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<!DOCTYPE html>
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<title>Fuzzing with Fandango — Fuzzing with Fandango</title>
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Fuzzing with Fandango
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<div id="jb-print-docs-body" class="onlyprint">
<h1>Fuzzing with Fandango</h1>
<!-- Table of contents -->
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<div>
<h2> Contents </h2>
</div>
<nav aria-label="Page">
<ul class="visible nav section-nav flex-column">
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#creating-a-name-database">Creating a Name Database</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#feeding-inputs-into-programs">Feeding Inputs Into Programs</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#feed-inputs-into-a-single-file">Feed Inputs into a Single file</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#feed-inputs-into-individual-files">Feed Inputs into Individual Files</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#invoking-programs-directly">Invoking Programs Directly</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#passing-inputs-on-the-command-line">Passing Inputs on the Command Line</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#passing-inputs-via-standard-input">Passing Inputs via Standard Input</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#calling-a-libfuzzer-style-harness-directly">Calling a libFuzzer style harness directly</a></li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#using-fandango-in-libafl">Using Fandango in LibAFL</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#executable-fan-files">Executable <code class="docutils literal notranslate"><span class="pre">.fan</span></code> files</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#running-fuzzing-campaigns">Running Fuzzing Campaigns</a></li>
</ul>
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</div>
<div id="searchbox"></div>
<article class="bd-article">
<section class="tex2jax_ignore mathjax_ignore" id="fuzzing-with-fandango">
<span id="sec-fuzzing"></span><h1>Fuzzing with Fandango<a class="headerlink" href="#fuzzing-with-fandango" title="Link to this heading">#</a></h1>
<section id="creating-a-name-database">
<h2>Creating a Name Database<a class="headerlink" href="#creating-a-name-database" title="Link to this heading">#</a></h2>
<p>Let us now come up with an input that is slightly more complex.
We want to create a set of inputs with <em>names of persons</em> and their respective <em>age</em>.
These two values would be <em>comma-separated</em>, such that typical inputs would look like this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Alice</span> <span class="n">Doe</span><span class="p">,</span><span class="mi">27</span>
<span class="n">John</span> <span class="n">Smith</span><span class="p">,</span><span class="mi">45</span>
<span class="o">...</span>
</pre></div>
</div>
<p>This makes the overall format of our input look like this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">start</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">person_name</span><span class="o">></span> <span class="s2">","</span> <span class="o"><</span><span class="n">age</span><span class="o">></span>
</pre></div>
</div>
<p>with <code class="docutils literal notranslate"><span class="pre"><age></span></code> again being a sequence of digits, and a <code class="docutils literal notranslate"><span class="pre"><person></span></code>’s name being defined as</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">person_name</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">first_name</span><span class="o">></span> <span class="s2">" "</span> <span class="o"><</span><span class="n">last_name</span><span class="o">></span>
</pre></div>
</div>
<p>where both first and last name would be a sequence of letters - first, an uppercase letter, and then a sequence of lowercase letters.
The full definition looks like this:</p>
<aside class="margin sidebar">
<p class="sidebar-title"></p>
<p>In Fandango specs, symbol names are formed like identifiers in Python - that is, they consist of letters, underscores, and digits.</p>
</aside>
<aside class="margin sidebar">
<p class="sidebar-title"></p>
<p>Use <a class="reference internal" href="Regexes.html#sec-regexes"><span class="std std-ref">regular expressions</span></a> to specifying character ranges.</p>
</aside>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <person_name> "," <age>
<person_name> ::= <first_name> " " <last_name>
<first_name> ::= <name>
<last_name> ::= <name>
<name> ::= <ascii_uppercase_letter><ascii_lowercase_letter>+
<age> ::= <digit>+
</pre></div>
</div>
</div>
</div>
<p>The symbols <code class="docutils literal notranslate"><span class="pre"><ascii_uppercase_letter></span></code>, <code class="docutils literal notranslate"><span class="pre"><ascii_lowercase_letter></span></code>, and <code class="docutils literal notranslate"><span class="pre"><digits></span></code> are predefined in the <a class="reference internal" href="Stdlib.html#sec-stdlib"><span class="std std-ref">Fandango Standard Library</span></a>; they are defined exactly as would be expected.</p>
<p>Create or download a file <a class="reference download internal" download="" href="_downloads/eb75cca3f691010aefb22548a3d912ba/persons.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">persons.fan</span></code></span></a> and run Fandango on it:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span>
</pre></div>
</div>
<p>Your output will look like this:</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Rwm Cult,845776327191306
Jppljtfqbmuk Acq,820898251675
Ikjnmuvuuhwkxsvvdcugn Ynbmsrpzytbo,91036973440310
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Lskfyc Yqcitmjdrwyqac,19759894362155
Qzsjmzjbffqfedw Gcfeqgihpqfyxanipub,318741314873532510
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Ermmbqjmp Ythkkagmewsxkgwzmg,916094413
Qwficiyadgkcqjjua Ikedajvnanstbinpndrk,2146045994580082
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Mfzbuxoozgtmmxnjkl Pypgtjnwtbalzyesmicb,7249504
Anvysiqxtnks Lscdfarskacd,830527074319
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Wxdcbxzzcgjrwooy Dnhmqemutgokqlveiu,87636848402
</pre></div>
</div>
</div>
</div>
<p>Such random names are a typical result of <em>fuzzing</em> – that is, testing with randomly generated values.
