docs/env_variables.md
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.
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_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
make
Setting AFL_CC, and AFL_CXX lets you use alternate downstream
compilation tools, rather than the default 'clang', or 'gcc' binaries
in your $PATH.
Setting AFL_COMPILER_LAUNCHER lets you prepend a command to the compiler
invocation. This is useful for tools like ccache that can speed up
compilation by caching object files. For example, setting
AFL_COMPILER_LAUNCHER=ccache will prepend ccache to all compiler calls.
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.
The optimization level can also be set with AFL_OPT_LEVEL, e.g.
AFL_OPT_LEVEL=z for -Oz, default is 3
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. Only LLVM CLASSIC pass can use this.
Setting AFL_INPUT_PLACEHOLDER to a string allows you to use that string
as a placeholder instead of "@@" in the target command line arguments.
Use this when "@@" conflicts with the parameters of your program.
For eg. AFL_INPUT_PLACEHOLDER=NEW_PLACEHOLDER afl-fuzz -i in -o out -- ./targetProgram NEW_PLACEHOLDER
AFL_NO_BUILTIN causes the compiler to generate code suitable for use with
libtokencap.so (but perhaps running a bit slower than without the flag).
AFL_PATH can be used to point a directory that contains LLVM/GCC plugins
for AFL++, AFL++'s runtime objects and QEMU/Frida support files.
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_CFISAN_VERBOSE=1 - outputs detailed information when control flow integrity violations occur, instead of simply terminating with "Illegal Instruction"
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
AFL_UBSAN_VERBOSE=1 - outputs detailed diagnostic information when undefined behavior is detected, instead of simply terminating with "Illegal Instruction"
. AFL_USE_RTSAN . activates the realtime sanitizer (realtime violations in deterministic run time constraints). (clang 20 minimum)
Note: both AFL_CFISAN_VERBOSE=1 and AFL_UBSAN_VERBOSE=1 are disabled by default as verbose output can significantly slow down fuzzing performance. Use these options only during debugging or when additional crash diagnostics are required
AFL_LLVM_ONLY_FSRV/AFL_GCC_ONLY_FSRV will inject forkserver but not pc instrumentation. Please note this is different compared to AFL_LLVM_DISABLE_INSTRUMENTATION, which will totally disable forkserver implementation. This env is pretty useful in two cases:
TMPDIR is used by afl-as for temporary files; if this variable is not set,
the tool defaults to /tmp.
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:
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/README.cmplog.md.
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/README.llvm.md#6) AFL++ Context Sensitive Branch Coverage.
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/README.instrument_list.md.
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/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_LTO_CALLER activates collision free CALLER instrumentationAFL_LLVM_LTO_CALLER sets the maximum number 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_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/README.lto.md.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/README.llvm.md#7) 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.
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).
Setting AFL_LLVM_DENY_EXEC=1 during compilation will cause the instrumented
binary to abort when any exec* family function is called. This is useful to
prevent coverage map corruption that can occur when a target calls exec*
functions, as the exec'd process will inherit the instrumentation but may not
be the intended fuzzing target. Only enable this if your target should never
call exec functions during normal operation.
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/README.instrument_list.md
for more information.
Setting AFL_GCC_DISABLE_VERSION_CHECK=1 will disable the GCC plugin
version check if the target GCC plugin differs from the system-installed
version, resolving issues caused by version mismatches between GCC and
the plugin.
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.
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.)
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.
Setting AFL_ALLOW_CORES will allow writing core files on crashes.
Not recommended unless you have crashes that do not reproduce stand-alone.
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
custom_mutators.md.
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 is basically
when the fuzzing state says state: finished
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.
AFL_FORCE_FASTRESUME on the other hand will load the fast resume file
(if it exists) even if the target binary was changed. Note that the
coverage map size must be exactly the same for this to work.
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 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_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 libdislocator.so.
AFL_SHA1_FILENAMES causes AFL++ to generate files named by the SHA1 hash
of their contents, rather than use the standard id:000000,... names.
Warning: this disables any syncing to any AFL instances!
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 127.0.0.1:8125. 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 rpc_statsd.md.
AFL_SYNC_TIME allows you to specify a different minimal time (in minutes)
between fuzzing instances synchronization. Default sync time is 20 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 binaryAFL_PERSISTENT enforces persistent mode even if none was detected
in the target binaryIf 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.
