CERT Basic Fuzzing Framework (BFF) 2.7

Change Log

See BFF 2.7 Release Notes

Requirements

• UbuFuzz requires VMWare Workstation 7 or later, or compatible virtualization software.
• The OS X installer requires Snow Leopard or later. 
  • Xcode Tools is recommended, which provides libgmalloc (Guard Malloc).

Demo Quick Start

UbuFuzz

1. Unzip BFF-2.7.zip to c:\fuzz or another folder to be shared as "fuzz"
2. Unzip UbuFuzz-2.7.zip
3. Open UbuFuzz.vmx
4. Create a snapshot in VMWare
5. Power on the VM

If you do not wish to use a shared folder, simply remove ~/bff
and unzip BFF-2.7.zip to the ~/bff directory.

OS X

1. Install BFF
2. Run BFF
3. Run ./batch.sh

To check the progress of the fuzzing campaign, run ~/bff/quickstats.sh

Look in ~/results to investigate the fuzzing results.

To use BFF without UbuFuzz

See BFF 2.7 Installation Notes

Additional information

The configuration for VMWare may prevent virtual machines from utilizing shared folders by default. You may need to manually enable shared folders for the VM after opening the VMX. If you chose to unzip scripts.zip to a folder other than c:\fuzz, then you will need to modify the properties of the shared folder in VMWare to point to the new location of the files. Alternatively, if you may unzip the BFF scripts into ~/bff if you do not wish to use a shared folder.

The fuzzing virtual machine is preconfigured to automatically begin a fuzzing run on several image format decoders provided by ImageMagick's "convert" program. An old (5.2.0) version of
ImageMagick is preloaded onto the VM. ImageMagick was built with debug symbols as well as non-optimized. This makes gdb provide more useful crash reports. ImageMagick was configured using the following command:

CFLAGS="-g -O0" ./configure --without-x

Analyzing results

When the fuzzing run encounters a crash, BFF will analyze the details of the crash. This involves capturing stderr, gdb, valgrind, and callgrind output. The gdb output contains several pieces of information, including the memory map, signal information, backtrace, registers, disassembly, as well as output from the CERT Triage Tools, which indicates possible exploitability of the crash. On the OS X platform, CrashWrangler is used instead of gdb. By looking at the backtrace, BFF will keep track of which test cases cause unique crashes. Each unique crash will be placed in the configured output directory.

results/
|-- bff.cfg
|-- bff.log
|-- crashers
| |-- <crash_id>
| | |-- <crash_id>.log
| | |-- <analysis_tools>.stderr
| | |-- minimizer_log.txt
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>.callgrind
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>.callgrind.annotated
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>.callgrind.calltree
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>.gdb
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>.stderr
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>-minimized.<seedfile_ext>.valgrind
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>.<seedfile_ext>
| | |-- sf_<seedfile_md5>-<zzuf_seed_number>.<seedfile_ext>.gdb
| | `-- sf_<seedfile_md5>.<seedfile_ext>
|-- seeds
| |-- <seedfile1_md5>
| | `-- zzuf_log.txt
| |-- <seedfile2_md5>
| | `-- zzuf_log.txt
| |-- seedfile_set.log
| |-- sf_<seedfile1_md5>.<seedfile1_ext>
| |-- sf_<seedfile2_md5>.<seedfile2_ext>
`-- uniquelog.txt

The UbuFuzz startup script, batch.sh, looks for the BFF code in /home/fuzz/bff, which by default is a soft link to the VMWare shared folder /mnt/hgfs/fuzz. The default results location is /home/fuzz/results, which in the UbuFuzz vm is a soft link to /home/fuzz/bff/results (thus /mnt/hgfs/fuzz/results). Either of these soft links can be changed if desired, or you can edit conf.d/bff.cfg to point BFF at a different destination.

BFF will copy its configuration to results/bff.cfg, and will log messages of level INFO or higher into results/bff.log. The config file copied here is just for recording purposes, another copy is made in /home/fuzz and it is this copy of the file that actually gets used by BFF.

