More Than 162,000 WordPress Sites Used for Distributed Denial of Service Attack

Distributed Denial of Service (DDOS) attacks are becoming a common trend on our blog lately, and that’s OK because it’s a very serious issue for every website owner. Today I want to talk about a large DDOS attack that leveraged thousands of unsuspecting WordPress websites as indirect source amplification vectors.

Any WordPress site with Pingback enabled (which is on by default) can be used in DDOS attacks against other sites. Note that XMLRPC is used for pingbacks, trackbacks, remote access via mobile devices and many other features you’re likely very fond of. But, it can also be heavily misused like what we are seeing.

The story

It all happened against a popular WordPress site that had gone down for many hours due to a DDOS. As the attack increased in size, their host shut them down, and then they decided to ask for help and subscribed to our CloudProxy Website Firewall.

Once the DNS was ported we were able to see what was going on, it was a large HTTP-based (layer 7) distributed flood attack, sending hundreds of requests per second to their server. The requests looked like this:

74.86.132.186 - - [09/Mar/2014:11:05:27 -0400] "GET /?4137049=6431829 HTTP/1.0" 403 0 "-" "WordPress/3.8; http://www.mtbgearreview.com"
121.127.254.2 - - [09/Mar/2014:11:05:27 -0400] "GET /?4758117=5073922 HTTP/1.0" 403 0 "-" "WordPress/3.4.2; http://www.kschunvmo.com" 
217.160.253.21 - - [09/Mar/2014:11:05:27 -0400] "GET /?7190851=6824134 HTTP/1.0" 403 0 "-" "WordPress/3.8.1; http://www.intoxzone.fr" 
193.197.34.216 - - [09/Mar/2014:11:05:27 -0400] "GET /?3162504=9747583 HTTP/1.0" 403 0 "-" "WordPress/2.9.2; http://www.verwaltungmodern.de" 
..

If you notice, all queries had a random value (like “?4137049=643182″) that bypassed their cache and force a full page reload every single time. It was killing their server pretty quickly.

But the most interesting part is that all the requests were coming from valid and legitimate WordPress sites. Yes, other WordPress sites were sending that random requests at a very large scale and bringing the site down.

WordPress Insecure Default Option = Very Large Botnet

Just in the course of a few hours, over 162,000 different and legitimate WordPress sites tried to attack his site. We would likely have detected a lot more sites, but we decided we had seen enough and blocked the requests at the edge firewall, mostly to avoid filling the logs with junk.

Can you see how powerful it can be? One attacker can use thousands of popular and clean WordPress sites to perform their DDOS attack, while being hidden in the shadows, and that all happens with a simple ping back request to the XML-RPC file:

$ curl -D -  "www.anywordpresssite.com/xmlrpc.php" -d '<methodCall><methodName>pingback.ping</methodName><params><param><value><string>http://victim.com</string></value></param><param><value><string>www.anywordpresssite.com/postchosen</string></value></param></params></methodCall>'

Yes, that simple command on Linux can start it all.

Is your site attacking others?

It might be and you have no idea. To verify, look through your logs for any POST requests to the XML-RPC file, similar to the one below. If you see a pingback to a random URL, you know your site is being misused.

93.174.93.72 - - [09/Mar/2014:20:11:34 -0400] "POST /xmlrpc.php HTTP/1.0" 403 4034 "-" "-" "POSTREQUEST:<?xml version=\x221.0\x22 encoding=\x22iso-8859-1\x22?>\x0A<methodCall>\x0A<methodName>pingback.ping</methodName>\x0A<params>\x0A <param>\x0A  <value>\x0A   <string>http://fastbet99.com/?1698491=8940641</string>\x0A  </value>\x0A </param>\x0A <param>\x0A  <value>\x0A   <string>yoursite.com</string>\x0A  </value>\x0A </param>\x0A</params>\x0A</methodCall>\x0A"

94.102.63.238 – - [09/Mar/2014:23:21:01 -0400] "POST /xmlrpc.php HTTP/1.0" 403 4034 "-" "-" "POSTREQUEST:\x0A\x0Apingback.ping\x0A\x0A \x0A \x0A http://www.guttercleanerlondon.co.uk/?7964015=3863899\x0A \x0A \x0A \x0A \x0A yoursite.com\x0A \x0A \x0A\x0A\x0A"

In these cases above, someone tried to use one of our honeypots to DDOS fastbet99.com and guttercleanerlondon.co.uk (a site we don’t know or service).