When using AFL_PRELOAD with a preload that disable fork() calls in
the target, the forkserver becomes unable to fork.
To overcome this issue, the AFL_PRELOAD_DISCRIMINATE_FORKSERVER_PARENT
permits to be able to check in the preloaded library if the environment
variable AFL_FORKSERVER_PARENT is set, to be able to use vanilla
fork() in the forkserver, and the placeholder in the target.
Here is a POC :
// AFL_PRELOAD_DISCRIMINATE_FORKSERVER_PARENT=1 afl-fuzz ...
pid_t fork(void)
{
if (getenv("AFL_FORKSERVER_PARENT") == NULL)
return 0; // We are in the target
else
return real_fork(); // We are in the forkserver
}
AFL_FORKSRV_UID allows you to specify the UID that should be used when
running the fork server. When setting this variable, user should ensure
afl-fuzz binary has enough privileges to modify the UID (e.g. CAP_SETUID
capability in Linux system).
AFL_FORKSRV_GID allows you to specify the GID and the supplementary group
IDs that should be used when running the fork server. When setting this
variable, user should ensure afl-fuzz binary has enough privileges to
modify the GIDs (e.g. CAP_SETGID capability in Linux system).
When both AFL_FORKSRV_UID and AFL_FORKSRV_GID are set, afl-fuzz binary
and the fork server no longer share any IDs. Thus, afl-fuzz binary changes
the group owner of the created files to ensure that the fork server can
still access them. In such case, user should ensure afl-fuzz binary has
enough privileges to modify the ownership of entities (e.g. CAP_CHOWN
capability in Linux system).
Setting AFL_FRAMESHIFT_DISABLE will disable the frameshift analysis stage.
Frameshift automatically discovers size/offset fields in structured inputs
and keeps them consistent as mutations insert or delete bytes. Disabling it
may be useful for targets that do not consume structured binary formats, or
when you want to avoid the overhead of the analysis entirely.
AFL_FRAMESHIFT_MAX_OVERHEAD controls the maximum fraction of total fuzzing
time that frameshift analysis is allowed to consume. The value is a float
between 0.0 and 1.0 (default 0.10, i.e. 10%). If the cumulative time
spent in frameshift analysis exceeds this fraction of the overall run time,
new analyses are skipped until the ratio drops back under the limit.
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.
The QEMU wrapper used to instrument binary-only code supports several settings:
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.
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/README.md.
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. You may
also provide the filename of the library. Excluding ranges takes priority
over any included ranges or AFL_INST_LIBS. See
qemu_mode/README.md#partial-instrumenation).
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.
You can switch to block coverage that has less chances of colliding (but
on the other hand coverage is on blocks, not edges) with
AFL_QEMU_BLOCK_COV.
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.
Using AFL_QEMU_IJON=test.conf allows qemu to call the ijon function, which allows aflpp to additionally gain awareness of changes in key variables of the target program, which is an excellent supplement to coverage-only feedback.(see qemu_mode/README.md for more details).
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.
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/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
support.
AFL_FRIDA_DEBUG_MAPS - See AFL_QEMU_DEBUG_MAPSAFL_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_EXCLUDE_RANGES - See AFL_QEMU_EXCLUDE_RANGESAFL_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_RANGES - See AFL_QEMU_INST_RANGESAFL_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/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_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_ADDR - See AFL_QEMU_PERSISTENT_ADDRAFL_FRIDA_PERSISTENT_CNT - See AFL_QEMU_PERSISTENT_CNTAFL_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_PERSISTENT_HOOK - See AFL_QEMU_PERSISTENT_HOOKAFL_FRIDA_PERSISTENT_RET - See AFL_QEMU_PERSISTENT_RETAFL_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.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.
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.
You can set AFL_ANALYZE_HEX to get file offsets printed as hexadecimal instead
of decimal.
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.
This library accepts AFL_TOKEN_FILE to indicate the location to which the
discovered tokens should be written.
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):
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_OPTIONS are 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_OPTIONS are set to:
exit_code=86 (required for legacy reasons)
abort_on_error=1
symbolize=0
msan_track_origins=0
allocator_may_return_null=1
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.