Additionally the following subdirectories are created in the results dir:

• crashers: Contains a subdir for each uniquely-crashing test case and its analyzed results
• seeds: Contains the original seedfiles as well as logs specific to that seedfile

Other files of note:

• results/uniquelog.txt – a log file that tracks the unique crashers found during the run

The "results/crashers" directory will contain the uniquely-crashing
test cases. The variants that have crashed the target application
will be stored here with the zzuf seed number appended to the
seed file name. For each uniquely-crashing case, there will also
be a .stderr, .gdb, .callgrind and .valgrind file that contains
the stderr, gdb, callgrind and valgrind output for that case,
respectively.

The CERT BFF minimizes crashers. In other words, each crashing
test case will have a number of bytes that have been modified
from the seed file. When a crasher is minimized, a test case is
generated with a minimal number of bytes that have changed from
the seed file. The minimized test case will have "-minimized"
inserted into the filename before the extension. Also, a
minimizer_log.txt file is created containing a log of the
minimization process. This is used by tools/minimizer_plot.py
to produce a chart showing the minimizer progress.

Aside from minimizing to the seed file, BFF also includes the
ability to minimize a crashing testcase to a string pattern.
This will show which bytes of a file may be altered without
affecting the crash. Due to performance issues, the
minimize-to-string option is disabled by default. However, the
minimizer can be run in a standalone mode.

Say you have a crashing test case, but you really need to get it to a
proof-of-concept exploit. The problem is when you load the crash into your
debugger you can't easily tell which registers, stack values, or memory
locations are under your control. But what if you could change the crashing
test case so that it had only the bytes required to cause that crash, and the
rest were all masked out with a fixed value, say "x" (0x78)? Then you'd know
that if you saw EIP=0x78787878, you may already be a winner. The
minimize-to-string option does just that.
To get command-line usage of the minimizer, run:
tools\minimize.py --help

To minimize a crashing testcase to the Metasploit string pattern, run:
tools\minimize.py --stringmode <crashing_testcase>

When minimizing to the Metasploit pattern, FOE will use the resulting byte map
to create an additional minimized file that uses a string of 'x' characters.
Note that this file is not guaranteed to produce the same crash as the
original string minimization.

Metasploit pattern enumeration:
Especially with larger files, you may notice that the Metasploit pattern
repeats several times over the length of a Metasploit-minimized crasher.
Given any particular dword, it may not be obvious which instance is the one
that you are dealing with. This is where the tools\mtsp_enum.py script comes
in handy. For example, let's say that you have a crasher.doc were EIP = "Aa0A"
If you run: tools\mtsp_enum.py Aa0A crasher.doc
You will end up with a file called crasher-enum.doc. With this file, every
instance of the byte pattern "Aa0A" will be replaced with a unique,
incrementing replacement. For example, "0a0A", "1a0A", "2a0A", etc. Now when
you open crasher-enum.doc, you could for example get EIP = "5a0A". If you
search for that pattern in the file, there should be only once instance of it.
Note that you can use a search pattern of any length and you can also search
for hex values. For example: "\x01\x02\x03\x04"

Analysis tools

The analysis directory contains a few tools for analyzing the
results of a fuzz run.
Try 'python <script> --help' for detailed usage options.

tools/bff_stats.py generates a concise summary of the fuzz run
results so far, including how many times each unique crash was
seen, the first seed number it was seen at, the most recent seed
number it was seen, and the bitwise and bytewise Hamming Distance
from the original seedfile for the minimized testcase.

tools/callsim.py can display crashes clustered by the
similarity of their called functions. This analysis is based on
the idea that crashes with similar call history are likely to be
related even if they result in unique crash hashes. The resulting
clusters of crashes can be useful in deciding which crashes to
investigate first given a large number of crashes. The source
data for this analysis is callgrind output generated for each
crash.

tools/create_crasher_script.py will generate a shell script
that in turn can be used to regenerate all the test cases for a
given crash id. The use of the '--destination' option is
highly recommended.

tools/minimizer_plot.py plots the minimizer data for a given
crash, showing how the minimizer tunes its parameters as it
progresses in order to find the optimal minimized test case.

tools/drillresults.py will search for crashing test cases
that are more easily exploitable than the others. It searches
based on the type of crash as well as whether the faulting
address matches patterns in the fuzzed file.

tools/repro.py will launch the specified application using the
same command-line parameters as configure for the fuzzing
campaign. This can be used to test crashing testcases
interactively.