To stop your WordPress website from being misused, you will need to disable the XML-RPC (pingback) functionality on your site. You can do this by removing the file, xmlrpc.php, or you can disable notifications in your settings. The biggest challenge you’ll find with removing the file is that on an update it’ll come right back, annoying, I know. Some preliminary tests are showing that we’re able to bypass the disable notification setting but we are still investigating.

Update: A better way to block it is by creating a plugin that adds the following filter:

add_filter( ‘xmlrpc_methods’, function( $methods ) {
   unset( $methods['pingback.ping'] );
   return $methods;
} );

This is a well known issue within WordPress and the core team is aware of it, it’s not something that will be patched though. In many cases this same issue is categorized as a feature, one that many plugins use, so in there lies the dilemma.

Online Tool to Check if your site was misused

Because of how prevalent an issue this is becoming, we’ve put together a little scanner that will check if your website has shown up in our logs. This scanner is only looking to see if your site has used to attack anyone within our network.

Use our WordPress DDOS Scanner to check if your site is DDOS’ing other websites.

Try it out and do your part making the Internet a safer place for everyone.

Scan your website for free:
About Daniel Cid

Daniel B. Cid is the CTO&Founder of Sucuri and the founder of the open source OSSEC HIDS. His interests range from intrusion detection, log analysis (log-based intrusion detection), web-based malware research and secure development.

You can find more about Daniel at his site dcid.me or on Twitter: @danielcid

  • Ines van Dijk

    Very interesting! Thank you for this.

  • jfarsen

    How will this affect (or not) the JetPack plugin ?

    • George Stephanis

      It will prevent Jetpack from functioning, as you’re hacking core — which is always a bad idea.

      What you should do, is `add_filter(‘xmlrpc_enabled’,’__return_false’);` or install http://wordpress.org/plugins/disable-xml-rpc/ instead.

      • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

        Hello. I see a lot of executions in one of our sites. How can I tell which ones are from JetPack servers?

        Can anyone confirm if the plugin Disable XML-RPC may negatively effect JetPack functions?

      • http://josephscott.org/ Joseph Scott

        You could do something very simple that just disables Pingbacks and not all of XML-RPC. Something like this should work ( not tested ):

        add_action( ‘xmlrpc_call’, function( $method ) {
        if ( $method === ‘pingback.ping’ ) {
        wp_die( ‘No pingbacks’, ‘Pingback is disabled’, array( ‘response’ => 403 ) );
        }
        } );

        There should be no need to hack core WordPress or disable all of XML-RPC.

      • Reed

        It looks to me like add_filter(‘xmlrpc_enabled’,’__return_false’) does NOT prevent pingbacks in wp 3.8.1, it’s only checked in wp_xmlrpc_server::login, which isn’t used for pingbacks.

        ‘xmlrpc_call’ is checked for pingbacks.

    • http://sucuri.net Andres Armeda

      I agree with George in his reply, you should not hack core and that wasn’t the intention of mentioning that. Our solution to disabling XML-RPC was adding the option in CloudProxy, our website firewall.

      XML-RPC is required in JetPack to auth with WordPress.com so if you disable XML-RPC, you will get a connection error when attempting to authenticate.

      • George Stephanis

        We actually use it after the initial authentication as well, for the ongoing two-way communication between WPCOM servers and the Jetpack site — for example, asynchronously pulling posts up into our elasticsearch index if the site has Related Posts turned on, for the WPCOM REST API module, and others.

        (just to specify that temporarily unblocking it for authorization and then reblocking can still have bad side effects)

        (if you’d like details on IP ranges that Jetpack requests may come in from to whitelist, just drop me a line on Skype — same as my Twitter handle)

  • Alex Shiels

    We’ve identified the source of the pingbacks, and we’re looking to see if the Akismet plugin can help to prevent it.

