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Ret2 systems wargame
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<i class="fa fa-calendar padding-small"></i><time datetime="2019-01-25T10:01:00+01:00"> Fri 25 January 2019</time>
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<p>This is a simple reversing challenge hosted <a href="https://wargames.ret2.systems/level/shmoo">here</a>.</p>
<p>The site offers a pretty nice interface with GDB, dissasembly view and python shell.</p>
<p>We are given part of 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"><string.h></span>
<span class="c1">// hidden files</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">"wargames.h"</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">"shmoo.h"</span>
<span class="kt">void</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="w"> </span><span class="o">*</span><span class="w"> </span><span class="n">argv</span><span class="p">[])</span><span class="w"> </span>
<span class="p">{</span>
<span class="w"> </span><span class="n">init_wargame</span><span class="p">();</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"------------------------------------------------------</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">"--[ ShmooCon 2019 - Coffee Cup Lockbox </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">"------------------------------------------------------</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span>
<span class="w"> </span><span class="kt">char</span><span class="w"> </span><span class="n">pin</span><span class="p">[</span><span class="mi">32</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </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">seed</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">argv</span><span class="p">[</span><span class="mi">1</span><span class="p">];</span><span class="w"> </span><span class="c1">// provided by server, unique per user</span>
<span class="w"> </span><span class="c1">// Prompt the user to enter the secret lockbox PIN</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"ENTER PIN: "</span><span class="p">);</span>
<span class="w"> </span><span class="n">fgets</span><span class="p">(</span><span class="n">pin</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">pin</span><span class="p">),</span><span class="w"> </span><span class="n">stdin</span><span class="p">);</span>
<span class="w"> </span><span class="n">pin</span><span class="p">[</span><span class="n">strcspn</span><span class="p">(</span><span class="n">pin</span><span class="p">,</span><span class="w"> </span><span class="s">"</span><span class="se">\n</span><span class="s">"</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="c1">// Check if the given PIN is valid for the RET2 lockbox</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">validate_pin</span><span class="p">(</span><span class="n">pin</span><span class="p">,</span><span class="w"> </span><span class="n">seed</span><span class="p">))</span>
<span class="w"> </span><span class="n">validation_failure</span><span class="p">();</span>
<span class="w"> </span><span class="k">else</span>
<span class="w"> </span><span class="n">unlock_door</span><span class="p">();</span>
<span class="p">}</span>
</code></pre></div>
<p>We see that we need to reverse <code>validate_pin</code> function. Let's look at the assembly code of this function:</p>
<div class="highlight"><pre><span></span><code><span class="c1">;validate pin</span>
<span class="err">0</span><span class="nl">x4009f6:</span><span class="w"> </span><span class="nf">push</span><span class="w"> </span><span class="no">rbp</span>
<span class="err">0</span><span class="nl">x4009f7:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rbp</span><span class="p">,</span><span class="w"> </span><span class="no">rsp</span>
<span class="err">0</span><span class="nl">x4009fa:</span><span class="w"> </span><span class="nf">sub</span><span class="w"> </span><span class="no">rsp</span><span class="p">,</span><span class="w"> </span><span class="mi">0x20</span>
<span class="err">0</span><span class="nl">x4009fe:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x18</span><span class="p">],</span><span class="w"> </span><span class="no">rdi</span>
<span class="err">0</span><span class="nl">x400a02:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x20</span><span class="p">],</span><span class="w"> </span><span class="no">rsi</span>
<span class="err">0</span><span class="nl">x400a06:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rax</span><span class="p">,</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x18</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400a0a:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rdi</span><span class="p">,</span><span class="w"> </span><span class="no">rax</span>
<span class="err">0</span><span class="nl">x400a0d:</span><span class="w"> </span><span class="nf">call</span><span class="w"> </span><span class="no">is_valid_length</span>
<span class="err">0</span><span class="nl">x400a12:</span><span class="w"> </span><span class="nf">test</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="no">eax</span>
<span class="err">0</span><span class="nl">x400a14:</span><span class="w"> </span><span class="nf">jne</span><span class="w"> </span><span class="mi">0x400a25</span>
<span class="err">0</span><span class="nl">x400a16:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rel</span><span class="w"> </span><span class="no">g_error</span><span class="p">],</span><span class="w"> </span><span class="mi">0x1</span>
<span class="err">0</span><span class="nl">x400a20:</span><span class="w"> </span><span class="nf">jmp</span><span class="w"> </span><span class="mi">0x400abb</span>
<span class="err">0</span><span class="nl">x400a25:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rax</span><span class="p">,</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x18</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400a29:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rdi</span><span class="p">,</span><span class="w"> </span><span class="no">rax</span>
