It sends a request to a host, without completing it, and then it waits. The problem comes not if we send no data, but we send some data, but painfully slowly. This will release the resources for that connection in order to serve someone else. The server will wait a certain amount of time, and then will let the connection go free. Say that we are browsing a site from our phone, and we lose internet connection for some reason. A server has a built-in timeout for each request. The attacker splits the HTTP GET request in as many packets as possible, and sends them as slow as possible.Īnd now Jimmy you might say, "Ok, I know that servers have a timeout built-in for each request. Instead of sending requests as fast as possible, it sends requests as slow as possible. The slow loris is a kind of slow and low attack invented by RSnake in 2009. Can I send requests so slowly, that I just bore them to death? And the answer is yes, yes you can! The idea of a slow loris attack is very simple. This marks the end of the headers, allowing the server to process the request the moment it receives this empty line. This conversation will continue back and forth until we load the page and all of its assets.Ī sample HTTP/1.1 GET request header will look like that:Įnter fullscreen mode Exit fullscreen mode And the server will send us the file we requested. Then we will send a new GET request and we will say "Hey I need logo.png now". We will send a GET request to the website, and we say "Hey I want index.html". Now let's make a quick and dirty recap on how HTTP/1.1 GET requests work. Server - Client Communication with HTTP/1.1 Since such attacks use legitimate requests traditional firewall filtering rules do not play well. Such a kind of attack is very difficult to mitigate, especially for smaller organizations with limited infrastructure. If you don't, it will just laugh in your face.
If you manage to send enough the machine will go down.
As many requests as possible all at the same time. Some modern attacks use techniques such as DNS amplification in order to maximize the impact of the attack.īut the main idea behind a DoS attack remains the same. Here instead of a single computer, you utilize a bunch of them in order to simultaneously send as many requests as possible to the machine on the other side. Since the machine has a finite amount of resources, if you manage to send more requests than it can handle, the server will go down.Ī distributed denial-of-service or DDoS attack, is the next level. The idea behind this attack is that you are trying to crash the machine that is on the business end of this attack, by sending as many requests as possible. In order to understand how such an attack works, let's refresh some basics ( too bored, show me now)Ī denial-of-service or DoS attack is an attack in which the attacker, seeks to make a machine or network unavailable to its intended users by disrupting the services of the host, either temporarily or indefinitely. Slow loris is a denial of service attack that can wreck havoc in unprotected thread-based web servers such as Apache, created in 2009, by a guy named RSnake (and is one of my favorite DoS attacks 😆)
#Slowloris attack service windows
NOTE: This question is for Apache servers as it is my understanding that Windows IIS servers are not affected.The cover image is a real life slow loris. Has anyone on ServerFault been experiencing attacks such as this? If so, what measures did you implement to defend/prevent it? Using mod_evasive to limit the number of connections from one host and use mod_security to deny requests that look like they were issued by slowloris seem to be the best defence so far. One other report indicates that using a reverse proxy (such as Perlbal) in front of the Apache server can help prevent the attack. This of course does nothing more than increase the requirements for the attacker's computer and does not actually protect the server 100%.
The best solution we have determined (so far) is to increase MaxClients. The basic concept of what slowloris does is not a new attack but given the recent attention I have seen a small increase in attacks against some of our Apache websites.Īt the moment there does not appear to be any 100% defence against this. Recently a script called "slowloris" has gained attention.
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