How do we get these into a program under test?</p>
</section>
<section id="feeding-inputs-into-programs">
<h2>Feeding Inputs Into Programs<a class="headerlink" href="#feeding-inputs-into-programs" title="Link to this heading">#</a></h2>
<p>So far, we have had Fandango simply <em>output</em> the strings it generates.
This is nice for understanding and debugging, but not necessarily useful for processing these inputs further.
Fandango provides a number of means to help you process its output.</p>
</section>
<section id="feed-inputs-into-a-single-file">
<h2>Feed Inputs into a Single file<a class="headerlink" href="#feed-inputs-into-a-single-file" title="Link to this heading">#</a></h2>
<p>Fandango has a <code class="docutils literal notranslate"><span class="pre">-o</span></code> option that directs its output into a single file.
To store the generated strings in a file named <code class="docutils literal notranslate"><span class="pre">persons.txt</span></code>, use <code class="docutils literal notranslate"><span class="pre">-o</span> <span class="pre">persons.txt</span></code>:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span><span class="w"> </span>-o<span class="w"> </span>persons.txt
</pre></div>
</div>
<p>By default, the generated strings are separated by newlines.
With the <code class="docutils literal notranslate"><span class="pre">-s</span></code> option, you can define an alternate separator.
To have the outputs generated separated by <code class="docutils literal notranslate"><span class="pre">:</span></code>, for instance, use</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span><span class="w"> </span>-o<span class="w"> </span>persons.txt<span class="w"> </span>-s<span class="w"> </span><span class="s1">':'</span>
</pre></div>
</div>
</section>
<section id="feed-inputs-into-individual-files">
<h2>Feed Inputs into Individual Files<a class="headerlink" href="#feed-inputs-into-individual-files" title="Link to this heading">#</a></h2>
<p>With the <code class="docutils literal notranslate"><span class="pre">-d</span></code> option, Fandango can store its strings into individual files in the directory given after <code class="docutils literal notranslate"><span class="pre">-d</span></code>.
They can then be picked up for post-processing.
To store all outputs in a directory named <code class="docutils literal notranslate"><span class="pre">persons</span></code>, for instance, use <code class="docutils literal notranslate"><span class="pre">-d</span> <span class="pre">persons</span></code>:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span><span class="w"> </span>-d<span class="w"> </span>persons
</pre></div>
</div>
<p>Fandango will create the directory and store the inputs as <code class="docutils literal notranslate"><span class="pre">Fandango-0001.txt</span></code>, <code class="docutils literal notranslate"><span class="pre">Fandango-0002.txt</span></code>, etc.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>If the directory is already present, Fandango will use that directory.</p>
</div>
<div class="admonition caution">
<p class="admonition-title">Caution</p>
<p>Fandango will overwrite files <code class="docutils literal notranslate"><span class="pre">Fandango-0001.txt</span></code>, <code class="docutils literal notranslate"><span class="pre">Fandango-0002.txt</span></code>, etc., but leave other files in the directory unchanged.</p>
</div>
<p>If you want a different file extension (for instance, because <code class="docutils literal notranslate"><span class="pre">.txt</span></code> is not suitable), Fandango provides a <code class="docutils literal notranslate"><span class="pre">--filename-extension</span></code> option to set a different one.</p>
</section>
<section id="invoking-programs-directly">
<span id="sec-invoke-targets"></span><h2>Invoking Programs Directly<a class="headerlink" href="#invoking-programs-directly" title="Link to this heading">#</a></h2>
<p>You can have Fandango invoke <em>programs directly</em>, and have Fandango feed them the generated inputs.