<span class="err">0</span><span class="nl">x400a2c:</span><span class="w"> </span><span class="nf">call</span><span class="w"> </span><span class="no">is_numeric_string</span>
<span class="err">0</span><span class="nl">x400a31:</span><span class="w"> </span><span class="nf">test</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="no">eax</span>
<span class="err">0</span><span class="nl">x400a33:</span><span class="w"> </span><span class="nf">jne</span><span class="w"> </span><span class="mi">0x400a41</span>
<span class="err">0</span><span class="nl">x400a35:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rel</span><span class="w"> </span><span class="no">g_error</span><span class="p">],</span><span class="w"> </span><span class="mi">0x2</span>
<span class="err">0</span><span class="nl">x400a3f:</span><span class="w"> </span><span class="nf">jmp</span><span class="w"> </span><span class="mi">0x400abb</span>
<span class="err">0</span><span class="nl">x400a41:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x10</span><span class="p">],</span><span class="w"> </span><span class="mi">0x0</span>
<span class="err">0</span><span class="nl">x400a48:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rax</span><span class="p">,</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x20</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400a4c:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">edx</span><span class="p">,</span><span class="w"> </span><span class="mi">0xa</span>
<span class="err">0</span><span class="nl">x400a51:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">esi</span><span class="p">,</span><span class="w"> </span><span class="mi">0x0</span>
<span class="err">0</span><span class="nl">x400a56:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rdi</span><span class="p">,</span><span class="w"> </span><span class="no">rax</span>
<span class="err">0</span><span class="nl">x400a59:</span><span class="w"> </span><span class="nf">call</span><span class="w"> </span><span class="no">strtoul</span>
<span class="err">0</span><span class="nl">x400a5e:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x8</span><span class="p">],</span><span class="w"> </span><span class="no">eax</span>
<span class="err">0</span><span class="nl">x400a61:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rax</span><span class="p">,</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x18</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400a65:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">edx</span><span class="p">,</span><span class="w"> </span><span class="mi">0xa</span>
<span class="err">0</span><span class="nl">x400a6a:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">esi</span><span class="p">,</span><span class="w"> </span><span class="mi">0x0</span>
<span class="err">0</span><span class="nl">x400a6f:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rdi</span><span class="p">,</span><span class="w"> </span><span class="no">rax</span>
<span class="err">0</span><span class="nl">x400a72:</span><span class="w"> </span><span class="nf">call</span><span class="w"> </span><span class="no">strtoul</span>
<span class="err">0</span><span class="nl">x400a77:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x4</span><span class="p">],</span><span class="w"> </span><span class="no">eax</span>
<span class="err">0</span><span class="nl">x400a7a:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x8</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400a7d:</span><span class="w"> </span><span class="nf">xor</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x4</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400a80:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x10</span><span class="p">],</span><span class="w"> </span><span class="no">eax</span>
<span class="err">0</span><span class="nl">x400a83:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0xc</span><span class="p">],</span><span class="w"> </span><span class="mi">0x0</span>
<span class="err">0</span><span class="nl">x400a8a:</span><span class="w"> </span><span class="nf">jmp</span><span class="w"> </span><span class="mi">0x400a97</span>
<span class="err">0</span><span class="nl">x400a8c:</span><span class="w"> </span><span class="nf">add</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x10</span><span class="p">],</span><span class="w"> </span><span class="mi">0x52455432</span>
<span class="err">0</span><span class="nl">x400a93:</span><span class="w"> </span><span class="nf">add</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0xc</span><span class="p">],</span><span class="w"> </span><span class="mi">0x1</span>
<span class="err">0</span><span class="nl">x400a97:</span><span class="w"> </span><span class="nf">cmp</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0xc</span><span class="p">],</span><span class="w"> </span><span class="mi">0x9</span>
<span class="err">0</span><span class="nl">x400a9b:</span><span class="w"> </span><span class="nf">jle</span><span class="w"> </span><span class="mi">0x400a8c</span>
<span class="err">0</span><span class="nl">x400a9d:</span><span class="w"> </span><span class="nf">rol</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x10</span><span class="p">],</span><span class="w"> </span><span class="mi">0x10</span>
<span class="err">0</span><span class="nl">x400aa1:</span><span class="w"> </span><span class="nf">xor</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x10</span><span class="p">],</span><span class="w"> </span><span class="mi">0xc011ec7</span>
<span class="err">0</span><span class="nl">x400aa8:</span><span class="w"> </span><span class="nf">cmp</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x10</span><span class="p">],</span><span class="w"> </span><span class="mi">0xc0ffee</span>
<span class="err">0</span><span class="nl">x400aaf:</span><span class="w"> </span><span class="nf">je</span><span class="w"> </span><span class="mi">0x400abb</span>
