Finally, a way to block those pesky WordPress DoS attacks on cPanel

A very common DOS attack on a cPanel server is against the WordPress API scripts, chiefly xmlrpc.php and wp-login.php.

If you have been subjected to this kind of attack in the past, and have attempted to prevent reoccurrence, you will likely know that the oft-quoted .htaccess solutions, such as:

<Files xmlrpc.php>
        order deny,allow
        deny from all
</Files>

Have limited success in mitigating this kind of attack.

The popular WordPress plugin Wordfence does mitigate this kind of attack, and I do often suggest that our customers install it, as it is a very comprehensive plugin, which prevents against all manner of attacks, and WILL bother you with update notifications!

However, I was looking for a way to permanently block attackers at firewall level.

I discovered recently that the popular software firewall ConfigServer Security & Firewall (csf) for cPanel supports wildcards in its custom logs, which has made this firewall-level blocking possible.

WARNING⌗

This may result in blocked IPs, for example, people using the WordPress App.

If the WordPress App is being used in your environment, Wordfence may be a better solution, as it does some ‘under-the-hood smarts’ to separate legitimate traffic from abuse.

Adding the magic⌗

Adding the account domain logs to lfd’s ‘watchlist’⌗

Open /etc/csf/csf.conf in an editor, and locate the line:

CUSTOM1_LOG = "/var/log/customlog"

Change this to read:

CUSTOM1_LOG =  "/usr/local/apache/domlogs/*/*"

Save and close this file.

Adding the rule to csf’s custom regex rule configuration⌗

First create a copy of the file:

cp /etc/csf/regex.custom.pm /etc/csf/regex.custom.pm.bak

Now open the file in an editor and replace the contents with the following:

#!/usr/local/cpanel/3rdparty/bin/perl
###############################################################################
# Copyright 2006-2016, Way to the Web Limited
# URL: http://www.configserver.com
# Email: [email protected]
###############################################################################

sub custom_line {
    my $line = shift;
    my $lgfile = shift;

    # DETECT AND BLOCK xmlrpc.php POST DOS attacks (requires: CUSTOM1_LOG = "/usr/local/apache/domlogs/*/*" in csf.conf)

    if (($globlogs{CUSTOM1_LOG}{$lgfile}) and ($line =~ /(.*) \- \- .*POST .*xmlrpc\.php.*/)) {
        return ("xmlrpc.php POST attack from",$1,"xmlrpc","20","80,443","1");
    }

    return 0;
}

1;

At this point, you can restart csf and lfd with: csf -ra.

Testing the rule⌗

HERE BE DRAGONS

YOU CAN AND WILL BLOCK YOUR IP FROM ACCESSING YOUR SERVER.

It may be worth following these instructions over a VPN, or from another server so your real IP is masked from lfd.

You can test if this rule has worked with the following bash one-liner:

while true; do curl -X POST http://www.example.com/xmlrpc.php ; done

This will simulate the attack, and will trigger the rule. You can confirm the rule has been trigged by checking /var/log/lfd.log - you will see a line similar to this:

Dec  7 10:18:16 servername lfd[22889]: (xmlrpc) xmlrpc.php POST attack from 198.51.100.45 (GB/United Kingdom/example.org): 20 in the last 3600 secs - *Blocked in csf* [LF_CUSTOMTRIGGER]

Unblocking blocked IPs⌗

To unblock an IP, the easiest and quickest method is on the CLI:

$ csf -dr 198.51.100.45
Removing rule...
DROP  all opt -- in !lo out *  198.51.100.45  -> 0.0.0.0/0
LOGDROPOUT  all opt -- in * out !lo  0.0.0.0/0  -> 198.51.100.45

And to whitelist that IP in future:

$ csf -a 198.51.100.45
Adding 198.51.100.45 to csf.allow and iptables ACCEPT...
ACCEPT  all opt -- in !lo out *  198.51.100.45  -> 0.0.0.0/0
ACCEPT  all opt -- in * out !lo  0.0.0.0/0  -> 198.51.100.45

If you would prefer, the block can be removed from the WebUI:

  1. WHM » Plugins » ConfigServer Security & Firewall
  2. Enter the blocked IP in the box adjacent to the ‘Quick Unblock’ button
  3. Click ‘Quick Unblock’

And whitelist the IP:

  1. WHM » Plugins » ConfigServer Security & Firewall
  2. Enter the blocked IP (and optional comment) in the box(es) adjacent to the ‘Quick Allow’ button
  3. Click ‘Quick Allow’

This article was also posted on the Memset Official Blog, here.