    • Daniel Cid

      That would be awesome. Any luck there?

  • http://www.fam.tuwien.ac.at/~schamane/ Andreas Schamanek

    As far as I am concerned you fail to explain the amplification. If it’s not amplified, I see no reason why I should care.

  • Anthony Hortin

    Just wondering if unchecking the “Attempt to notify any blogs linked to from the article” &
    “Allow link notifications from other blogs (pingbacks and trackbacks)” options (on the SETTINGS>DISCUSSION page) would perform the same functionality as unsetting the Pingback with the Filter you mentioned above? Or would it still actually get around this?

    • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

      Now that’s a very good question. I’m interested in reading an answer to this too.

    • http://willnorris.com/ willnorris

      unchecking “Allow link notifications from other blogs” will certainly turn off pingbacks for future posts, though you would still need to go in and edit existing posts to disable it for them (bulk edit is your friend here). Filtering ‘xmlrpc_methods’ is just a much faster way to disable pingbacks site-wide.

      The “Attempt to notify any blogs linked to from the article” controls whether you ping other sites that you’ve linked to in your posts. It still generates network traffic to those sites, but cannot be triggered remotely as this article is discussing. Leaving this enabled is fine.

      • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

        ok, and if the site dont need pingbacks, would recommend the “add_filter(‘xmlrpc_enabled’,’__return_false’);” method?

        • http://willnorris.com/ willnorris

          if you don’t need XML-RPC at all, then sure. But it’s used for more than just pingback… notably Jetpack and any other service that you may have that posts to WordPress for you.

          • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

            I know Will, but AFAIK, XML RPC is ONLY used when you publish from third parties TO WordPress, not in the opposite way. I deactivated XML RPC and didnt notice any malfunction in JetPack; I use Stats, Comments and Publish to social networks. What JetPack features would require XML RPC enabled? Post by Email? Monitor? JSON API?
            Maybe I’m blind, but I don’t think XML RPC is a feature every site cannot live without. Please clarify if I’m wrong.

      • Anthony Hortin

        Thanks for the clarification Will :-)

  • rcain

    great article. very informative. thanks. the wp filters around ping backs, etc gives me the idea to create a plugin – that for example allows admin to determine blacklist & white list for ping-backers, track-backers, etc. might be neatly implemented as simple extension to existing wordpress firewall type plugins – eg: wp-anti-spammer-anti-splogger plugin. will look into it.

  • Anonym

    Some simple PHP lines in the beginning of the code on the attacked site might would have changed this whole story. Something like this maybe:

    [code][/code]

  • pzriddle

    Thanks for this. Unfortunately the code in your Update isn’t a complete solution: it contains erroneous “smartquotes,” it doesn’t work in PHP 5.2, and it’s not packaged as a plugin (although I guess an alternative would be to put it in a theme’s functions.php file, right?).

    Has anybody put together a proper plugin, preferably compatible with PHP 5.2?

    • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

      Try “Disable XML RPC” ;) Worked for me under PHP 5.4 and 5.2 :)

  • http://www.sonatype.com/ Jessica Dodson

    It blows my mind how sometimes something so simple can cause so much trouble. One little line of code in the wrong place at the wrong time can wreck havoc for thousands of people!

  • Bea

    Thanks so much for sharing this information, Daniel. I checked my site and breathed a sigh of relief. Cheers!

  • http://robcubbon.com/ Rob Cubbon

    I’ve found one of my sites is being used in DDOS. I have installed “Disable XML-RPC Pingback” hopefully that’ll do the trick. I don’t use JetPack and I don’t think I’ll need the pingback.

  • Subtuse

    How long were companies impacted (unable to access the internet) during this attack? How do these attacks affect productivity and corporate image? I have found a service from TW Telecom which sits at the peering points will remove packets during the attack but NOT interfere with the internet traffic flow so the company continues to function as if it were not happening…thoughts?

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  • Josh

    Ok to sum it up will disabling “Allow link notifications from other blogs”, but leaving enabled “Attempt to notify any blogs linked to from the article” still present security risk to my site or do I need to disable both and maybe install a 3rd party plugin?

