Are you ready for slow reading?

   Imagine a line at a fast food restaurant that serves two types of burgers, and a customer at the cashier is stuck for a while deciding what he wants to order, making the rest of the line anxious, slowing down the business. Now imagine a line at the same restaurant, but with a sign saying "think ahead of your order," which is supposed to speed things up. But now the customer orders hundreds of burgers, pays, and the line is stuck again, because he can take only 5 burgers at time to his car, making signs ineffective.

While developing the slowhttptest tool, I thought about this burger scenario, and became curious about how HTTP servers react to slow consumption of their responses. There are so many conversations about slowing down requests, but none of them cover slow responses. After spending a couple of evenings implementing proof-of-concept code, I pointed it to my so-many-times-tortured Apache server and, surprisingly, got a denial of service as easily as I got it with slowloris and slow POST.

Let me remind you what slowloris and slow POST are aiming to do: A Web server keeps its active connections in a relatively small concurrent connection pool, and the above-mentioned attacks try to tie up all the connections in that pool with slow requests, thus causing the server to reject legitimate requests, as in first reastaurnt scenario.

The idea of the attack I implemented is pretty simple: Bypass policies that filter slow-deciding customers, send a legitimate HTTP request and read the response slowly, aiming to keep as many connections as possible active. Sounds too easy to be true, right?
 

Crafting a Slow Read

Let’s start with a simple case, and send a legitimate HTTP request for a resource without reading the server’s response from the kernel receive buffer.

We craft a request like the following:

GET /img/delivery.png HTTP/1.1
Host: victim
User-Agent: Opera/9.80 (Macintosh; Intel Mac OS X 10.7.0; U; Edition MacAppStore; en) Presto/2.9.168 Version/11.50
Referer: http://code.google.com/p/slowhttptest/

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