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Layer 7 DDOS – Blocking HTTP Flood Attacks

26/11/2015 Comments off

There are many types of Distributed Denial of Service (DDOS) attacks that can affect and bring down a website, and they vary in complexity and size. The most well known attacks are the good old SYN-flood, followed by the Layer 3/4 UDP and DNS amplification attacks.

Today though, we’re going to spend a little time looking at Layer 7, or what we call an HTTP Flood Attack.

An HTTP flood attack is a type of Layer 7 application attack that utilizes the standard valid GET/POST requests used to fetch information, as in typical URL data retrievals (images, information, etc.) during SSL sessions. An HTTP GET/POST flood is a volumetric attack that does not use malformed packets, spoofing or reflection techniques. – DDoSAttacks.biz

If you’re wondering, yes, we deal with these every day, and we protect our client websites via our Website Firewall.

Today I’m going to share with you some details on a rather large DDoS attack that leveraged the following HTTP request flood attack to wreak havoc on a clients website. I’ll also share the steps we took to mitigate the issue.

Layer 7 DDoS – HTTP Flood Attacks

The first thing to understand about Layer 7 attacks is that they require more understanding about the website and how it operates. The attacker has to do some homework and create a specially crafted attack to achieve their goal. Because of this, these types of DDoS attacks require less bandwidth to take the site down and are harder to detect and block.

Layer 7 DDoS – Part 1: Random URLs

This specific client came to us after his site was down for almost a week. They tried other services to protect their website with not much luck. As soon as he switched his DNS to us, we gained a much deeper appreciation and started to see why.

He was getting thousands of requests like these every second:

75.118.29.205 - - [20/Jan/2014:19:32:06 -0500] "GET /?458739416183768700 HTTP/1.1" 200 440 "http://movies.netflix.com/WiPlayer?movieid=70136122&trkid=7882978&t=Weeds" ""
173.245.56.201 - - [20/Jan/2014:19:32:06 -0500] "GET /?458726993617499500 HTTP/1.1" 200 440 "http://landing.pcwhatsap.com/1/?offer_id=3534&aff=1788&url_id=5618&sub_id=whatsapp_pop" ""
79.19.41.22 - - [20/Jan/2014:19:32:06 -0500] "GET /?458741338856272200 HTTP/1.1" 200 440 "http://www.rumoreweb.it/index.php?option=com_news_portal&view=category&id=21&Itemid=259" ""
190.121.64.3 - - [20/Jan/2014:19:32:06 -0500] "GET /?458722169268652700 HTTP/1.1" 200 440 "http://www.eldivisadero.cl/noticias/?task=show&id=37352" ""
186.54.141.146 - - [20/Jan/2014:19:32:06 -0500] "GET /?458741274224646000 HTTP/1.1" 200 440 "http://badoo.com/01244944965/?r=37.4&p=1" ""

To be more exact, he was getting 5,233 HTTP requests every single second. From different IP addresses around the world.

What is important to note here is how this worked against the client’s platform. The client’s website was built on WordPress. The uniqueness of the requests were bypassing the caching system, forcing the system to render and respond to every request. This was bringing about system failures as the server quickly became overwhelmed by the requests.

For illustration purposes, here is a quick geographic distribution of the IP’s hitting the site. This is for 1 second in the attack. Yes, every second these IP’s were changing.

Stopping the DDoS: Once we identified the type of attack, blocking was easy enough. By default, they were not passing our anomaly check, causing the requests to get blocked at the firewall. One of the many anomalies we look for are valid user agents, and if you look carefully you see that the requests didn’t have one. Hopefully, you’ll also noticed that the referrers were dynamic and the packets were the same size, another very interesting signature. Needless to say, this triggered one of our rules, and within minutes his site was back and the attack blocked.

Lire la suite…