The programs are specified as arguments on the command line.</p>
<p>There are three ways to pass the input into programs, on the command line, via standard input, and in-process to a shared object with a libFuzzer style harness.</p>
<p>Refer to <a class="reference external" href="https://github.com/fandango-fuzzer/fandango/tree/main/evaluation/experiments/libfuzzer-harness">this example</a> for further details on how a harness can be compiled to interface with each mode.</p>
<section id="passing-inputs-on-the-command-line">
<h3>Passing Inputs on the Command Line<a class="headerlink" href="#passing-inputs-on-the-command-line" title="Link to this heading">#</a></h3>
<p>When Fandango invokes programs, you can pass input as a file name as last argument on the program’s command line.
This is the default, and can also be obtained with the <code class="docutils literal notranslate"><span class="pre">--input-method=filename</span></code> option.</p>
<p>For instance, to test the <code class="docutils literal notranslate"><span class="pre">wc</span></code> program with its own <code class="docutils literal notranslate"><span class="pre">-c</span></code> option and the inputs Fandango generates, use</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span><span class="w"> </span>wc<span class="w"> </span>-c
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">wc</span></code> program is then invoked as <code class="docutils literal notranslate"><span class="pre">wc</span> <span class="pre">-c</span> <span class="pre">FILE_1</span></code>, <code class="docutils literal notranslate"><span class="pre">wc</span> <span class="pre">-c</span> <span class="pre">FILE_2</span></code>, etc., where each <code class="docutils literal notranslate"><span class="pre">FILE</span></code> contains an individual input from Fandango.</p>
</section>
<section id="passing-inputs-via-standard-input">
<h3>Passing Inputs via Standard Input<a class="headerlink" href="#passing-inputs-via-standard-input" title="Link to this heading">#</a></h3>
<p>As an alternative, Fandango can pass inputs via standard input.
For this, use the <code class="docutils literal notranslate"><span class="pre">--input-method=stdin</span></code> option.</p>
<p>For instance, to test the <code class="docutils literal notranslate"><span class="pre">cat</span></code> program with its own <code class="docutils literal notranslate"><span class="pre">-n</span></code> option and the inputs Fandango generates, use</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span><span class="w"> </span>--input-method<span class="o">=</span>stdin<span class="w"> </span>cat<span class="w"> </span>-n
</pre></div>
</div>
<p>The <code class="docutils literal notranslate"><span class="pre">cat</span></code> program is then invoked repeatedly, each time passing a new Fandango-generated input as its standard input.</p>
</section>
<section id="calling-a-libfuzzer-style-harness-directly">
<span id="sec-libfuzzer"></span><h3>Calling a libFuzzer style harness directly<a class="headerlink" href="#calling-a-libfuzzer-style-harness-directly" title="Link to this heading">#</a></h3>
<p>Fandango further supports calling a libFuzzer style harness directly. If you are able to compile your binary to a shared object (<code class="docutils literal notranslate"><span class="pre">.so</span></code> on Linux or <code class="docutils literal notranslate"><span class="pre">.dylib</span></code> on macOS), Fandango can load the binary and directly call the function. This requires some extra work initially but removes the need to start a new process for each input evaluation, thus improving performance significantly. Fandango will exit on the first crashing input. <code class="docutils literal notranslate"><span class="pre">--input-method=libfuzzer</span></code> is only supported in combination with <code class="docutils literal notranslate"><span class="pre">--file-mode=binary</span></code>, since libFuzzer style harnesses expect binary data.</p>
<p>Fandango is invoked as follows:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ fandango fuzz -f persons.fan -n 10 --input-method=libfuzzer --file-mode=binary ./harness.{so,dylib}
</pre></div>
</div>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>If your target crashes when using the libFuzzer interface, the fuzzer process will exit as well, losing the input if not otherwise specified. Consider using the <a class="reference internal" href="#sec-libafl"><span class="std std-ref">LibAFL integration</span></a> for much more robust target and fuzzing process handling.</p>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The libFuzzer interface is untested on Windows so far due to a lack of hardware. If you would like to test it, try un-skipping <code class="docutils literal notranslate"><span class="pre">test_output_with_libfuzzer_harness</span></code> in <code class="docutils literal notranslate"><span class="pre">test_cli.py</span></code>. You may need to adjust how the C file is compiled.</p>
</div>
</section>
</section>
<section id="using-fandango-in-libafl">