===== Fuzzing on your own =====

When the UbuFuzz VM is powered on, it will automatically
execute the batch.sh script in the VMWare shared folder. In order
to power on the virtual machine without it beginning a fuzzing
campaign, you should rename batch.sh. This will allow you to
power on the virtual machine to install the target software. Once
BFF has started a fuzzing run, it will copy bff.cfg to the
/home/fuzz directory in the virtual machine. This configuration
file will be used for subsequent fuzzing runs, rather than the
copy in the shared folder. This is why it is important to make
a snapshot of the VM in its clean state. If you wish to reset
a fuzzing machine to a clean state, e.g. to start a new fuzzing
campaign or if you've change fuzzing parameters or seed files,
you should run the ~/bff/reset_bff.sh script.

The first step to beginning a fuzzing run is to obtain and
install the target software. Usually this process will involve
downloading the application source code and compiling a debug
version of it. Rather than using Ubuntu's package repository,
this will ensure that you will be fuzzing the latest version of
the target application. It will also give you the ability to have
debug symbols, as well as the ability to use a non-optimized
build if you like.

Create a new snapshot of the VM after the target software has
been installed.

The bff.cfg file contains all of the parameters for the fuzzing
run. This file must be edited to suit the software that you will
be fuzzing. The bff.cfg file is annotated and should be
relatively self-explanatory.

The default bff.cfg file will start a fuzzing run that invokes:
convert $SEEDFILE /dev/null

This will use ImageMagick's "convert" program to process the seed
file, outputting to /dev/null. Because BFF will mangle the seed
file, this fuzzing run will exercise ImageMagick's decoding
capabilities.

You should choose a command line that efficiently exercies the
capabilities of the application that you wish to test. Ideally,
this will involve a command-line application that terminates
immediately upon completion. In such cases, the fuzzing run will
be CPU-bound, as BFF will invoke the application as fast as it
can. Applications that have a GUI can still be fuzzed using this
framework. However, in most cases, each invocation of the target
application will need to wait for the specified timeout to elapse
before the process is terminated.

BFF analyzes gdb (or CrashWrangler on OS X) backtraces to
determine uniquely-crashing testcases. The default setting is to
hash the last five code locations in the backtrace to determine a
hash for the crash. We have found this value to be effective for
most applications, however you can adjust the value to fit your
needs.

Another important bff.cfg option is the "copymode" setting. By
default, zzuf will use LD_PRELOAD to hook into an application to
perform input mangling and intercept signals. In some cases, the
target application will not behave properly in this mode. The
file may fail to be mangled, or the target application may fail
to run properly at all. If this is the case, set "copymode" to 1
and zzuf will create a temporary file and then open that file
with the target application. Most OS X applications require copy
mode, so the default bff.cfg provided on OS X will default to this
mode.

There are several options within bff.cfg related to application
timeouts. "progtimeout" is the maximum time that BFF will allow
a single invocation of the target application to run before
terminating it. In the case of the "convert" component of
ImageMagick, it is reasonable to expect the program to finish
within a few seconds. Depending on the application you are
fuzzing, this value will need to be adjusted. Especially with GUI
applications, the goal is to allow the application to run long
enough to process the input file, but not so long that the
application is sitting idle for an amount of time for each
invocation. The "killprocname" and "killproctimeout" options are
used for the external process killer. When running some analyzers,
the application that you are fuzzing may be left running after
the analyzer is terminated. The external process killer
(killproc.sh) is a script that polls running processes and makes
sure that no single instance of the specified application is
allowed to run for longer than the specified time.

Once you have configured the options in bff.cfg, power on the
UbuFuzz VM. It should begin fuzzing automatically. If the
target is a GUI application, you should see the application
launch repeatedly as it is being fuzzed. If it is a command-line
application, you should notice increased CPU usage in the top
window. The first seed_interval that BFF executes will also
display stderr to the terminal window to let you see if something
is obviously wrong. If you need to tweak your settings, such as
by using a smaller progtimeout value to improve throughput, you
can edit the bff.cfg file and restore the VM to its previous
snapshot. Alternatively, you can run ~/bff/reset_bff.sh and
restart X to cause the VM to re-read the shared bff.cfg file and
start fuzzing again.