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Uphigh CTF lvl. 4
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<i class="fa fa-calendar padding-small"></i><time datetime="2018-07-23T10:01:00+02:00"> Mon 23 July 2018</time>
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<p>This is fourth level of CTF found at
<a href="http://uphigh.com.au/4/">http://uphigh.com.au/4/</a></p>
<p>In this challenge, we have a simple website asking for background color code.</p>
<p><img alt="challenge start screen" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_7_23_Vonex.png"></p>
<p>Looking at the page source code we don’t see much:</p>
<div class="highlight"><pre><span></span><code><span class="p"></</span><span class="nt">div</span><span class="p">><</span><span class="nt">form</span> <span class="na">id</span><span class="o">=</span><span class="s">"form"</span> <span class="na">action</span><span class="o">=</span><span class="s">"/4/"</span> <span class="na">method</span><span class="o">=</span><span class="s">"post"</span><span class="p">></span>
Background colour: <span class="p"><</span><span class="nt">input</span> <span class="na">type</span><span class="o">=</span><span class="s">"text"</span> <span class="na">name</span><span class="o">=</span><span class="s">"bgcolour"</span> <span class="na">value</span><span class="o">=</span><span class="s">'#ffffff'</span><span class="p">><</span><span class="nt">button</span> <span class="na">id</span><span class="o">=</span><span class="s">"submit"</span> <span class="na">type</span><span class="o">=</span><span class="s">"submit"</span><span class="p">></span>Go<span class="p"></</span><span class="nt">button</span><span class="p">></span>
<span class="p"></</span><span class="nt">form</span><span class="p">></span>
<span class="p"><</span><span class="nt">br</span><span class="p">></span>
<span class="p"><</span><span class="nt">div</span> <span class="na">id</span><span class="o">=</span><span class="s">'text'</span><span class="p">></span><span class="cm"><!-- key --></span>Vonex is hiring<span class="p"></</span><span class="nt">div</span><span class="p">><</span><span class="nt">style</span><span class="p">>#</span><span class="nn">resume</span><span class="w"> </span><span class="p">{</span><span class="k">margin-left</span><span class="p">:</span><span class="w"> </span><span class="mi">38</span><span class="kt">%</span><span class="p">;</span><span class="w"> </span><span class="k">margin-top</span><span class="p">:</span><span class="mi">7</span><span class="kt">%</span><span class="p">;}</</span><span class="nt">style</span><span class="p">></span>
</code></pre></div>
<p>Trying to inject commands instead of color, gives us no results but looking at network requests/responses we see interesting cookie:</p>
<div class="highlight"><pre><span></span><code>V0n3XL7d=011101010000111000001010000000110001100101000011000011010101000001000001000011000000111100010011000010100000101001000100;
</code></pre></div>
<p>Now looking at the html source and the cookie value, could it be some sort of Vigenère cipher?</p>
<div class="highlight"><pre><span></span><code><span class="kn">import</span> <span class="nn">binascii</span>
<span class="n">cookie</span> <span class="o">=</span> <span class="sa">b</span><span class="s2">"011101010000111000001010000000110001100101000011000011010101000001000001000011000000111100010011000010100000101001000100"</span>
<span class="n">comment</span> <span class="o">=</span> <span class="sa">b</span><span class="s2">"Vonex is hiring"</span>
<span class="n">cookie_hex</span> <span class="o">=</span> <span class="nb">hex</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">cookie</span><span class="p">,</span><span class="mi">2</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Cookie in hex: "</span> <span class="o">+</span> <span class="n">cookie_hex</span><span class="p">)</span>
<span class="n">comment_hex</span> <span class="o">=</span> <span class="s2">"0x"</span><span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">binascii</span><span class="o">.</span><span class="n">hexlify</span><span class="p">(</span><span class="n">comment</span><span class="p">))[</span><span class="mi">2</span><span class="p">:</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Comment in hex: "</span> <span class="o">+</span> <span class="n">comment_hex</span><span class="p">)</span>
<span class="n">res_hex</span> <span class="o">=</span> <span class="nb">hex</span><span class="p">(</span><span class="nb">int</span><span class="p">(</span><span class="n">comment_hex</span><span class="p">,</span> <span class="mi">16</span><span class="p">)</span> <span class="o">^</span> <span class="nb">int</span><span class="p">(</span><span class="n">cookie_hex</span><span class="p">,</span> <span class="mi">16</span><span class="p">))</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Result hex: "</span> <span class="o">+</span> <span class="n">res_hex</span><span class="p">)</span>
<span class="n">res_ascii</span> <span class="o">=</span> <span class="nb">str</span><span class="p">(</span><span class="n">binascii</span><span class="o">.</span><span class="n">unhexlify</span><span class="p">(</span><span class="n">res_hex</span><span class="p">[</span><span class="mi">2</span><span class="p">:]))[</span><span class="mi">2</span><span class="p">:</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span>
<span class="nb">print</span><span class="p">(</span><span class="s2">"Result: "</span> <span class="o">+</span> <span class="n">res_ascii</span><span class="p">)</span>
</code></pre></div>
<p>As result we get:</p>
<div class="highlight"><pre><span></span><code>Cookie in hex: 0x750e0a0319430d50410c0f130a0a44
Comment in hex: 0x566f6e657820697320686972696e67
Result hex: 0x236164666163642361646661636423
Result: #adfacd#adfacd#
</code></pre></div>
<p>Trying result we obtained as color gives us our flag :D</p>
<p><img alt="solved challenge screen" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_7_23_VonexResult.png"></p>
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