Slow Loris Attack using JavaScript on PHP Server

Leave the post, let’s begin with what this title is about! This is a web security based article which would get into basics on how HTTP works and a simple attack which exploits the way HTTP protocol works.

What is HTTP?

HTTP – HyperText Transfer Protocol, is the protocol used by web for communication. Your device, when you use a browser, uses this particular protocol, to send request to remote servers to request data from them.

It’s basically like you saying to your mom, “Hey mom, I need to eat the item in the fridge present at shelf 2, could you give it to me?”

And your mom says, “Sure, here you go”, and sends you that item. Now, HTTP is the way you were able to communicate this information to your mom, more like the language you used for communication.

How HTTP Works?

Here’s a TL;DR video if you’re a video person. Otherwise, proceed with the article:

HTTP (Layer 7) is built on the top of TCP protocol (Layer 4). We can use nc (netcat) utility to open raw TCP socket to any website running on HTTP (usually port 80). See the following example on how we connect to HTTP port 80 for using netcat:

See the data we sent:

GET / HTTP/1.1
X-header-1: somemoredata
X-header-2: somemoredata
<empty line>

Ignore the extra X-header-* headers, they’re just random headers you can send with your request. The important header to include in HTTP/1.1 spec is the Host header.

And the response we got:

HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=UTF-8
Date: Tue, 01 Oct 2019 23:24:13 GMT
Expires: Thu, 31 Oct 2019 23:24:13 GMT
Cache-Control: public, max-age=2592000
Server: gws
Content-Length: 219
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
Accept-Ranges: none
Via: HTTP/1.1 forward.http.proxy:3128
Connection: keep-alive

<HTML><HEAD><meta http-equiv="content-type" content="text/html;charset=utf-8">
<H1>301 Moved</H1>
The document has moved
<A HREF="">here</A>.

Thus, HTTP is a plaintext protocol consisting of the request information sent by client and the response as shown above.

Slow Loris Attack

Slow Loris attack exploits the fact that I could make a HTTP request very very slowly, i.e. I can initiate an HTTP request to server and keep sending data to server very slowly in order to keep that connection alive, but at the same time, never end that connection and open multiple such connections to exhaust the connection pool of the server.

Disclaimer – Penetration testing any online/offline service not owned by you without prior written permission is illegal and I’m not responsible for any damages caused. Use this content for educational purposes only.

Slow Loris Demonstration:

This means, I could keep on sending additional data to server in the form of headers. Now, I’ll start a simple PHP development server on my machine:

And I use a simple Node script to perform what we discussed above, on my local server:

You could find the Node script used here.

After some time, you’ll see that our PHP server actually crashes!

This is because there are way too many open connections and PHP cannot handle any more open connections (due to memory/hardware limits).

Now, of course this works flawlessly on a local development server, but if you’re able to find a server which does not implement protections against slow loris attack, it is a big problem for them.

Protections against Slow Loris attack

  1. Use solutions like Cloudflare in front of your servers to prevent DoS/DDoS
  2. Rate limit number of simultaneous connections open by a particular IP address to a small number.


A lot of servers (nginx/apache2 new versions) comes with slow loris attack protections by default, but a lot of internal services, servers might be vulnerable to this simple attack. You might want to check your services and implement the fixes. Web security is an exciting area, and I plan to do web series on it on codedamn. You could connect with me on twitter for updates too. Till then, be safe!

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