<span class="err">0</span><span class="nl">x400ab1:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rel</span><span class="w"> </span><span class="no">g_error</span><span class="p">],</span><span class="w"> </span><span class="mi">0x3</span>
<span class="err">0</span><span class="nl">x400abb:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="no">dword</span><span class="w"> </span><span class="p">[</span><span class="no">rel</span><span class="w"> </span><span class="no">g_error</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400ac1:</span><span class="w"> </span><span class="nf">leave</span><span class="w"> </span>
<span class="mi">0x400ac2</span><span class="p">:</span><span class="w"> </span><span class="no">retn</span><span class="w"> </span>
</code></pre></div>
<p>We also see it makes call to is <code>is_valid_length</code> function:</p>
<div class="highlight"><pre><span></span><code><span class="c1">;is_valid_length</span>
<span class="err">0</span><span class="nl">x400966:</span><span class="w"> </span><span class="nf">push</span><span class="w"> </span><span class="no">rbp</span>
<span class="err">0</span><span class="nl">x400967:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rbp</span><span class="p">,</span><span class="w"> </span><span class="no">rsp</span>
<span class="err">0</span><span class="nl">x40096a:</span><span class="w"> </span><span class="nf">sub</span><span class="w"> </span><span class="no">rsp</span><span class="p">,</span><span class="w"> </span><span class="mi">0x10</span>
<span class="err">0</span><span class="nl">x40096e:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x8</span><span class="p">],</span><span class="w"> </span><span class="no">rdi</span>
<span class="err">0</span><span class="nl">x400972:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rax</span><span class="p">,</span><span class="w"> </span><span class="no">qword</span><span class="w"> </span><span class="p">[</span><span class="no">rbp-0x8</span><span class="p">]</span>
<span class="err">0</span><span class="nl">x400976:</span><span class="w"> </span><span class="nf">mov</span><span class="w"> </span><span class="no">rdi</span><span class="p">,</span><span class="w"> </span><span class="no">rax</span>
<span class="err">0</span><span class="nl">x400979:</span><span class="w"> </span><span class="nf">call</span><span class="w"> </span><span class="no">strlen</span>
<span class="err">0</span><span class="nl">x40097e:</span><span class="w"> </span><span class="nf">cmp</span><span class="w"> </span><span class="no">rax</span><span class="p">,</span><span class="w"> </span><span class="mi">0xa</span><span class="w"> </span><span class="c1">; len 10</span>
<span class="err">0</span><span class="nl">x400982:</span><span class="w"> </span><span class="nf">sete</span><span class="w"> </span><span class="no">al</span>
<span class="err">0</span><span class="nl">x400985:</span><span class="w"> </span><span class="nf">movzx</span><span class="w"> </span><span class="no">eax</span><span class="p">,</span><span class="w"> </span><span class="no">al</span>
<span class="err">0</span><span class="nl">x400988:</span><span class="w"> </span><span class="nf">leave</span><span class="w"> </span>
<span class="mi">0x400989</span><span class="p">:</span><span class="w"> </span><span class="no">retn</span><span class="w"> </span>
</code></pre></div>
<p>This is a simple function calling <code>strlen</code> and checking if the length is 10.</p>
<p>If we look at register values during <code>strtoul</code> calls we see that <code>qword [rbp-0x20]</code> is <code>seed</code> and that <code>qword [rbp-0x18]</code> is our input. Both get converted from string to 32 bit int. </p>
<p>After conversion to int, <code>seed</code> gets saved to <code>dword [rbp-0x8]</code> and our input to <code>dword [rbp-0x4]</code>.</p>
<p>We can use GDB they provided to read <code>seed</code> value.</p>
<p>If we look at the rest of the function we see that we have a loop that adds <code>0x52455432</code> 10 times after which we <code>rol</code>, <code>xor</code> and finally comparison with <code>0xc0ffee</code>.</p>
<p>We can write C program to follow these steps backwards and generate the required input.</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"><stdint.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">uint32_t</span><span class="w"> </span><span class="n">res1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">0xc0ffee</span><span class="w"> </span><span class="o">^</span><span class="w"> </span><span class="mh">0xc011ec7</span><span class="p">;</span>
<span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">res2</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">uint32_t</span><span class="w"> </span><span class="n">seed</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mh">0x3273b96d</span><span class="p">;</span>
<span class="w"> </span><span class="kr">__asm</span><span class="p">(</span>
<span class="w"> </span><span class="s">"ror $0x10, %1;"</span>
<span class="w"> </span><span class="o">:</span><span class="s">"=r"</span><span class="p">(</span><span class="n">res2</span><span class="p">)</span>
<span class="w"> </span><span class="o">:</span><span class="s">"r"</span><span class="p">(</span><span class="n">res1</span><span class="p">)</span>
<span class="w"> </span><span class="p">);</span>
<span class="w"> </span><span class="kt">uint32_t</span><span class="w"> </span><span class="n">inputXor</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">res2</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="p">(</span><span class="mh">0x52455432</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">uint32_t</span><span class="w"> </span><span class="n">input</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">inputXor</span><span class="w"> </span><span class="o">^</span><span class="w"> </span><span class="n">seed</span><span class="p">;</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"Potential input: %u</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">input</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>
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