HTTP Post Denial Of Service: more dangerous than initially thought

What’s special about this denial of service attack is that it’s very hard to fix because it relies on a generic problem in the way HTTP protocol works. Therefore, to properly fix it would mean to break the protocol, and that’s certainly not desirable. The authors are listing some possible workarounds but in my opinion none of them really fixes the problem.

The attack explained

An attacker establishes a number of connections with the web servers. Each one of these connections contains a Content-Length header with a large number (e.g. Content-Length: 10000000). Therefore, the web server will expect 10000000 bytes from each one of these connections. The trick is not to send all this data at once but to send it character by character over a long period of time (e.g. 1 character each 10-100 seconds). The web server will keep these connections open for a very long time, until it receives all the data. In this time, other clients will have a hard time connecting to the server, or even worse will not be able to connect at all because all the available connections are taken/busy.

In this blog post, I would like to expand on the effect of this denial of service attack against Apache.

First, I would like to start with one of their affirmations:

“Hence, any website which has forms, i.e. accepts HTTP POST requests, is susceptible to such attacks.”

At least in the case of Apache, this is not correct. It doesn’t matter if the website has forms or not.
Any Apache web server is vulnerable to this attack. The web server doesn’t decide if the resource can accept POST data before receiving the full request.

Read more: acunetix

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