<span id="sec-libafl"></span><h2>Using Fandango in LibAFL<a class="headerlink" href="#using-fandango-in-libafl" title="Link to this heading">#</a></h2>
<p>With libafl-fandango-pyo3 (<a class="reference external" href="https://crates.io/crates/libafl-fandango-pyo3">crates.io</a>, <a class="reference external" href="https://github.com/riesentoaster/libafl-fandango-pyo3">repository</a>), it is possible to use Fandango in conjunction with the <a class="reference external" href="https://github.com/aflplusplus/libafl">LibAFL</a> ecosystem. This makes target handling and evaluation much more flexible.</p>
<p>Refer to the <a class="reference external" href="https://github.com/riesentoaster/libafl-fandango-pyo3">documentation of libafl-fandango-pyo3</a> for instructions on how to use it and a couple of example fuzzers showing how to integrate it into LibAFL-based fuzzers.</p>
</section>
<section id="executable-fan-files">
<h2>Executable <code class="docutils literal notranslate"><span class="pre">.fan</span></code> files<a class="headerlink" href="#executable-fan-files" title="Link to this heading">#</a></h2>
<p>On a Unix system, you can turn a <code class="docutils literal notranslate"><span class="pre">.fan</span></code> file into an <em>executable file</em> by placing a line</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env -S fandango fuzz -f</span>
</pre></div>
</div>
<p>at the top.</p>
<p>If you set its “executable” flag with <code class="docutils literal notranslate"><span class="pre">chmod</span> <span class="pre">+x</span> <span class="pre">FILE</span></code>, you can then directly execute the <code class="docutils literal notranslate"><span class="pre">.fan</span></code> file as a command as if it were prefixed by <code class="docutils literal notranslate"><span class="pre">fandango</span> <span class="pre">fuzz</span> <span class="pre">-f</span></code>.</p>
<p>As an example, let us create a file <a class="reference download internal" download="" href="_downloads/a75afeeeb7b45ef908dad92a1ad23b02/fuzz-persons.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">fuzz-persons.fan</span></code></span></a>:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span><span class="o">(</span><span class="nb">echo</span><span class="w"> </span><span class="s1">'#!/usr/bin/env -S fandango fuzz -f'</span><span class="p">;</span><span class="w"> </span>cat<span class="w"> </span>persons.fan<span class="o">)</span><span class="w"> </span>><span class="w"> </span>fuzz-persons.fan
$<span class="w"> </span>chmod<span class="w"> </span>+x<span class="w"> </span>fuzz-persons.fan
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
</div>
<p>You can now invoke the file, even with extra arguments:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>./fuzz-persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">1</span>
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>Tftvfckrez Nnahozqeq,363854708719630429
</pre></div>
</div>
</div>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>The <code class="docutils literal notranslate"><span class="pre">env</span></code> command varies greatly across Unix flavors and versions.
If the above does not work on your system, try skipping the <code class="docutils literal notranslate"><span class="pre">-S</span></code> option, such that the first line reads:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env fandango fuzz -f</span>
</pre></div>
</div>
</div>
</section>
<section id="running-fuzzing-campaigns">
<h2>Running Fuzzing Campaigns<a class="headerlink" href="#running-fuzzing-campaigns" title="Link to this heading">#</a></h2>
<p>So far, we have only produced a limited number of outputs. This can be done in two ways: By limiting the number of generations for the evaluation or by capping the number of solutions:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span><span class="w"> </span><span class="c1"># to limit the number of solutions</span>
$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>-N<span class="w"> </span><span class="m">10</span><span class="w"> </span><span class="c1"># to limit the number of generations</span>
</pre></div>
</div>
<p>To run fuzzing continuously, consider specifying <code class="docutils literal notranslate"><span class="pre">--infinite</span></code> to keep Fandango producing inputs indefinitely. You likely want to combine this with one of the configurations where Fandango <a class="reference internal" href="#sec-invoke-targets"><span class="std std-ref">executes your target directly</span></a> like so:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>persons.fan<span class="w"> </span>--infinite<span class="w"> </span>--input-method<span class="o">=</span>libfuzzer<span class="w"> </span>--file-mode<span class="o">=</span>binary<span class="w"> </span>./harness.<span class="o">{</span>so,dylib<span class="o">}</span>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#creating-a-name-database">Creating a Name Database</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#feeding-inputs-into-programs">Feeding Inputs Into Programs</a></li>
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