  • http://bigemployee.com Norik Davtian

    This is very interesting :) Essentially with this you don’t even need zombies for DDOS, and its so easy to find WordPress websites these days, its almost all over the web.

  • michaelmd

    maybe the site should do a dns check to see if the ip in of the pingback request matches the domain in the included url *before* doing a request to that url to check for links.(most likely it wouldn’t match in the case of such an attack if the attacker isn’t also doing some kind of dns hack (far less likely) or using the same same host (also far less likely) – stopping it right there

    I think its a small thing that could make such misuse harder to do

    also the way that caching is done obviously needs to be looked at too.
    (as that in itself sounds like it presents a DOS risk for the site (not other sites though) – given the ram each process handling a php request needs on 64 bit platforms these days it wouldn’t need many concurrent requests to send most sites into hardcore swap if too many of them can’t be handled very quickly from cache.

  • Sol

    Over the last 3 days, I have noticed a massive amount of traffic hitting my VPS client sites xmlrpc file. At one stage there were more than 100 bots per second. This flood successfully brought down two of our busy ecommerce sites, leading to client calls (clients don’t really know what a DDOS is and don’t care, they just want their site to work. Though they were patient, this kind of issue can cast a shadow on your service).

    We tried a whole range of things to try to mitigate the issue, SYNFLOOD in our CSF firewall (no effect), Wordfence plugin (no effect), BRUTE protect plugin (no use).

    Finally we blocked the file using Order deny,allow Deny from all but the bots simply turned to the theme index file and continued their DOS attack.

    Then we came up with a mod_sec rule which had more success:


    SecRule REQUEST_LINE "POST .*wp-login.*" "pass,initcol:ip=%{REMOTE_ADDR},setvar:ip.maxlimit=+1,deprecatevar:ip.maxlimit=1/600,nolog,id:35011"
    SecRule IP:MAXLIMIT "@gt 10" "log,deny,id:350111,msg:'wp-bruteforce: denying %{REMOTE_ADDR} (%{ip.maxlimit} connection attempts)'"

    SecRule REQUEST_LINE "POST .*xmlrpc.*" "pass,initcol:ip=%{REMOTE_ADDR},setvar:ip.maxlimit=+1,deprecatevar:ip.maxlimit=1/600,nolog,id:35012"
    SecRule IP:MAXLIMIT "@gt 2" "log,deny,id:350112,msg:'wp-bruteforce-xmlrpc: denying %{REMOTE_ADDR} (%{ip.maxlimit} connection attempts)'"

    The problem with disabling xmlrpc is that a lot of plugins and sites use it legitimately. Removing it will break wordpress. I haven’t tried your suggested plugin / function, will give that a go.

    • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

      Interesting approach, limiting the amount of connections by IP could work, but what about and attack with 500+ source IP. It could overflow the iptables list if your server has not enough RAM memory available.
      Are those websites using some kind of cache? CDN? Using it may help to mitigate the cpu load and disk stress. But obviously, it must be correctly configured.

      • Sol

        Yes, IP table list is limited to 500 entries, we also use cloudlinux to manage consecutive connections so that any one account cannot chew up the RAM.

        We’ve also installed bruteprotect plugin and that is definitely helping.

        We are now looking at cloudflare and cloudproxy and considering which to try. Cloudflare seems more comprehensive and cheaper than cloudproxy and I’m not sure why? What do you use?

        • http://www.marcelopedra.com.ar/blog/ Marcelo Pedra

          Glad you asked. I do choose a variety of things: MySQL cache in the server + W3 Total Cache to generate html versions + CloudFlare to allocate those html versions + Amazon Cloudfront to allocate all the other files (images/js/css). This combination is a relief for Apache. Plugins: CSF in the server + WordFence + iThemes Security. All these elements provide a resilient configuration and almost 0% downtime. Obviously counting on a very reliable hosting service is the cornerstone for your online buildings.

          If you need to solve the configuration with a professional approach don’t hesitate in contacting me.

          All the best