIME allows 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_NO_SYNC disables any syncing whatsoever and takes priority on all other syncing parameters.

  • Setting AFL_TARGET_ENV causes 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_PATH but 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_ENV environment variables will apply to QEMU, as well as the target binary. Therefore, in this case, you might want to use QEMU's QEMU_SET_ENV environment 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_SIZE allows you to override the size of #define TESTCASE_CACHE in config.h. Recommended values are 50-250MB - or more if your fuzzing finds a huge amount of paths for large inputs.

  • AFL_TMPDIR is used to write the .cur_input file 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.

  • Setting AFL_TRY_AFFINITY tries 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_FORKSRV enforces a deferred forkserver even if none was detected in the target binary
    • AFL_PERSISTENT enforces 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 :-)

  • Set AFL_PIZZA_MODE to 1 to enable the April 1st stats menu, set to -1 to disable although it is 1st of April. 0 is the default and means enable on the 1st of April automatically.

  • If you need a specific interval to update fuzzer_stats file, you can set AFL_FUZZER_STATS_UPDATE_INTERVAL to the interval in seconds you'd the file to be updated. Note that will not be exact and with slow targets it can take seconds until there is a slice for the time test.

6) Settings for afl-qemu-trace

The QEMU wrapper used to instrument binary-only code supports several settings:

  • Setting AFL_COMPCOV_LEVEL enables 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/

    There are two levels at the moment, AFL_COMPCOV_LEVEL=1 that instruments only comparisons with immediate values / read-only memory and AFL_COMPCOV_LEVEL=2 that instruments all the comparisons. Level 2 is more accurate but may need a larger shared memory.

  • AFL_DEBUG will 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-qemu-trace ./program.

  • AFL_ENTRYPOINT allows 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.

  • Setting AFL_INST_LIBS causes the translator to also instrument the code inside any dynamically linked libraries (notably including glibc).

  • You can use AFL_QEMU_INST_RANGES=0xaaaa-0xbbbb,0xcccc-0xdddd to just instrument specific memory locations, e.g. a specific library. Excluding ranges takes priority over any included ranges or AFL_INST_LIBS.

  • You can use AFL_QEMU_EXCLUDE_RANGES=0xaaaa-0xbbbb,0xcccc-0xdddd to NOT instrument specific memory locations, e.g. a specific library. Excluding ranges takes priority over any included ranges or AFL_INST_LIBS.

  • It is possible to set AFL_INST_RATIO to skip the instrumentation on some of the basic blocks, which can be useful when dealing with very complex binaries.

  • Setting AFL_QEMU_COMPCOV enables the CompareCoverage tracing of all cmp and sub in x86 and x86_64. This is an alias of AFL_COMPCOV_LEVEL=1 when AFL_COMPCOV_LEVEL is not specified.

  • With 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_ADDR=start addr.

  • With 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 addr environment variable. With this variable assigned, instead of patching the return address, the specified instruction is transformed to a jump towards start addr.

  • With AFL_QEMU_PERSISTENT_RETADDR_OFFSET, you can specify the offset from the stack pointer in which QEMU can find the return address when start addr is hit.

  • With 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.

  • Normally a README.txt is written to the crashes/ directory when a first crash is found. Setting AFL_NO_CRASH_README will prevent this. Useful when counting crashes based on a file count in that directory.

8) Settings for afl-frida-trace

The FRIDA wrapper used to instrument binary-only code supports many of the same options as afl-qemu-trace, but also has a number of additional advanced options. These are listed in brief below (see frida_mode/ for more details). These settings are provided for compatibility with QEMU mode, the preferred way to configure FRIDA mode is through its scripting support.

  • AFL_FRIDA_DRIVER_NO_HOOK - See AFL_QEMU_DRIVER_NO_HOOK. When using the QEMU driver to provide a main loop for a user provided LLVMFuzzerTestOneInput, this option configures the driver to read input from stdin rather 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_DYNAMIC_LOAD - Don't instrument the code loaded late at runtime. Strictly limits instrumentation to what has been included.
  • AFL_FRIDA_INST_NO_OPTIMIZE - Don't use optimized inline assembly coverage instrumentation (the default where available). Required to use AFL_FRIDA_INST_TRACE.
  • 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.
  • 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_NO_OPTIMIZE.
  • AFL_FRIDA_INST_TRACE_UNIQUE - As per AFL_FRIDA_INST_TRACE, but each edge is logged only once, requires AFL_FRIDA_INST_NO_OPTIMIZE.
  • 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/ for details.
  • AFL_FRIDA_OUTPUT_STDOUT - Redirect the standard output of the target application to the named file (supersedes the setting of AFL_DEBUG_CHILD)
  • AFL_FRIDA_OUTPUT_STDERR - Redirect the standard error of the target application to the named file (supersedes the setting of AFL_DEBUG_CHILD)
  • AFL_FRIDA_PERSISTENT_DEBUG - Insert a Breakpoint into the instrumented code at AFL_FRIDA_PERSISTENT_HOOK and AFL_FRIDA_PERSISTENT_RET to 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_INTERVAL has 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 gcore or 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-fuzz killing the process whilst it is being dumped.

9) Settings for afl-cmin

The corpus minimization script offers very little customization:

  • AFL_ALLOW_TMP permits 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_TRACES makes the tool keep traces and other metadata used for minimization and normally deleted at exit. The files can be found in the <out_dir>/.traces/ directory.

  • Setting AFL_PATH offers a way to specify the location of afl-showmap and afl-qemu-trace (the latter only in -Q mode).

  • AFL_PRINT_FILENAMES prints each filename to stdout, as it gets processed. This can help when embedding afl-cmin or afl-showmap in other scripts.

10) Settings for afl-tmin

Virtually nothing to play with. Well, in QEMU mode (-Q), 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 -e flag.

11) Settings for afl-analyze

You can set AFL_ANALYZE_HEX to get file offsets printed as hexadecimal instead of decimal.

12) Settings for libdislocator

The library honors these environment variables:

  • AFL_ALIGNED_ALLOC=1 will force the alignment of the allocation size to max_align_t to be compliant with the C standard.

  • AFL_LD_HARD_FAIL alters 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_MB caps 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.

  • AFL_LD_NO_CALLOC_OVER inhibits abort() on 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_VERBOSE causes the library to output some diagnostic messages that may be useful for pinpointing the cause of any observed issues.

13) Settings for libtokencap

This library accepts AFL_TOKEN_FILE to indicate the location to which the discovered tokens should be written.

14) 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:

  • By default, ASAN_OPTIONS are set to (among others):


    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_OPTIONS are set to:


    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_OPTIONS are set to:

    exit_code=86 (required for legacy reasons)
  • By default, LD_BIND_NOW is 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_LAZY beforehand, but it is almost certainly pointless.